Practical solutions guide to arc flash hazards

Practical Solution Guide
to
Arc Flash Hazards

Conrad St. Pierre
David Castor, P.E.
Chet Davis, P.E.
Robert Luo, PhD
Satish Shrestha

© EasyPower LLC, 2013.
Published by EasyPower LLC
No part of this publication may be reproduced, stored in a retrieval system, or
transmitted, in any form or by any means, electronic, mechanical, photocopying, or
otherwise, without the prior permission of the publisher.

Disclaimer
Warning - Disclaimer: The calculation methods listed in the book are based
on theoretical equations derived from measured test results. The test
results are a function of specific humidity, barometric pressure,
temperature, arc distance, and many other variables. These parameters
may not be the same in your facility or application. The results calculated
from these equations may not produce conservative results when applied to
your facility. PPE recommended by any calculation method will NOT
provide complete protection for all arc hazards. Injury can be expected
when wearing recommended PPE. The results should be applied only by
engineers experienced in the application of arc flash hazards. The authors
make no warranty concerning the accuracy of these results as applied to
real world scenarios.

Arc flash as given in NFPA-70E and IEEE Std-1584-2002TM is
concerned with personal injury when a worker is near or working on
energized equipment. Working on energized conductors should only be
done when it is impossible to shut down the equipment. This book does not
condone working on energized equipment.

Using the methods in NFPA 70E or IEEE Std-1584 does not insure that a
worker will not be injured by burns from an arc-flash. Following the NFPA
70E and IEEE 1584 procedures and wearing the proper protective
equipment will greatly reduce the possibility of burns. Using the incident
energy equations developed from the arc flash tests, it is expected that the
personal protective equipment (PPE) classification per the tables in NFPA
70E will be adequate for 95% of the classifications based on test results.

Forward
EasyPower LLC is pleased to bring you the “Practical Solution Guide to Arc Flash
Hazards” version 1.0. We believe this will be a valuable tool for electrical engineers,
safety managers, or anyone responsible for implementing and maintaining an arc flash
hazard safety program.
The guide was designed to walk you through the necessary steps of implementing an arc
flash assessment as part of your overall safety program requirements. It will help you
and your team make important decisions concerning the safety of your employees and
how to manage the complex tasks of OSHA and NFPA-70E compliance for arc flash
hazards.
Arc flash hazard analysis and safety program development to protect against arc flash
hazards is in its infancy. Research into the arcing phenomena is ongoing as industry tries
to better understand and model arcing faults. Standards and recommended practices are
changing constantly in order to reflect the added understanding we are gaining and to
better protect workers. Personal protective equipment (PPE) is also changing at a rapid
pace as new and better technology is developed. EasyPower LLC has created an Arc
Flash Resource Center at the website www.easypower.com to keep you up to date as new
information becomes available and industry advancements are made. Look for new
versions of this guide as we continue to enhance and add new technology to the arc flash
assessment process.
EasyPower LLC is committed to providing industry with the most advanced state of the
art technology in our EasyPower software product line. We believe the EasyPower
software provides the self-documenting solution capabilities to keep your safety program
current and in compliance with OSHA and NFPA-70E regulations. EasyPower LLC can
also provide detailed engineering studies and arc flash assessment programs to help your
company get started.
We hope that the “Practical Solution Guide to Arc Flash Hazards” becomes a valued
resource to your library.

Table of Contents
1
1.1
1.2
1.3
1.3.1
1.4
1.5
1.6
1.7
1.7.1
1.7.2
1.7.3
1.7.4
1.7.5
1.7.6
1.8
1.9
1.10

Introduction ...................................................................................................... 1
Causes of Electric Arcs .......................................................................................... 1
The Nature of Electrical Arcs ................................................................................. 2
Hazards of Arcing Faults ........................................................................................ 3
Probability of Survival ........................................................................................ 4
Impacts of Arc Flash .............................................................................................. 4
Potential Exposure to Arc Flash............................................................................. 4
Recent Developments in Addressing Arc Flash Hazard ........................................ 5
NFPA 70E and Arc Flash Hazard .......................................................................... 7
Protection Boundaries ....................................................................................... 7
Flash Protection Boundary ................................................................................ 7
Personal Protective Equipment ......................................................................... 8
Classification of Hazard/Risk Category ............................................................. 8
Determining Flash Protection Boundary and Hazard Category ........................ 9
Difference between NFPA 70E and IEEE 1584 Calculations............................ 9
Hazard Assessment Methods .............................................................................. 10
Reducing Exposure to Arc Flash Hazard ............................................................. 11
Arc Flash Hazard Program .................................................................................. 12

2.1
2.2
2.2.1
2.2.2
2.2.3
2.2.4
2.2.5
2.2.6
2.3
2.3.1
2.3.2
2.3.3
2.4
2.5
2.5.1
2.5.2
2.5.3
2.5.4
2.5.5
2.5.6
2.6
2.7

Planning for an Arc Flash Hazard Program ................................................ 15
Overview of the Planning Process ....................................................................... 15
Review of Existing Practices and Resources ...................................................... 15
Benefits of Existing Practices Review ............................................................. 16
Determining Significant Exposure to an Arc Flash Hazard ............................. 16
Assessing Existing Safety Program................................................................. 17
Assessing Available Human Resources .......................................................... 19
Power System Size and Records .................................................................... 19
Financial Resources ........................................................................................ 20
Values, Goals and Objectives .............................................................................. 20
Reasons to Address Arc Flash Hazard ........................................................... 20
Goals of Arc Flash Hazard Program................................................................ 21
Objectives ........................................................................................................ 21
Selection of Resource Persons............................................................................ 22
Selection of Arc Flash Hazard Assessment Method............................................ 23
Simplified NFPA 70 Tables .............................................................................. 23
Hand Calculations............................................................................................ 24
Spreadsheet Calculator ................................................................................... 24
Commercial Integrated Software - EasyPower............................................. 24
Accuracy and Conservatism ............................................................................ 25
Overprotection ................................................................................................. 26
Estimate of Human Efforts ................................................................................... 27
Cost Estimates ..................................................................................................... 28

2

3

Arc Flash Calculation Methods .................................................................... 29
3.1 IEEE Std 1584-2002 ............................................................................................ 29
3.1.1
Step 1: Estimate of Arcing Current .................................................................. 29
3.1.2
Step 2: Estimate of Normalized Incident Energy ............................................. 30
3.1.3
Step 3: Estimate of Incident Energy ................................................................ 30
3.1.4
Step 4: Flash Protection Boundary .................................................................. 31
3.2 NFPA 70E Protection Boundaries........................................................................ 31
3.2.1
Flash Protection Boundary .............................................................................. 31

Limited Approach Boundary ............................................................................ 32
3.2.2
3.2.3
Restricted Approach Boundary ........................................................................ 32
3.2.4
Prohibited Approach Boundary ........................................................................ 32
3.3 NFPA 70E – May 2003 ROP ............................................................................... 32
3.3.1
Annex B Method .............................................................................................. 33
3.3.1.1
Arc Flash Boundary ..................................................................................... 33
3.3.1.2
Incident Energy............................................................................................ 33
3.3.2
Annex C Method (2003)................................................................................... 33
3.4 NFPA 70E Tables ................................................................................................ 34
3.4.1
Flash Protection Boundary .............................................................................. 34
3.4.2
Hazard/Risk Category Classifications ............................................................. 34
3.5 Arc Blast Pressure ............................................................................................... 37
4

Practical Steps to Arc Flash Calculations................................................... 39
4.1 Step 1 – Identification of Locations/Equipment for AFH ...................................... 40
4.2 Step 2 – Data Collection ...................................................................................... 40
4.2.1
Equipment Data for Short Circuit Analysis ...................................................... 40
4.2.2
Equipment Data For Protective Device Characteristics .................................. 41
4.2.3
Equipment Data For Arc Flash Study .............................................................. 41
4.2.4
Determine All Possible Operating Conditions.................................................. 42
4.3 Step 3 – Prepare single-line diagram of the system ............................................ 43
4.4 Step 4 – Short Circuit Study ................................................................................. 43
4.4.1
Calculate Bolted Fault Current ........................................................................ 44
4.4.2
Calculate Contributing Branch Currents .......................................................... 45
4.5 Step 5 – Determine Expected Arc Current........................................................... 45
4.5.1
Calculate Arc Current ...................................................................................... 45
4.5.2
Consider a range of Arc Current ...................................................................... 45
4.5.2.1
Tolerance due to Random Variation based on IEEE-1584 ......................... 45
4.5.2.2
Tolerance due to Random Variation based on Actual Data ........................ 46
4.5.2.3
Limits for Arc Currents ................................................................................. 48
4.5.3
Calculate Branch Currents Contributing to the Arc Current ............................ 49
4.6 Step 6 – Determine Arcing Time .......................................................................... 50
4.6.1
Evaluate Protective Device Performance ........................................................ 52
4.6.2
Trip time for multiple feeds .............................................................................. 53
4.7 Step 7 – Determine Incident Energy .................................................................... 54
4.7.1
Tolerance of Calculated Incident Energy......................................................... 55
4.8 Step 8 – Determine Hazard/Risk Category .......................................................... 56
4.9 Step 9 – Determine Flash Protection Boundary .................................................. 57
4.10 Step 10 – Document The Arc Flash Hazard Assessment ................................... 58
4.10.1
Documentation in Reports ........................................................................... 59
4.10.2
Documentation in One-Line Diagrams ........................................................ 59
4.10.3
Documentation on Equipment ..................................................................... 61

5

Limiting Arc Exposure .................................................................................. 65
5.1 Avoiding Arc Flash Accidents .............................................................................. 65
5.1.1
Preventive Maintenance .................................................................................. 65
5.1.2
Working on Live Equipment ............................................................................. 67
5.2 Reducing Incident Energy on Worker .................................................................. 67
5.2.1
Reducing the Level of Fault ............................................................................. 67
5.2.1.1
System Configuration .................................................................................. 67
5.2.1.2
Current Limiting Fuses/Breakers ................................................................. 68
5.2.1.3
Current Limiting Reactors............................................................................ 68
5.2.2
Reducing Arcing Time ..................................................................................... 69
5.2.2.1
Reducing Breaker Response Time Safety Margin ...................................... 69
5.2.2.2
Bus Differential Protection ........................................................................... 69
5.2.2.3
Temporary Instantaneous Setting ............................................................... 70

Retrofit Instantaneous Trip Device .............................................................. 70
5.2.2.4
5.2.2.5
Optical Sensor Trip...................................................................................... 73
5.2.2.6
Fuse Size and Speed .................................................................................. 73
5.2.2.7
Temporary Relay Settings ........................................................................... 73
5.2.2.8
Protective Device Coordination Study ......................................................... 74
5.2.3
Remote Operation and Racking ...................................................................... 74
6

Personal Protective Equipment ................................................................... 77
6.1 Standards on Personal Protective Equipment ..................................................... 77
6.1.1
OSHA ............................................................................................................... 78
6.1.2
NFPA 70E ........................................................................................................ 79
6.1.3
ASTM ............................................................................................................... 79
6.2 Fire Resistant Clothing ......................................................................................... 80
6.2.1
Factors Affecting Protection Level From Arc ................................................... 80
6.2.2
Care of FR Clothing ......................................................................................... 81
6.2.3
Useful Life of PPE............................................................................................ 81
6.2.4
Selection of PPE .............................................................................................. 82
6.2.5
Types of FR Clothing ....................................................................................... 83
6.3 Other PPE ............................................................................................................ 84

7
7.1
7.2
7.2.1
7.2.2
7.2.3
7.2.4
7.2.5
7.2.6
7.2.7
7.2.8
7.3
7.3.1
7.3.2
7.3.3
7.4
7.5
7.6

Arc Flash Hazard Program ........................................................................... 87
Electrical Safety Program .................................................................................... 87
Training ................................................................................................................ 87
Awareness ....................................................................................................... 87
Standards and Codes ...................................................................................... 88
Understanding of Arc Flash Quantities ............................................................ 88
PPE ................................................................................................................ 88
Reading and Following Warning Signs and Labels ......................................... 89
Methods to Reduce Risk While Working on Live Exposed Parts .................... 89
Arc Flash Hazard Assessment ........................................................................ 89
Documentation................................................................................................. 89
Safety Audit .......................................................................................................... 89
Purpose of Safety Audit ................................................................................... 90
Arc Flash Hazard and Safety Audit ................................................................. 90
Methods of Obtaining Information for Audit ..................................................... 91
Safety Meetings ................................................................................................... 91
Documentation ..................................................................................................... 91
Personal Protective Equipment............................................................................ 92

Appendices
A.
B.

Uncertainties in Arc Flash Hazard Analysis ......................................................... 97
Implementing the New Arc Flash Standards ..................................................... 113

Index ..................................................................................................................... 129

Chapter 1. Introduction

1 Introduction
This chapter provides an overview of arc flash hazards and briefly describes the various
causes, nature, results, standards and procedures associated with arc flash hazards. In
order to deal with the hazard, it is first necessary to develop an understanding of the
phenomena. Details are provided in the following chapters.
An electric arc or an arcing fault is a flashover of electric current through air in electrical
equipment from one exposed live conductor to another or to ground. Arc flash hazard is
the danger of excessive heat exposure and serious burn injury due to arcing faults in
electrical power systems. Electric arcs produce intense heat, sound blast and pressure
waves. They have extremely high temperatures, radiate intense heat, can ignite clothes
and cause severe burns that can be fatal.
The demand for continuous supply of power has brought about the need for electrical
workers to perform maintenance work on exposed live parts of electrical equipment.
Besides the existence of electrical shock hazard that results from direct contact of live
conductors with body parts, there also exists a possibility of electric arcs striking across
live conductors. Although electrical safety programs have existed since the beginning of
electricity, arc flash hazard has not been prominently addressed until recently.

1.1 Causes of Electric Arcs
Arcs can be initiated by the following:


Glow to arc discharge:
 Dust and impurities: Dust and impurities on insulating surfaces can
provide a path for current, allowing it to flashover and create arc discharge
across the surface. This can develop into greater arcs. Fumes or vapor of
chemicals can reduce the breakdown voltage of air and cause arc flash.
 Corrosion: Corrosion of equipment parts can provide impurities on
insulating surfaces.
Corrosion also weakens the contact between
conductor terminals, increasing the contact resistance through oxidation or
other corrosive contamination. Heat is generated on the contacts and
sparks may be produced, this can lead to arcing faults with nearby exposed
conductors of different phase or ground.



Condensation of vapor and water dripping can cause tracking on the surface of
insulating materials. This can create a flashover to ground and potential escalation
to phase to phase arcing1.



Spark discharge:

1

Practical Solution Guide to Arc Flash Hazards

 Accidental touching: Accidental contact with live exposed parts can
initiate arc faults.
 Dropping tools: Accidental dropping of tools may cause momentary short
circuit, produce sparks and initiate arcs.


Over-voltages across narrow gaps: When air gap between conductors of different
phases is very narrow (due to poor workmanship or damage of insulating
materials), arcs may strike across during over-voltages.



Failure of insulating materials.

Electric arcs are also caused by the following:


Improperly designed or utilized equipment.



Improper work procedures.

Figure 1.1: (a) Arc blast in box2 ; (b) Arcing fault in electrical panel board

1.2 The Nature of Electrical Arcs




2

Electric arcs produce some of the highest temperatures known to occur on earth –
up to 35,000 degrees Fahrenheit3. This is four times the surface temperature of the
sun.
The intense heat from arc causes the sudden expansion of air. This results in a
blast with very strong air pressure (Lightning is a natural arc).




All known materials are vaporized at this temperature. When materials vaporize
they expand in volume (Copper – 67,000 times, Water–1670 times4). The air blast
can spread molten metal to great distances with force.



For a low voltage system (480/277 V), a 3 to 4-inch arc can become “stabilized”
and persist for an extended period of time.



Energy released is a function of system voltage, fault current magnitude and fault
duration.



Arcs in enclosures, such as a Motor Control Center (MCC) or switchgear, magnify
blast and energy transmitted as the blast is forced to the open side of the enclosure
and toward the worker.

1.3 Hazards of Arcing Faults

Figure 1.2: (a) Hand burned by arc flash5; (b) Clothed areas can be burned more
severely than exposed skin
Some of the hazards of arcing faults are:


Heat: Fatal burns can occur when the victim is several feet from the arc. Serious
burns are common at a distance of 10 feet6. Staged tests have shown temperatures
greater than 437F on the neck area and hands for a person standing close to an arc
blast7.



Objects: Arcs spray droplets of molten metal at high-speed pressure.
shrapnel can penetrate the body.



Pressure: Blast pressure waves have thrown workers across rooms and knocked
them off ladders8. Pressure on the chest can be higher than 2000 lbs/ sq. ft.

Blast

3




Clothing can be ignited several feet away. Clothed areas can be burned more
severely than exposed skin.



Hearing loss from sound blast. The sound can have a magnitude as high as 140 dB
at a distance of 2 feet from the arc9.

1.3.1 Probability of Survival
Injuries due to arc flash are known to be very severe. According to statistics from the
American Burn Association, the probability of survival decreases with the increasing age
of the arc flash burn victim.

25% Body Burn
100
50% Body Burn

% Survival

80

75% Body Burn

60
40
20
0
20 - 29.9

30 - 39.9

40 - 49.9

50 - 59.9

Age Range, Years

Figure 1.3: Burn Injury Statistics – Probability of Survival (Source: American Burn
Association, 1991-1993 Study; Revised March 2002)

1.4 Impacts of Arc Flash
Treatment can require years of skin grafting and rehabilitation. The victim may never
return to work or retain the same quality of life. Some of the direct costs are:


Treatment can exceed $1,000,000/case.



Litigation fees.



Production loss.

1.5 Potential Exposure to Arc Flash
Although it may appear that arc flash incidents are uncommon, statistics show that the
damage they cause is considerable. Bureau of Labor Statistics data for 1994 show 11,153

4


cases of reported days away from work due to electrical burns, electrocution/electrical
shock injuries, fires and explosions.
The Census of Fatal Injuries noted 548 employees died from the causes of electrical
current exposure, fires and explosions of 6,588 work related fatalities nationwide.
In the US Chemical Industry, 56% of the fatalities that occurred over a 5-year period
were attributed to burns, fires and explosions, with many of the ignition sources being
related to electrical activity.
Capelli-Schellpfeffer, Inc. of Chicago reported that there are 5 to 10 arc flash injuries per
day resulting in hospitalization. Many arc flash accidents/injuries occur that do not
require a stay or are not properly documented for national tracking purposes. The
number of arc flash accidents is greater than many engineers realize since most arc flash
accidents do not make the daily news.
IEEE Standard 1584, IEEE Guide for Performing Arc Flash Hazard Calculations,
provides 49 arc flash injury case histories in Annex C. A brief description is provided for
each case on incident setting, electric system, equipment, activity of worker, event,
apparel worn by the worker and the outcome of the incident. Readers are encouraged to
read these case histories to gain insights on various conditions leading to such incidents.
The exposure to arc flash depends on the following:


Number of times the workers work on exposed live equipment.



Complexity of the task performed, need to use force, available space and safety
margins, reach, etc.



Training, skills, mental and physical agility, coordination with helper.



Tools used.



Condition of equipment.

1.6 Recent Developments in Addressing Arc Flash
Hazard
Historically, the National Electric Code (NEC) and other safety codes have been
primarily concerned with protection from fire, electrocution, and shock hazard – arc flash
hazards were not addressed. This is now changing. The 2002 NEC now has requirements
for warning labels. The National Fire Protection Association (NFPA) is responsible for
the NEC. Since the NEC was concerned mainly with electrical design, construction and
inspection, it could not be adopted by employers and employees with regard to
implementing standards for workplace safety. In order to bridge this gap, a new standard,
NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces10, was

5


developed. NFPA 70E is intended for use by employers, employees, and the
Occupational Safety and Health Administration (OSHA). The publication NFPA 70E
(2000) and its proposed revision (May 2003 ROP11) include arc flash hazard as a
potential danger to workers near and around live exposed electrical parts. NFPA 70E and
IEEE Std 1584-2002TM provide guidance on implementing appropriate safety
procedures and arc flash calculations. For the actual wording, see section 6.1.2.
NEC Article 110.16 requires "field marking" of potential arc flash hazards for panels
likely to be serviced or examined in an energized condition. This article also contains a
fine print note (FPN) regarding proper signage and an FPN referencing NFPA 70E.
These FPNs are not technically part of the NEC, but are recommended practices.
OSHA has not specifically addressed arc flash hazards, however, there exists adequate
safety requirements for employers to follow to ensure the safety of the worker in the
workplace (General Duty clause). Some of these are outlined in Table 6.1 in Chapter 6.
The Code of Federal Regulations (Standards – 29 CFR) Part 1910 deals with
occupational safety and health standards. Standards on personal protective equipment
(PPE) are outlined in subpart 132. In response to an inquiry on OSHA's stand on arc
flash hazard, Richard S. Terrill, the Regional Administrator for Occupational Safety and
Health, US Department of Labor for the Northwest Region at Seattle, concluded as
follows:
"Though OSHA does not, per se, enforce the NFPA standard, 2000 Edition,
OSHA considers NFPA standard a recognized industry practice. The employer
is required to conduct assessment in accordance with CFR 1910.132(d)(1). If
an arc flash hazard is present, or likely to be present, then the employer must
select and require employees to use the protective apparel. Employers who
conduct the hazard/risk assessment, and select and require their employees to
use protective clothing and other PPE appropriate for the task, as stated in the
NFPA 70E standard, 2000 Edition, are deemed in compliance with the Hazard
Assessment and Equipment Selection OSHA standard."
12

In 2002, unionized electricians, contractors and federal regulators in Columbus, Ohio,
forged an agreement to protect electrical workers on the job by using NFPA-70E. It is
claimed that this agreement could serve as a model for the nation and is expected to apply
to the 2,500 unionized electrical workers in the Columbus area. The Columbus office of
the U.S. Occupational Safety and Health Administration (OSHA), the Central Ohio
chapter of the National Electrical Contractors Association (NECA), and Locals 683 and
1105 of the International Brotherhood of Electrical Workers (IBEW) collaborated to
develop this pioneering program. The National Joint Apprentice and Training Committee
(NJATC), the training arm of IBEW and NECA, provided technical expertise and will be
responsible for development and coordination of training for this effort.

6


1.7 NFPA 70E and Arc Flash Hazard
1.7.1 Protection Boundaries
70E defines a series of boundaries relating to electrical safety when working on
energized equipment. Only "qualified" people can enter these boundaries and they are
required to wear appropriate PPE within these boundaries.
The four protection boundaries are:
1. Flash Protection Boundary
2. Limited Approach Boundary
3. Restricted Approach Boundary
4. Prohibited Approach Boundary
The flash protection boundary is briefly described in this chapter. For further details and
description on the other boundaries, see Chapter 4.

Limited
Approach
Boundary

Flash
Protection
Boundary

Prohibited
Approach
Boundary

Restricted
Approach
Boundary

Energized
Equipment

Figure 1.2: Protection boundaries
1.7.2 Flash Protection Boundary
The flash protection boundary is the distance from the arc source (energized exposed
equipment) at which the potential incident heat energy from an arcing fault falling on the
surface of the skin is 1.2 calories/cm2. An exposure to 1.2 cal/cm2 would normally result
in a curable second-degree burn. Within this boundary workers are required to wear
protective clothing like fire resistant (FR) shirts and pants, and other equipment to cover
various parts of the body. This distance may vary from equipment to equipment since it

7


is a function of the available fault current of the system at that point, the voltage and the
tripping characteristics of the upstream protective device as well as some other
parameters. See Chapter 4 for details.
1.7.3 Personal Protective Equipment
NFPA specifies the requirement of personal protective equipment (PPE) for workers
within the flash protection boundary. All parts of the body which may be exposed to the
arc flash, need to be covered by the appropriate type and quality of PPE. The entire PPE
set may be comprised of FR clothing, helmet or headgear, face shield, safety glasses,
gloves, shoes, etc. depending upon the magnitude of the arc energy. The amount of PPE
required and its quality needs to be determined on the basis of the calculated incident
energy on the worker's body. The calculations need to be performed by a qualified
person such as an engineer. The protective clothing should limit the incident energy
reaching the chest/face of the worker to less than 1.2 cal/cm2. FR clothing provides
thermal insulation and is also self-extinguishing. Protective clothing is rated in cal/cm2.
For details on PPE, see Chapter 6.
1.7.4 Classification of Hazard/Risk Category
NFPA 70E defines 5 levels of risk category for arc flash hazard based upon the calculated
incident energy at the working distance, as shown in Table 1.1. Examples of typical
protective clothing that cover the torso are also provided in this table. Other PPE are also
required to protect various parts of the body.
Table 1.2: Hazard/risk classification as per NFPA 70E-2000
Category
0

N/A

1

5 cal/cm2

FR shirt and FR pants

2

8 cal/cm2

Cotton underwear plus FR shirt & pants

3

25 cal/cm2

Cotton underwear plus FR shirt & pants plus FR
coverall

4

8

Energy Level

Typical PPE Examples

40 cal/cm2

Cotton underwear plus FR shirt & pants plus
double layer switching coat and pants

Non-melting, flammable materials (e.g. untreated
cotton, wool, rayon, etc.)


1.7.5 Determining Flash Protection Boundary and Hazard Category
NFPA 70E-2000 provides two methods of determining flash protection boundary as
outlined in Part II section 2-1.3.3.2:
 A fixed distance of 4.0 ft. for a fault with the product of fault current and fault
duration less than 5000 ampere seconds.
 Ralph Lees' equation.
In the proposed draft, NFPA 70E-May 2003 ROP, three (3) acceptable methods of
determining flash protection boundary and hazard categories are provided. They are
listed below. (see Chapter 4 for details):
 Simplified NFPA 70E tables: proposed draft Table 220.2(B)(2)(C) for flash
protection boundary and Table 220.6(B)(9)(A) for hazard category.
 Calculations based on NFPA 70E Annex B.
 Calculations based on IEEE Standard 1584.
IEEE Standard 1584 recommends that the person performing arc flash hazard assessment
should understand the limitations of the method. Some of the limitations are:
 The equations used in the standards are based on tests performed in a laboratory and
the conditions may differ from those in the plant where the application of the
standards is sought.
 The equations are based on a range of test values such as available fault current, arc
gap, enclosure size, etc. It is necessary to check whether parameters existing for the
plant are within the same range.
 The random nature of arcs makes it very difficult to model the arc precisely. The
estimate provided by the equation for arc current in the standards is an “average”
value.
All of the known standards or methods have some limitation. The tables may be easy to
use and require less or no computation. However, these are based on typical equipment
and systems and are very approximate. Detailed analysis yields different results than the
tables do. Therefore, whatever standard you may choose, it is necessary to understand its
limitations. Further detailed analysis can overcome some of these limitations.
1.7.6 Difference between NFPA 70E and IEEE 1584 Calculations
NFPA 70E method estimates incident energy based on a theoretical maximum value of
power dissipated by arcing faults, based on Ralph Lee's work. This is believed to be
generally conservative. In contrast, IEEE 1584 estimates incident energy with empirical

9


equations developed from statistical analysis of measurements taken from numerous
laboratory tests. The IEEE method was intended to be more realistic rather than
conservative, and aims to avoid accidents due to limitations provided by over-protection
to workers. Over-protection can also lead to the restriction of visibility and movement,
discomfort and reduces worker productivity.

1.8 Hazard Assessment Methods
Arc flash hazard calculation can be carried out in several ways. The choice of method
may be based on available information, volume of calculation work, necessity for
accuracy, availability of resources and quality of arc flash hazard mitigation program.
Whatever method is used, the qualified person performing the assessment should be
aware of the limitations of the method employed, and should perform further engineering
analysis to achieve best results.
1. Table 3-3.9.1 Hazard Risk Category Classifications in NFPA 70E-2000 provides a
simple way to determine the hazard category. Simplified tables of proposed NFPA
70E-May 2003 ROP: You can look up Table 220.2(B)(2)(C) and Table
220.6(B)(9)(A) to perform hazard assessment for small radial distribution systems.
This method requires the least time and is suitable when limited information is
available on the power system. This is the least accurate method because it is very
generalized. These tables do not provide you with the exact PPE rating that are
required in cal/cm2.
2. Hand calculations: You can perform hand-calculations using NFPA 70E equations or
IEEE 1584 equations for small radial distribution systems. This is very time
consuming and is not suitable for large systems. While performing many hand
calculations, unnoticed errors may be introduced in the calculations.
3. Spreadsheet calculator: IEEE Standard 1584 comes with a spreadsheet calculator in
Excel® that can be used to assess arc flash hazards. Similar spreadsheets can be easily
built using NFPA 70E equations. This calculator requires the user to enter available
fault current data for each point of assessment. Required data for each point includes
short circuit current and protective device trip times for each source. Because of the
inability of the spreadsheet calculator to determine the trip time and short circuit
currents and because of the time-consuming nature of this process, assumptions and
approximations have to be made, which compromise accuracy. This method is limited
to radial single source systems and errors increase with the size of the system.
4. Commercial integrated software: This is practical for all systems with multiple power
sources and multiple scenarios of interconnections where better accuracy is desired
and where the system goes through ongoing changes over time. Once the data is
entered into the software, carrying out hazard assessment takes very little time. The
results are instantly observed. Graphical integrated power system software like
EasyPower can perform multiple aspects of engineering studies at once. For instance,
EasyPower can perform short circuit calculations, protective device coordination, and

10


arc flash calculations and display results graphically with simple mouse clicks.
EasyPower provides an active self-documenting arc flash assessment program to meet
the needs of today’s changing electrical systems.
The results of the assessment can show up on one-line drawings, detailed arc flash
reports and warning labels that can be placed on the hazardous location or equipment.
An additional advantage of EasyPower software is the ability to simulate and modify
the protective device settings in order to reduce exposure to arc flash hazard. The
software can automatically obtain the accurate arcing time from the trip
characteristics of the protective devices. All other methods lack this ability and
therefore need to rely on some approximate value for arcing time.
TX-2

BUS-4
BL-2
67.1" AFB
10.4 cal / cm² @ 18"
#3 @ 18"

BL-1
99.6" AFB
18.7 cal / cm² @ 18"
#3 @ 18"
BL-3
31.5" AFB
2.7 cal / cm² @ 18"
#1 @ 18"

BL-4
67.1" AFB
10.4 cal / cm² @ 18
#3 @ 18"

M-1

Figure 1.3: Example of arc flash hazard calculation results on one-line diagram in
the integrated software EasyPower®.

Figure 1.4: Example of detailed arc flash hazard report for equipment shown in
Figure 1.3 produced by the integrated software EasyPower®.

1.9 Reducing Exposure to Arc Flash Hazard
In order to reduce exposure of workers to arc flash hazard, companies and employees can
take the following steps:
1. Understand arc flash and its associated hazards and acknowledge the existence of the
hazard.
2. Assess the magnitude of the hazard.
11


3. Develop an arc flash hazard program and integrate it into the safety program.

1.10 Arc Flash Hazard Program
An arc flash hazard program is implemented as part of the electrical safety program,
which in turn is part of the overall safety program of the company. The main objective of
the program is to prevent or minimize injuries to workers from arc flash. Since arc flash
hazard mitigation is a fairly new concept in the industry, it is expected that considerable
efforts and allocation of resources will be required to provide an initial thrust to
effectively launch the program. The amount of additional resource allocation required
for the program and the likely success of the program may depend on what resources are
already available in the company and the related existing safety practices. Chapter 2
deals with reviewing existing practices and available resources. This is the first step in
carrying out an arc flash hazard program.
The arc flash hazard program consists of the following steps:
1. Hazard assessment: A qualified person performs calculations based on power system
parameters to determine the flash protection boundary, the incident energy a worker
may be subject to and the hazard/risk category. The basic equations and steps are
outlined in Chapters 3. An important task in the assessment is reviewing available
technical data and collecting the remaining necessary data. The standards themselves
do not provide the various practical issues in carrying out hazard assessment. The
practical considerations are outlined in Chapter 4.
2. Documentation: It is necessary to document the results of arc flash hazard assessment
in reports and drawings, and also provide signs and labels on equipment and at
hazardous areas. Documentation is also a part of the planning process before working
on live equipment and after, if work changes are made to the equipment or system.
Also required is the documentation of training provided to workers. Chapter 7
provides some insights to documentation. NFPA 70E-2000 requires up-to-date
single-line drawings of electrical systems, and the proposed 2004 update provides
recommendations for short circuit and protective device study requirements.
3. Personal protective equipment (PPE) plan: Based upon the hazard assessment the
appropriate PPE must be selected and provided to the workers. Workers must wear
the PPE properly, provide care and maintenance of the PPE, inspect it before every
use and dispose of it after its useful life has expired. Chapter 6 describes the various
aspects of PPE.
4. Development of procedures to minimize hazard: The potential hazards can be
minimized by developing safer working methods, providing protective shields, proper
work planning, etc. The exposure to arc flash can also be reduced by improving
system designs, using current-limiting devices and solid state relays, and adjusting
relay and trip device to safer settings. Chapter 5 outlines some procedures to
minimize risk.

12


5. Training for workers: Workers who are exposed to arc flash hazard should be well
trained to understand what the hazard is, how it is initiated, how to read the
documents and warning labels, how to properly wear PPE, and how the hazard can be
reduced with safer working procedures. Different tasks will require different work
practices.
6. Continual improvement: It is expected with more research and development in arc
flash hazard, that there will be further additions to what we already know. The arc
flash hazard program can be continually improved by including new developments in
standards, industry practices and PPE. Since the power system within a company can
keep changing with time, it is necessary to update arc flash assessment information on
a regular basis. Also, experience can bring in new ideas from workers that can be
included in the program. For this reason, it is necessary to keep the program ongoing
rather than implement it as a one-time project.
7. Safety audit: Safety audits should be performed regularly to evaluate various aspects
of a safety program. The safety audit should include arc flash hazard. If the arc flash
hazard program is in its initial stages, then a closer examination is required.
8. Corporate-wide plan: Corporate-wide plan should be implemented to ensure
consistency in safety practices. It is not advisable to have each plant or division
implement the safety program differently. Communication channels should be
established and responsibility should be distributed between various plants or
divisions, taking a unified approach.
1

Ralph Lee, "Pressures Developed by Arcs", IEEE Transactions on Industry Applications, Vol.
IA-23, No. 4. July/August 1987, page 760-764.
2

Source: Thomas E. Neal, Presentation "Insight Into The Arc Hazard", IEEE-PCIC Electrical
Safety Workshop, February, 2003; © DuPont Company.
3

Ralph Lee, "The Other Electrical Hazard: Electrical Arc Blast Burns", IEEE Transactions on
Industry Applications, Vol. IA-18, No. 3 May/June 1987, page 246-251.

4

See endnote 1.

5

Source: Danny P. Ligget, Presentation "Electrical Hazards – Taking Basics to the Future", IEEEPCIC Electrical Safety Workshop, February, 2003.
6

See endnote 3.

7

Ray A. Jones, et al, "Staged Tests Increase Awareness of Arc-Flash Hazards in Electrical
Equipment", IEEE Transactions on Industry Applications, Vol. 36, No. 2, March/April 2000, page
659-667.
8

See endnote 1.

9

See endnote 7.

10

NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces10, 2000
Edition, National Fire Protection Association.

13


11

NFPA 70E – May 2003 ROP is only a proposed version. The 2003 ROP Revision of the
standard is scheduled to be published as the 2004 version after multiple changes in January of
2004. Readers should note the differences and follow the published standard after it is released.
12

NFPA Online, "Landmark agreement to use NFPA 70E protects electricians in Columbus OSHA, IBEW and NECA contractors forge pact that could lead the nation", September 27, 2002;
( http://www.nfpa.org/PressRoom/NewsReleases/ Landmark/ Landmark.asp).

14

Chapter 2. Planning for Arc-flash Hazard Program

2 Planning for an Arc Flash Hazard Program
2.1 Overview of the Planning Process
The planning process for the arc flash hazard program determines the following:
1. Requirements -What is to be done?
2. Implementation - Who is responsible for what?
3. Scheduling - When and how long?
4. Methodology and procedures – How to go about it?
5. Budget - How much will it cost?
6. End Result - What is to be achieved?
The components of the planning process are:
1. Review of existing practices and resources:
a. The magnitude of exposure to hazard.
b. Existing safety program.
c. Human resources.
d. Power system size and available technical records.
e. Financial resources.
2. Formulation of values, goals and objectives for the program.
3. Selection of resource persons.
4. Selection of assessment method.
5. Estimate for human efforts (hours of work).
6. Cost estimate.
The cost of PPE can be determined only after completion of the hazard assessment,
however, the tentative initial costs can be estimated for budgeting purposes.

2.2 Review of Existing Practices and Resources
Before commencing an arc flash hazard program, it is necessary to assess the safety
practices and exposure to hazards. An arc flash hazard program is generally part of the
electrical safety program, which in turn, is part of the overall safety program within the
company. The company and its workers must follow other safety practices along with
the procedures designed to reduce exposure to arc flash hazard. Arc flash procedures are
integral with existing electrical safety procedures such as lockout/tag out.

15


2.2.1 Benefits of Existing Practices Review
A review of existing situations will provide the following benefits:
1. Provide an idea of what manpower or outsourcing may be required, and how
responsibility may be delegated within the existing work groups.
2. Provide an idea of what additional training is required. If the existing training level is
inadequate for other safety procedures, arc flash training can be combined with other
safety training so that the overall cost, time and efforts are minimized. The training
program can also be designed to suit the understanding and experience level of the
workers.
3. A snapshot assessment of company safety policies: A companies view on safety
matters, and its commitment and priorities are likely to govern the shape of any safety
program it is about to implement. The policies would include vision and value
statements with regard to safety matters, goals and measurements, corporate safety
department/division and delegation of managerial responsibility for safety and budget
allocations. This review would provide a platform for changing policies if needed.
4. Provide an idea of how much investment is required. The exact amount of resources
required will be clear only after a detailed plan is designed. However, the review is a
starting point for carrying out a program. If a company already has a very strong
safety program, then including the arc flash hazard program will not take as many
resources as it would take in a company where marginal programs, or none at all,
have been implemented. The total investment required also depends on the extent of
the safety program a company may desire to implement.
2.2.2 Determining Significant Exposure to an Arc Flash Hazard
The establishment of an arc flash hazard mitigation program would depend on the level
of risk workers may be exposed to. In order to determine the level of risk, the following
considerations should be made:
1. Policies on working on energized equipment: Some companies do not allow electrical
workers to work on energized equipment – work is carried out only after deenergizing the equipment. Working on de-energized equipment should be the goal
of every company. In such cases, there is minimal arc flash hazard. However, it
must be remembered that arc flash can occur while switching off a circuit breaker or a
switch to de-energize equipment. Arcs can also be initiated by sparks from corroded
electrical parts. Workers nearby may be exposed to arc energy even though they may
not be working on the equipment.
2. Number of workers working on or near live equipment: The higher the number of
workers, the greater the risk. Therefore, the need to have a more elaborate arc flash
hazard mitigation program. Consideration must be given to integrate contractors to
comply with a program.
16


3. Number of times workers are exposed to live equipment: When the frequency of
exposure to live equipment is small, an elaborate program may not be needed – a few
simple procedures may suffice.
4. Voltage level: For low voltage equipment (240V or less) being fed by small
transformers (125kVA or less), the potential hazard is small, and therefore does not
need to be included in arc flash hazard assessment. The higher the voltage or the size
of transformers, the greater the risk.
5. Continuous processes: Continuously operated facilities may require work on
energized equipment like MCC's and panels. The exposure to risk is higher for such
plants. When possible, schedule work during plant shutdowns.
6. System size: Large systems are likely to have greater arc flash hazard due to the
higher fault currents.
7. System condition: Systems that do not receive periodic planned maintenance are
likely to have a higher risk of arc flash incidents.
8. Changes in electrical system: Since the level of risk depends on the possible
magnitude of arc current, which in turn depends on the interconnections within the
power system, a system that changes with time due to the requirements of the
company will need review of arc flash hazard when the changes are implemented.
Additional effort will need to be incorporated into the safety program to address the
changes. A static electrical power system will require the assessment only once and
the safety procedures will remain the same unless the fault level of the utility changes
or OSHA and NFPA regulations change.
9. Environmental conditions: Are the exposed live parts of electrical equipment subject
to corrosive vapors (such as in chemical plants, sea-side, etc.), oxidation, bees, dust,
rodents or birds causing electrical disturbances resulting in spark and eventually arc
flash? The chance of arc flash exposure is higher in such cases.
2.2.3 Assessing Existing Safety Program
When implementing a new arc flash hazard mitigation program, the additional efforts,
manpower, budget and time that is required will depend largely upon what is already in
place and what resources are available to the company. As mentioned in the previous
sections, the arc flash hazard program is an integral extension of the existing safety
program and is not about just sticking labels on equipment and wearing flash suits. The
following points should be considered in the preliminary planning stage.


Existence of an electrical safety program: If no such program exists, the company
will need to implement the program from scratch. Companies with rigorous safety
programs can easily incorporate the arc flash hazard program since much of the
process is similar in principle to the electric shock hazard. The main differences
lie in the PPE and the assessment process that requires detailed calculations that

17


should be carried out by an experienced electrical engineer familiar with power
systems calculations. Some questions to ask are:
 Corporate:
1. Does the company have a safety division or department?
2. If the company is small, does it have a safety team with a safety
coordinator?
 Are safety meetings conducted periodically and before commencing
work?
 Do workers receive safety training?
documented?

How often?

Is the training

 Has assessment of electrical hazards been carried out? How often? Are
warning labels posted in these areas?
 Has safety audit been performed? How often?
 Is PPE provided to workers? Is the PPE adequate? Is PPE properly used
and maintained?
 Does each facility have up-to-date electrical drawings, short circuit, and
protective device coordination studies?
 Is each facility modeled with state of the art graphical power system
software to self-document the system and safety assessment in compliance
with NFPA-70E?
 Has the company developed any procedures for safety? Do the workers
follow them?
 Are contractors required to follow the same or similar safety procedures?
 Is safety training provided to outsourced professionals or contractors?
 Willingness of workers to comply with changes in safety program:
Accepting the arc flash program and wearing arc flash PPE is a significant
change to operating habits. Experience shows that workers do not like to
comply with additional clothing.


18

Existence of arc flash hazard program: If some kind of arc flash hazard program is
already in place, then improving the program will not be difficult since the basic
concepts will already have been implemented. Any improvement will come in the
form of better accuracy in hazard evaluation, better documentation, training and


selection of PPE. Since the basic data for calculations will be readily available, the
assessment can be conducted relatively quickly.
Which of the following methods does the company use to assess arc flash hazards:
1.

Simple NFPA tables?

2.

Hand calculations using IEEE 1584 or NFPA 70E equations?

3.

Spreadsheet using the above equations?

4.

Integrated software?

2.2.4 Assessing Available Human Resources


Does the company employ safety professionals?



Does the company employ electrical engineers? How much time can they devote
to arc flash hazard program? Can they manage this project? Are they trained or
experienced in short circuit studies, protective device coordination and arc flash
hazard assessment?



Does the company employ electrical technicians or use outside technicians for
routine maintenance?



Does the company have safety coordinators for different locations? How much
time can they devote to an arc flash hazard program, for both learning and
implementing?



Does the company have personnel for managing the entire arc flash safety
program?



How much time can workers devote to learning about arc flash hazard and its
prevention as well as implementation of procedures at work?

2.2.5 Power System Size and Records


The source of power supply, multiple grid connection, co-generation and multiple
generators affect the available fault level, the complexity of the arc flash hazard
assessment and the number of scenarios that will be required for analysis. Data is
required from the serving utilities.



Radial distribution versus loops: Radial distribution systems are easier to deal with
and hand calculations can be performed for small systems. Looped distribution
systems require more rigorous calculations.

19




Number of voltage levels: The nature of arcs and therefore the calculation method
varies with the voltage level, and so does the risk.



Number of connection points (buses): Each bus needs to be assessed for arc flash
hazard, and therefore contributes to the total project size.



Number and types of equipment/load: Different data sets are required for different
equipment. The calculation details may also change.



Diversity in settings and characteristics of protective devices (i.e. fuses, relays,
circuit breakers). This affects data collection and estimation of arcing time.



Frequency of changes in the system: If the system is modified frequently, then
documentation, assessment and PPE upgrading may be required for each
modification. A robust and simpler method of achieving all of the requirements
must be selected.



Does the company have up-to-date drawings and equipment data readily available?
Has a short circuit and protective device coordination study been performed
recently for the existing system? This determines how much extra work may be
needed.

2.2.6 Financial Resources


What financial resources (budget) is being allocated for safety?



Is this adequate to meet the safety goals?



What further allocations can the company make?



Does the company view this as an expense or investment?



What are the costs of insurance and worker's compensation?

2.3 Values, Goals and Objectives
Companies usually define their values for human life and safety/well being in their safety
policies. These values form the guiding principle for most of their actions. The
following reasons for carrying out an arc flash hazard program tie in with company
values.
2.3.1 Reasons to Address Arc Flash Hazard


20

Protect the workers from potential harm and prevent loss of life.




Comply with Occupational Safety and Health Administration (OSHA) codes and
with National Fire Protection Association (NFPA) standards on employee safety,
NFPA-70E.



Prevent loss to organizations through loss of skilled manpower, litigation fees,
higher insurance costs, and loss of morale.



Increase process uptime by reducing accidents.



Required by insurance carrier.

2.3.2 Goals of Arc Flash Hazard Program


Educate all electrical employees on the potential danger.



Avoid arc flash related accidents.



Reduce exposure of body parts to arc flash in case of accidents.

2.3.3 Objectives
Objectives are basically an extension of the goals, but are more specific. Typically
companies associate measurable statistics with objectives. Some objectives may be
associated with the end result, (for instance, zero accidents), whereas some may be
objectives of the process involving "what to do" or "how to do".
For example, the process related objectives could be:


Train 50% of workers with a basic level course and 50% of workers with an
advanced level course within 6 months.



Develop rigorous procedures to avoid accidents within 3 months.



Accomplish arc flash hazard assessment in 25% of the plant locations within 6
months.



Select and purchase 50% of the required PPE in three months and complete
distribution in six months.



Provide consistent programs to all plants for OSHA compliance.

Similarly, the result related objectives could be:


Reduce the lost work day case incident rate (LWDCIR) by 50% within 1 year.



Reduce OSHA recordable incidents by 30% in the year 2004.

21




Reduce the insurance costs by 10% within 2 years.

These are general examples for safety related programs, but specific goals may be set for
arc flash hazards.
Some objectives may be set to tie in with the regular safety audits. This is useful for
ongoing programs. For example,


Reduce improper use of PPE to zero.



Reduce improperly labeled or non-labeled hazardous locations to zero.



Reduce system change documentation time to one week. In many facilities,
documentation is not done after change is implemented in the system. This may
lead to accidents. Some companies inspect their facilities to update the drawings
every few years. The best practice is to document the changes as soon as they are
completed.

The role of defining objectives is critical because program design and resource allocation
are based on the objectives.

2.4 Selection of Resource Persons
Arc flash hazard assessment can be very complex. There are a number of factors that are
dependent on power system conditions and the random nature of arcs make it necessary
to consider multiple scenarios based on statistical data. It is necessary to develop a good
understanding of the outcomes of the various methods before selecting the method.
Therefore, it is necessary to involve skilled and knowledgeable persons in an arc flash
hazard program.
There are 3 ways to select resource persons for the arc flash hazard program:
1. In-house staff: Large companies with high exposure to hazards typically employ
qualified and experienced engineers, safety professionals and trainers. These
resource people can help launch and implement the program.
2. Consultants: Outsourcing is typical in small and mid-sized companies that do not
employ adequate experts. Some large companies do not have the necessary
staffing or expertise. Consultants work closely with the company staff providing
most, or all, of decision-making roles in the program.
3. Combined implementation: Companies that eventually want their own employees
to handle all the work in the future, hire consultants for a designated period. The
consultant oversees the process and provides guidance and training. The internal
resource persons get trained during the initial phases of the program.

22


2.5 Selection of Arc Flash Hazard Assessment Method
The four different methods of arc flash hazard assessment were briefly described in
Chapter 1. Further details are provided in this section. It is necessary to understand each
method before selecting the which method is best for your company.
Table 2.1: Guide for selection of assessment method
Attribute

NFPA
Tables

Hand
SpreadCalculations sheet

Integrated
software

Number of Buses

< 25

< 25

< 50

50+

Number of Voltage Levels

1-2

1-2

2-3

3+

Radial/Loop Distribution

Radial

Radial

Radial

Either

Power Sources

1

1

1

Multiple

Frequent Changes in
System

No

No

No

Yes

Diversity in Protective
Devices

Small

Small

Medium

Large

Need for Accuracy

Low

Medium

Medium

High

Separate Short Circuit
Studies

Yes

Yes

Yes

No

Separate Coordination
Studies

Yes

Yes

Yes

No

Table 2.1 provides a guideline for the selection of assessment method based upon various
system attributes. This guide has been prepared taking the following considerations


Total engineering time for determination of hazard/risk category and boundary.



Accuracy required.



Possibility of errors in the various methods.



Ease of documentation and production of single-line diagrams.



Overall cost of various methods.

2.5.1 Simplified NFPA 70 Tables
Table 220.2(B)(2)(C) in the proposed NFPA 70E – May 2003 ROP provides the flash
protection boundary based on system voltage and enclosure type. This provides very
broad ranges and may be too high or too low at times. This table is shown in Chapter 3.

23


Table 220.6(B)(9)(A) in the proposed NFPA 70E – May 2003 ROP provides the
hazard/risk category based on available fault current, voltage, fault clearing time, type of
work to be performed and type of equipment. The available fault current is typically not
known until someone calculates it from the system data. If the available fault current is
not known, then calculations will need to be performed anyway. Once the fault currents
are known, then it is fairly easy to look up the tables to find the hazard category. The
fault clearing time may not be the same as it is assumed in this method. This could lead
to erroneous results and therefore could be risky. The summary of this table for working
on live equipment and for testing voltages is provided in Chapter 3.
2.5.2 Hand Calculations
Hand calculations can be performed either using NFPA 70E Annex B or Annex C
methods, or using IEEE Standard 1584 equations. Please read the individual standards
for details. The summary of the calculation methods for both are provided in Chapter 3.
2.5.3 Spreadsheet Calculator
The IEEE 1584 spreadsheet calculator provides a quick way to obtain arc flash hazard
results. However, like both of the previous methods, this requires the input of available
fault current and the fault clearing time. For some typical protective devices, the total let
through energy can be computed without the need for entering the clearing time. Also if
the protective device is a current-limiting device for which characteristics has been
determined, the reduced arc current and the associated reduced arc flash energy is
calculated. The calculator requires users to enter what percent of arc current is flowing
through the protective device. This can only be determined from short circuit studies.
Often, loads such as motors, contribute to the arcing fault current. The calculator does
not take this into account.
2.5.4 Commercial Integrated Software - EasyPower
EasyPower, power system software provides an extensive array of capabilities to
minimize the effort level required to obtain accurate arc flash analysis in compliance with
NFPA-70E and IEEE-1584. A brief summary of its integrated features are provided
below.
Integrated software may contain several capabilities that reduce the engineering effort to
a single arc flash assessment task. Commercial integrated software can have the
following features that are necessary for arc flash hazard assessment.


24

Short circuit analysis: A single mouse click can calculate the exact short circuit
current at every point (bus/equipment), along with the contributing currents from
every branch including the branch that contains the upstream protective device.
Calculations are provided in accordance with ANSI and IEEE Standards ensuring
proper arc current for both total energy and tripping times.




Protective device coordination analysis: Protective device coordination in
EasyPower is as simple as click-and-drag to obtain the proper setting. Automated
protection boundaries guide your settings to ensure proper selectivity. Arc flash
results are computed and displayed as you set your devices so you can fine tune for
both selectivity and arc flash protection.



Arc flash analysis: EasyPower provides total arc flash integration in both short
circuit and protective device coordination modes. Arc flash boundaries, incident
energies, PPE requirements and more, are calculated as you change system
parameters so you instantly know the effect/danger of any system or parameter
change. Assessment reports can be instantly generated and results can also
viewed on the one-line diagrams.



Graphical one-line (single-line) diagrams: EasyPower’s graphical one-line
diagrams are easy to read and follow. Output of the arc flash assessment results on
the one-line diagram are very useful for engineers and electricians before working
on the equipment.



Scenario Manager: EasyPower’s Scenario ManagerTM provides total scenario
control for analyzing arc flash results for any system operating condition including
generation changes, shutdown configurations, open and closed tie breakers, and
any type of “what-if” analysis. This feature saves hundreds of hours of analysis
time and ensures that all modes of operation are accounted for.



Direct printout of warning signs and labels.

Just like any other method, the basic data is required to operate the software. Once the
necessary data has been entered into the software, results can be obtained with a few
clicks of the mouse button. EasyPower’s fast algorithms, accuracy, ability to perform
complex calculations and consider multiple scenarios for various possible connections
make it ideal for both large and small systems and those that have frequent additions or
changes.
EasyPower’s complete integration for all critical aspects of arc flash analysis from data
repository, one-line documentation and arc flash results make it the clear choice for a self
documenting safety program. A single source program to maintain compliance with the
many aspects of NFPA-70E arc flash requirements can greatly simplify a safety program.
2.5.5 Accuracy and Conservatism
Before performing an arc flash hazard assessment, it is necessary to determine how
accurate or conservative the assessment needs to be. Arc flash assessment methods rely
on theoretical and empirical equations. It has been observed that there is some random
behavior of arcing faults that may result in actual occurrences that differ from predicted
values. Although the theoretically maximum arcing power has been used by NFPA
methods (which is believed to be safer) arcing currents can vary randomly. This affects
the fault clearing time, and hence the incident energy to which workers may be exposed.
25


Therefore, when carrying out the assessment, it is necessary to cover all aspects of
variability in order to be truly conservative. This requires us to consider a range of
possible values rather than just a single value obtained from short circuit studies. A quick
way to consider a whole range is described in Chapter 4. Appendix A provides more
details on how to deal with the random behavior of arcing faults.
Additional analysis time is required to consider a range of values instead of a single
value. However, this eliminates chances of error and provides greater accuracy.
Recently, it was proposed that over-protection of workers could cause greater chances of
accidents as the workers' movement could be limited due to excess PPE13. The IEEE
Standard 1584 was developed using test results to avoid over-protection from theoretical
equations. Although tests performed may have shown that the theoretical maximum arc
power was not achieved during the tests, the possibility of its occurrence cannot be ruled
out. Therefore, taking theoretical equations to be conservative is valid.
Although not always true, the chart below provides general observations on various
calculations methods.

Table 2.2: General observations on various methods
Method

Description

NFPA 70E Annex B

Conservative

IEEE 1584

Statistically Probable

Scenario Analysis

Improved Accuracy

Scenario analysis is a standard feature in EasyPower and provides easy analysis of all
system configurations and operating conditions and for bracketing arcing current ranges.
2.5.6 Overprotection
When arc flash hazard assessment is too conservative, the assessed hazard/risk category
or the incident energy may require the workers to wear more protective gear than is
practically necessary. Extra layers of thick fire resistant clothing, face shields, and thick
gloves may render the work rather difficult. This situation has the following
disadvantages:
1. The difficulty provided by the PPE may lead to accidents that can be avoided by
slightly less but adequate PPE.

26


2. Longer time is taken by a worker to execute a task when wearing heavier PPE,
therefore reducing overall productivity. Safety should not be compromised to increase
productivity, however, over-protection cannot achieve greater safety.
3. Workers may be discouraged with the chore of having to wear extra PPE.

2.6 Estimate of Human Efforts
Table 2.3 provides a guide for estimating the hours of work required for various types of
work for an expert level. Beginners and trainees should allow for additional 50-100% of
time to complete the assessment. Table 2.4 shows an estimate of hours of work required
for arc flash hazard assessment for a typical plant. This is only an example and the
requirements for human effort may vary with company, complexity of system, and
availability of information. In this example, all the analysis is done from scratch, i.e. no
short circuit analysis was done previously. Some companies may have up-to-date records
of the power system with short circuit and coordination studies already completed. For
such companies arc flash hazard assessment is just another step.
The estimates provided include detailed written analysis for each aspect of the study.
This type of detailed documentation provides the basis for a complete safety program,
and ensures documented reasoning and application should OSHA or your insurer initiate
an audit due to an incident.
Table 2.3: A guide for estimating time to complete arc flash hazard assessment.

Task

Category

Per
Equipment

2

Substation

0.1

Data Collection

Hours

Load

Data Entry &
Verification

Short Circuit

0.15 – 0.25

Bus

Protective Device Data

0.1 – 0.25

Device

Short Circuit
Study

Analysis

0.1 – 0.25

Bus

Report

0.1 – 0.25

Bus

Protective Device
Coordination

Analysis

0.4

Device

Report

0.15 – 0.4

Device

Arc flash Hazard

3-Scenario Analysis

0.25

Bus

1-Scenario Analysis

0.1 – 0.25

Bus

Report

0.1 – 0.25

Bus

Warning Labels (by program)

0.05

Equipment

27


Similarly, estimates can be made for other activities such as worker training, safety audit,
documentation (much of the documentation will already have been done during the
assessment), procurement and distribution of PPE and development of safety procedures
for arc flash hazard.
Table 2.4: Estimate of hours for arc flash hazard assessment for a plant with 350
buses at 4 different voltage levels and 56 substations, using commercial integrated
software for computation.
Task

Hours

% of Total

Data Collection

136

18%

Data Entry and Verification

64

9%

Short Circuit Analysis

80

11%

Protective Device
Coordination

336

46%

Arc flash Hazard
Assessment

120

16%

Total

736

2.7 Cost Estimates
Cost estimates for human efforts can be obtained from the hours of work needed for the
arc flash hazard program. The PPE cost can be estimated using the highest likely risk
category. This can be roughly determined using the table method for the highest
available fault current. Other costs will include consulting fees for safety training and/or
management, printing warning labels, etc.
13

IEEE-PCIC Electrical Safety Workshop, February 2003, Houston Texas.

28

Chapter 3. Arc-flash Calculation Methods

3 Arc Flash Calculation Methods
3.1 This chapter provides an overview of arc flash hazard
calculations recommended by IEEE and NFPA. All
equations, data, and calculation methods listed in this
chapter are the property of the IEEE and NFPA. You are
encouraged to read the standards for details. IEEE Std
1584-2002
The following procedures are recommended by IEEE Standard 1584-2002 in the evaluation of
arc flash hazard. The empirically derived equations were developed by IEEE working group on
arc flash. These equations are based on test results and are applicable for the following
conditions.
Table 3.1: Conditions for which the IEEE 1584 equations are applicable
Parameter

Applicable Range

System voltage (kV)

0.208 to 15 kV

Frequencies (Hz)

50 or 60 Hz

Bolted fault current (kA)

0.7 to 106 kA

Gap between electrodes (mm)

13 to 152 mm

Equipment enclosure type
Grounding type

Open air, box, MCC, panel, switchgear, cables
Ungrounded, grounded, high resistance
grounded

Phases

3 Phase faults

3.1.1 Step 1: Estimate of Arcing Current
For low voltage systems (<1 kV), the arc current is given by equation (3.1).
Ia  10

{K 0.662 log(Ibf ) 0.0966V 0.000526G  0.5588V *log(Ibf ) - 0.00304G *log (Ibf )}

…

(3.1)

where
log is the log10
Ia = arcing current (kA)

29


K

=
–0.153;
open configuration
=
–0.097;
box configuration
Ibf = bolted fault current for three-phase faults (symmetrical RMS) (kA)
V = system voltage (kV)
G = gap between conductors, (mm)
For medium voltage systems (>1 kV), the arc current is given by equation (3.2).
Ia  10

{0.00402  0.983 log (Ibf )}

(3.2)

3.1.2 Step 2: Estimate of Normalized Incident Energy
The normalized incident energy, based on 0.2 second arc duration and 610 mm distance from the
arc, is given by equation (3.3)
E n  10

{K 1  K 2  1.081 * log(I a )  0.0011G}

(3.3)

where
En = incident energy normalized for time and distance (J /cm2)
K1 = -0.792;
= -0.555;

open configuration
box configuration

K2 = 0; ungrounded and high resistance grounded systems
= -0.113; grounded systems
G = gap between conductors (mm)
3.1.3 Step 3: Estimate of Incident Energy
The normalized incident energy is used to obtain the incident energy at a normal surface at a
given distance and arcing time with equation (3.4).
E  4.184 C f E n

 t   610 
 

 0.2   D 

x

(3.4)

where
E = incident energy (J / cm2)
Cf = Calculation factor

30

= 1.0; voltage > 1kV


= 1.5; voltage < 1kV
t = arcing time (seconds)
D = working distance from arc (mm)
x = distance exponent as shown in Table 3.2.
Table 3.2: Distance Factor (x) for various voltages and enclosure types
Enclosure Type

0.208 to 1 kV

>1 to 15 kV

Open air

2

2

Switchgear

1.473

0.973

MCC and Panels

1.641

Cable

2

2

3.1.4 Step 4: Flash Protection Boundary
The flash protection boundary is the distance at which a person without personal protective
equipment (PPE) may get a second-degree burn that is curable.

 t  1
D B  610 *  4.184C f E n 


 0.2  E B


1

 x




(3.5)

where
DB = distance of the boundary from the arcing point (mm)
Cf
= calculation factor = 1.0; voltage > 1 kV
= 1.5; voltage < 1 kV
En = incident energy normalized
EB = incident energy at the boundary distance (J/cm2); EB can be set at 5.0 J/cm2 (1.2 Cal/cm2)
for bare skin.
t = arcing time (seconds)
x = the distance exponent from Table 3.2.
Ibf = bolted fault current (kA).

3.2 NFPA 70E Protection Boundaries


Serious injury due to arc flash burns can occur within this area unless appropriate PPE is
used.



Anyone within this area must wear appropriate PPE regardless of what they are doing.

31




The distance from the arc source at which the on-set of a second degree burn occurs.



1.2 Cal/cm² > 0.1 sec. is considered a second degree burn threshold.



Medical treatment may still be required if bare skin is exposed to this level of flash. Full
recovery expected.

3.2.2 Limited Approach Boundary


Defines a boundary around exposed live parts that may not be crossed by “unqualified”
persons unless accompanied by “qualified” persons.



May be closer than flash boundary.



Defined solely based on the nominal voltage.

3.2.3 Restricted Approach Boundary


Boundary near exposed live parts that may be crossed only by “qualified” persons using
appropriate shock prevention techniques and equipment.



Concern is a shock hazard.



Defined solely based on the nominal voltage.

3.2.4 Prohibited Approach Boundary
A shock protection boundary to be crossed by only “qualified” persons using same protection as
if direct contact with live part is planned. Defined solely based on the nominal voltage.

3.3 NFPA 70E – May 2003 ROP
The proposed edition of NFPA 70E, Standard for Electrical Safety Requirements for Employee
Workplaces, 2004 Edition, has recommended in Annex B and C of the standard, the following
methods for evaluating arc flash hazard. The final version has been changed to “Electrical
Safety Requirements for Employee Workplaces” and is scheduled to be released in January of
2004. Methods shown here may differ from the final release version. Two different methods
are described.

32


3.3.1 Annex B Method
3.3.1.1

Arc Flash Boundary

The theoretical maximum arc power in MW is half the bolted 3-phase fault MVA14,15. This
occurs when the arc current is 70.7% of the bolted fault current. Based on this, the flash
protection boundary is calculated as:
DB 

2.65 * 1.732 * V * Ibf * t

where
DB =
V =
Ibf =
t =

distance of the boundary from the arcing point (inches)
rated system voltage L-L (kV)
bolted fault current (kA)
arcing time (seconds)

3.3.1.2

(3.6)

Incident Energy

Arc in open air – 0.6 kV or below, 16-50 kA short circuit current
E  5271D

 1.9593

2

t [ 0.0016 * Ibf  0.0076 * Ibf  0.8938 ]

(3.7)

Arc in box – 0.6 kV or below, 16-50 kA short circuit current
E  1038.7 D

 1.4738

2

t [ 0.0093 * Ibf

 0.3453 * Ibf  5.9675 ]

(3.8)

Arc in open air – Above 0.6 kV
E  793 D

2

(3.9)

V Ibf t

where

E
Ibf
t
D

=
=
=
=

incident energy (cal / cm2)
working distance from arc (inches)

Equations (3.7) and (3.8) are part of the 2000 edition, and equation (3.9) was proposed in the 2003 draft.

3.3.2 Annex C Method (2003)
Table 3.3: Equations for arc in box for calculating arc current, incident energy and flash
protection boundary.
V < 1 kV

1 kV < V < 5 kV

V > 5 kV

33

0.85 Ibf – 0.004 Ibf2

Ia =

DB =

(416 Ia t / 1.2)0.625

Ibf

21.8 Ia t D-0.77

416 Ia t D-1.6

E =

0.928 Ibf

16.5 Ia t D-0.77

(21.8 Ia t / 1.2)1.3

(16.5 Ia t / 1.2)1.3

The equations in Table 3.3 apply only to arc in box for short circuit currents between 0.6 kA
and 106 kA.
where
E =
Ibf =
Ia =
t =
D =
DB =

incident energy (cal / cm2)
arc current (kA)
working distance from arc (inches)
distance of the flash protection boundary from the arcing point (inches)

3.4 NFPA 70E Tables
Table 220.2(B)(2)(C) of the proposed NFPA 70E – 2003 ROP provides a simple method of
determining flash protection boundary. This is shown in Table 3.5 below. Note that this table is
presented here for information purposes only, and should not be applied unless it is published as
part of the standard. This table is extremely approximate – the actual arc flash boundaries may
depend upon numerous factors such as available fault level and trip characteristics of the
upstream protective device. Therefore, this table is not recommended for use.
Table 3.5: Simple method of determining flash protection boundary as per Table
220.2(B)(2)(C) of the proposed NFPA 70E – 2003 ROP.
Arc Location

System Voltage

Flash Protection
Boundary (feet)

Arc in Air

200 to 1000 volts

4

Arc in Enclosure

200 to 1000 volts

10

Arc in Enclosure

1000 volts and up

20

3.4.2 Hazard/Risk Category Classifications
The draft of NFPA 70E – 2003 ROP proposes the option of using Table 220.6(B)(9)(A) Hazard
Risk Category Classifications16. This table does not provide the flash protection boundary, but
only prescribes the hazard/risk category number. The table also specifies the requirement of V34


rated gloves and V-rated tools. The classification of risk category is based on several factors such
as voltage, type of equipment, type of work to be performed, available short circuit current,
circuit breaker tripping time or fuse clearing time and the position of the enclosure doors. The
various types of work mentioned in the table are; operating circuit breakers or fuses, working on
live parts, voltage testing, removing and installing bolted covers, applying safety grounds,
working on control circuits, etc.
An example of what 70E (2004) Table 220.6(B)(9)(A) may look like is summarized for two
items in Table 3.4: working on live parts and voltage testing. This table is preliminary and is for
reference purposes only. Refer to NFPA-70E (2004) for final application guidelines.
The exact short circuit currents for three phase bolted faults can be calculated using commercial
software. A simple approximation described in Annex B.2 of proposed NFPA 70E – 2003 ROP
draft is using the upstream transformer data in the following equation. The actual short circuit
current will be less than this calculated value due to the impedance of the system upstream to
transformer.
 MVA Base   100 



 1.732 V   %Z 

ISC  


(3.10)

where
ISC = 3-phase bolted fault current
MVA Base = rated MVA of the upstream transformer
V = line-to-line voltage at the secondary side of the transformer
%Z = percentage impedance of the transformer.

35


Table 3.4 (a): Hazard / Risk Category Classification for Working on Live Parts as per
Table 220.6(B)(9)(A) of proposed NFPA 70E – 2003 ROP (Note: This table is only a
proposed draft and had not been approved or published at the time of writing this book. Please
refer to the upcoming edition of NFPA 70E (2004).)
Fault
Clearing
Time (s)

0.24 kV

42

0.03

1

< 10

0.03

0

< 36

0.1

2

65

0.03

2

< 10

0.03

1

Bus

42

0.33

4

Bus

52

0.2

4

Bus

65

0.1

4

Bus

62

0.33

5

Bus

76

0.2

5

Bus

102

0.1

5

Bus

< 10

0.1

3

Bus

< 10

0.33

4

35

0.5

4

42

0.33

4

52

0.2

4

65

0.1

4

< 25

0.33

3

62

0.33

5

76

0.2

5

102

0.1

5

35

0.5

4

42

0.33

4

52

0.2

4

65

Equipment Type

Short Circuit
Current (kA)

0.1

4

55

0.35

Equipment Side

Panel Board

MCC 0.6 kV Class
Load Side of
Breaker / fuse

Switchgear 0.6 kV Class

Other Equipment 0.6 kV Class

NEMA E2 Motor Starters MV

0.277 to 2.3 to 1 to 38
0.6 kV 7.2 kV
kV

1

5

Metal Clad Switchgear, MV

5

Other Equipment

5

36


Table 3.4 (b) Hazard / Risk Category Classification for Voltage Testing as per Table
220.6(B)(9)(A) of proposed NFPA 70E – 2003 ROP.
Fault
Clearing
Time (s)

0.24 kV

42

0.03

1

< 10

0.03

1

< 36

0.1

2

65

0.03

2

< 10

0.03

1

Bus

42

0.33

2

Bus

52

0.2

2

Bus

65

0.1

2

Bus

62

0.33

2

Bus

76

0.2

2

Bus

102

0.1

2

Bus

< 10

0.1

1

Bus

< 10

0.33

1

35

0.5

2

42

0.33

2

52

0.2

2

65

0.1

2

< 25

0.33

1

62

0.33

2

76

0.2

2

102

0.1

2

35

0.5

2

42

0.33

2

52

0.2

2

65

Equipment Type

Short Circuit
Current (kA)

0.1

2

55

0.35

Equipment Side

Panel Board

MCC 0.6 kV Class
Load Side of
Breaker / fuse

Switchgear 0.6 kV Class

Other Equipment 0.6 kV Class

NEMA E2 Motor Starters MV

0.277 to 2.3 to 1 to 38
0.6 kV 7.2 kV
kV

2

Metal Clad Switchgear, MV

5

Other Equipment

5

3.5 Arc Blast Pressure
Another item associated with an electric arc is the blast energy or pressure. This hazard is not
presently covered in NFPA 70E or IEEE Standard 1584. This force can be significant and can
blow workers away from the arc causing falls and injuries that may be more severe than burns. In
Ralph Lee’s second IEEE paper17, Pressures Developed by Arcs in 1987, he cites several case

37


histories. In one case, with approximately 100-kA fault level and arc current of 42 kA, on a 480V system, an electrician was thrown 25 feet away from the arc. Being forced away from the arc
reduces the electricians’ exposure to the heat radiation and molten copper, but can subject the
worker to falls or impact injuries. The approximate initial impulse force at 24 inches was
calculated to be approximately 260 lb/ft2 as determined from the equation below.
Pressure 

11.58 * Iarc
D

0.9

(3.11)

where,
Pressure is in pounds per square foot.
D = Distance from arc in feet.
Iarc = Arc current in kA.

14

Ralph Lee's, "The Other Electrical Hazard: Electrical Arc Blast Burns," IEEE Transactions on Industry
Applications, Vol. 1A-18, No. 3, Page 246, May/June 1982.

15

This conclusion is supported by the Maximum Power Transfer Theorem: The power transferred to a
load (an arc in our case) is maximum when the impedance of the load is equal to the Thevenin
impedance of the source. The theorem was first developed by Moritz Hermann Jacobi.
16

Proposed NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces, 2003
ROP Edition, National Fire Protection Association.
17

Ralph Lee, "Pressures Developed by Arcs", IEEE Transactions on Industry Applications, Vol. IA-23, No.
4. July/August 1987, page 760-764.

38

Chapter 4. Practical Steps to Arc-flash Calculations

4 Practical Steps to Arc Flash Calculations
Several methods for arc flash calculations as recognized by proposed NFPA 70E-2004 were
described in Chapter 3. Electrical engineers typically perform the calculations, since an
understanding of the short circuit behavior of power systems is necessary. If the available short
circuit values at various equipment and the associated fault clearing times are known, then a
trained person can also perform arc flash hazard assessment (AFH) by using the appropriate
NFPA 70E tables. However, this approach has significant drawbacks due to gross
oversimplification inherent in this simplified approach. For larger power systems with multiple
sources and possible operating conditions, it is preferable to do a detailed engineering analysis
that can determine the worst-case arc flash hazard conditions that can occur. As will be seen,
this will not necessarily be the conditions with the highest fault current.
The following steps are involved in detailed arc flash study. Prior to beginning the data
collection, it should be determined and agreed upon which method of calculation will be used.
1. Identify all locations/equipment for AFH assessment.
2. Data Collection:
a. Equipment data for short circuit analysis;: voltage, size (MVA/kVA), impedance, X/R
ratio, etc.
b. Equipment data for protective device characteristics; type of device, existing settings for
relays, breakers and trip units, rating amps, time-current curves, total clearing time.
c. Equipment data for arc flash study; type of equipment, type of enclosure (open air, box,
etc.), gap between conductors, grounding type, number of phases, and approximate
working distance for the equipment.
d. All power system equipment, their existing connections and possible alternative
connections.
3. Prepare a single-line diagram of the system.
4. Short circuit study:
a. Calculate bolted (available) three-phase fault current for each equipment.
b. Calculate every contributing branch/load currents.
5. Calculate estimated arc current:
a. Calculate arc current using empirical formula (NFPA, IEEE, or other standards).
b. Calculate branch currents contributing to the arc current from every branch.

39


6. Estimate arcing time from the protective device characteristics and the contributing arc
current passing through this device for every branch that significantly contributes to the arc
fault.
7. Estimate the incident energy for the equipment at the given working distances.
8. Determine the hazard/risk category (HRC) for the estimated incident energy level.
9. Estimate the flash protection boundary for the equipment.
10. Document the assessment in reports, one-line diagrams and with appropriate labels on the
equipment.

4.1 Step 1 – Identification of Locations/Equipment for AFH
Arc flash hazard assessment is needed only for those locations where workers are exposed to the
risk. Therefore, it may not be necessary to perform the assessment for each and every piece of
equipment in the power system. Panels and switchboards rated 208 volts or less can generally be
ignored if the service transformer is less than 125 kVA. The arc will not likely be sustainable at
lower voltages and smaller available fault currents. All panels with breakers and fuses should be
included in the assessment if there is potential for significant arc flash injury. Incidents may
occur when operating the breakers or fused disconnects, even with the door closed. You can
consult the existing one-line diagrams for determining the equipment that require assessment. If
such a diagram does not exist, it should be constructed as discussed in steps 2 and 3.

4.2 Step 2 – Data Collection
4.2.1 Equipment Data for Short Circuit Analysis
Although some equipment may not require arc flash hazard assessment, data about this
equipment may be required in a short circuit analysis. Typical data required for the study is
shown in Table 4.1. Short circuit analysis requires data on utility, generators, transformers,
cables, transmission lines, motors, etc. The name-plate of the equipment can provide most of the
necessary data. In the absence of particular data, it may be possible to obtain the information
from the manufacturers or their representatives. Also, typical data can be assumed by referring
to books and product literature. Power system software such as EasyPower has an extensive
library of manufacturers data covering most electrical equipment in use today. This book is not
meant to be a guide for short circuit studies. Refer to standard literature18, 19, 20 for short circuit
studies.

Table 4.1: Typical data needed for equipment for short circuit analysis

Description

40

Data


Equipment Type
Voltage
MVA/KVA
Impedance
X/R Ratio
Phases/connection
4.2.2 Equipment Data For Protective Device Characteristics
Obtain data on the various protective devices that will determine the arcing time. Table 4.2
shows what kind of information is required. This data may be obtained from existing drawings,
relay calibration data, coordination studies and from field inspection. Obtain from the
manufacturers the time-current characteristics (TCC) for these devices. Determine whether the
protective device is reliable enough. This can be done by asking the operators, or by testing if
necessary. Some companies have periodic relay testing programs. If the protective device is
deteriorating, the data provided by the manufacturer may not be applicable. If the fault
interruption does not occur as expected then the arc flash assessment cannot be accurate. It will
be necessary to repair or replace such equipment.
Table 4.2: Protective device data to gather
Protective
Device

Data to Gather

Relay

Type, CT ratio, pickup (tap) setting, delay type (curve)
and setting (time dial).

Fuse

Type, amp rating, voltage, peak let-through current.

Breaker

Type, fault clearing time, pickup setting, delay curve,
delay setting.

4.2.3 Equipment Data For Arc Flash Study
Depending on the method of calculation selected, the following equipment data is required for an
arc flash hazard study.

Table 4.3: Equipment data for AFH study
Description

Data

Type of enclosure (open air, box, etc.)
41


Gap between exposed conductors*
Grounding type*
Phases/Connection
Working Distance
Data required for IEEE 1584 method has been marked with ' * '. The working distance is an
approximate measure that should be based on the type of work being performed and the type of
equipment. It may vary based on manufacturer’s design and work practices. Working distances
should be documented for various work practices and equipment as part of a complete safety
program.
4.2.4 Determine All Possible Operating Conditions
Make note of all possible connections (system operating modes) using diagrams and tables. The
circuit breaker/switch/fuse status may change during abnormal operations. Parallel feeds can
greatly increase the available fault current and resulting arc flash hazard. The contribution of
connected motors to the fault will increase the hazard as well. Assessment should include both
the normal operating condition as well as the worst possible arc flash scenario. In general, the
higher the available fault current, the greater the arc flash energy. However, since arc flash
energy is a function of the arc duration as well as the arc current, it cannot be automatically
assumed that the highest fault current will always be the worst-case AFH. Figure 4.1 shows the
available fault currents for two scenarios of connections: (a) everything connected and (b)
generator normally not running and the motor turned off for maintenance. The difference in fault
currents can clearly be seen. Table 4.4 is an example worksheet for this case, considering
multiple connections.
Table 4.4: Example worksheet for connection scenarios
Equipment
Connected

Normal
Operation

Co-generation

Maintenance
Schedule A

Maintenance
Schedule B

Utility

ON

ON

OFF

ON

Generator

OFF

ON

ON

OFF

Motor

ON

ON

ON

OFF

M-2

ON

ON

ON

ON

42


UTILITY

UTILITY

0.48KV BUS

OPEN

0.142

0.346

0.142

Faulted
Bus
TRANSFORMER

MOTOR

OPEN

4.16KV BUS

1.
52
9

1.388

2.
93
4

1.388
4.16KV BUS

Faulted
Bus

GENERATOR

1.067

GENERATOR

TRANSFORMER

MOTOR

0.48KV BUS

M-2

M-2

Figure 4.1: EasyPower example of connected equipment for two possible cases

4.3 Step 3 – Prepare single-line diagram of the system
Single-line (one-line) diagrams are powerful tools for documenting and communicating
information about power systems. They are easy to read, show the connections and status of
equipment and contain data required for analysis. The results of analysis such as short circuit
studies and arc flash hazard assessment can easily be placed on the diagrams. Most existing
plants should already have one-line diagrams. The accuracy of these should be verified before
commencing the assessment. If a new diagram is required, it can be prepared using the data
collected.

4.4 Assessment using a commercial integrated software like
EasyPower requires entry of data to build a power
system model. EasyPower provides an advanced
graphical drag-and-drop one-line diagram completely
integrated with short circuit, protective device
coordination, and arc flash analysis. EasyPower provides
an easy way to create, update and maintain your power
system one-line in compliance with NFPA-70E
requirements. Step 4 – Short Circuit Study
A complete tutorial on performing short circuit studies is not provided here. But additional
considerations related to AFH when performing short circuit studies are described briefly in the
sections that follow. Only 3-phase faults are considered when performing AFH. This may seem

43


odd, but it is consistent with the recommendations in IEEE-1584 and NFPA-70E. There are
several reasons for this. One is that 3-phase faults generally give the highest possible short
circuit energy and represent a worst-case. Another important reason is that experience has
shown that arcing faults in equipment or air that begin as line-to-ground faults, can escalate very
rapidly into 3-phase faults as the air ionizes across phases. This progression from single-phase to
3-phase happens generally within a few cycles. Because of this, most testing done on arc flash
energy has been based on 3-phase faults. For singe-phase systems, IEEE-1584 recommends that
calculations be done for an equivalent 3-phase system and states that this will yield conservative
results. Based on the data collected for various system operating modes, arc flash calculations
should be performed for each possible case. Traditionally, when performing short circuit
calculations to determine maximum short circuit current, extremely conservative estimates and
assumptions are used. This makes sense if the goal is to determine maximum breaker or
equipment duties. However, for AFH, using overly conservative short circuit data can yield nonconservative results since a very high fault current may produce a very short arc duration due to
the operation of instantaneous trip elements. The highest fault current does not necessarily imply
the highest possible arc flash hazard because the incident energy is a function of arcing time,
which may be an inversely proportional function of the arcing current. For AFH determinations,
short circuit calculations should be conservative, but not overly conservative.
4.4.1 Calculate Bolted Fault Current
Calculate the 3-phase bolted fault current in symmetrical rms amperes for all buses or
equipment, and for each possible operating mode. Check for the following while considering
various interconnections at the concerned bus or equipment:


Multiple utility sources that may be switched in or out of service.



Multiple local generator sources that are operated in parallel or isolated depending on the
system configuration.



Emergency operating conditions. This may be with only small backup generators.



Maintenance conditions where short circuit currents are low but arc duration may be long.



Parallel feeds to Switchgear or MCC’s.



Tie breakers which can be operated open or closed.



Large motors or process sections not in operation.

A short circuit/arc flash case should be developed for each operating mode. This can be a
daunting task for most software or spreadsheet calculators. EasyPower Scenario Manager
provides a simple and easy method to document and analyze each operating mode for quick
repeatable analysis.

44


Hand calculations and spreadsheet calculators may typically neglect the transient short circuit
values that last for a few cycles. These are higher than the sustained short circuit values. The
generator and motor transients during the fault contribute to the arc fault. To account for these:


Use sub-transient and transient impedances of the generators to find the bolted fault
current if the arcing time is small. For long arcing times, use the sustained short circuit
values. This suggests an iterative process, since the arcing time depends upon the fault
current passing through the upstream protective device.



Include contribution of connected motors in the calculations.

4.4.2 Calculate Contributing Branch Currents
Contributing branch currents to faults are calculated to estimate the contributing arc currents by
various branches, which again, are used to determine the trip times of the protective devices on
the branches. The protective device upstream to the fault sees only the current passing through
it. The fault current may be greater than the current passing through the upstream protective
device. Therefore, the total fault current cannot be used to find the trip time unless other branch
currents are significantly smaller than the upstream current. Similarly, for parallel feeds, the
contributing currents from each feed must be calculated to determine the trip time.
Special care is needed when computing branch currents through transformers. This could be
one of the common source of errors since the branch current needs to be adjusted by the
transformation ratio. When a fault occurs on the low voltage side of a transformer, the protective
device on the high voltage side of the transformer sees a smaller current due to the transformer
turns ratio.

4.5 Step 5 – Determine Expected Arc Current
4.5.1 Calculate Arc Current
Calculate arc current for every required equipment or bus using one of the empirical formulae
recognized by the NFPA-70E (NFPA, IEEE, or other standards). These are described in Chapter
3. The arc current may be a function of the bolted fault current, the open circuit voltage, the type
of enclosure and the gap between conductors depending on the calculation method selected.
4.5.2 Consider a range of Arc Current
4.5.2.1

Tolerance due to Random Variation based on IEEE-1584

To cover the variance that can occur in arcs, IEEE procedure suggests the following.
1. Calculate the maximum expected bolted fault condition.

45


2. Calculate the minimum expected bolted fault condition. The minimum bolted fault
current could be a light load condition with many motor loads or generators not running.
3. Calculate the arcing current at 100% of IEEE 1584 estimate for the above two conditions.
4. Calculate the arcing current at 85% of IEEE 1584 estimate for the two above conditions.
5. At these four arcing currents calculate the arc flash incident energy and use the highest of
the incident energies to select PPE. The minimum fault current could take longer to clear
and could result in a higher arc flash incident energy level than the maximum-fault
current condition.
The fault current in the main fault current source should be
determined since the current in this device may determine the fault clearing time for the
major portion of the arc flash incident energy.
Note: Although IEEE recommends considering a range of 85% to 100% of the estimated arc
current, IEEE test data shows that the measured values of arc current vary from 67% of the
estimate to 157% of the estimate. Further analysis of the IEEE test data was performed by the
authors and the results are discussed in detail in Appendix A. Careful application of tolerances is
required for the following reasons:
i. The tripping time of inverse-time protective devices is influenced by the arc current.
ii. The incident thermal energy is more sensitive to arcing time than it is to arc current.
iii. A more realistic and reasonably conservative estimate of arcing time can be obtained by
proper selection of tolerances of arc current.
The following section provides guidelines based on statistical analysis of test data for various
voltage levels and enclosure types.
4.5.2.2

Tolerance due to Random Variation based on Actual Data

Because of the random nature of arc currents, the actual arc current may take any value within a
range of possible values. The calculated arc current is only a single estimate within the range
possible values.
The calculated arc current or the highest possible arc current may not
necessarily produce the highest incident energy to which workers may be exposed. The arcing
time may depend upon the arc current due to the tripping characteristics of the protective device.
Therefore, the incident energy may be greater for smaller arc currents if the contributing branch
current of the arc current lies in the inverse-time section of trip characteristics.

46


Table 4.5: Minimum and maximum tolerances for arc currents obtained from IEEE 1584
test data for confidence level of 95%.
Voltage

Enclosure Type

Minimum Arc
Current

Maximum Arc Current

LV

Open

-26.5%

26.0%

LV

Box

-26.9%

33.0%

MV

Open

-6.7%

10.2%

MV

Box

-20.8%

12.3%

When considering the range of the calculated arc current, the simplest way is to take a tolerance.
This tolerance is a percentage of the calculated arc current. The tolerance is obtained from
statistical analysis of the test data. The tolerances differ for IEEE 1584 method and NFPA 70E
methods. For further description see Appendix A. Table 4.5 provides tolerances for IEEE 1584
arc current estimate for a confidence level of 95%. A confidence level of 95% means that there is
a probability of 95% that the arc current will be in the tolerance range. To be more conservative,
you could also take a confidence level of 99%. This would widen the tolerance range.
Example
For a 0.48 kV equipment in open air, the calculated arc current (Iarc) using IEEE 1584 estimate
was 40 kA. What is the possible range of the arc current?
From Table 4.5 the minimum and maximum tolerances of arc current are –27.5% and +31.9%
respectively.
Minimum arc current = Iarc * (100 + Min. Tolerance %)/100
= 40 * (100 – 26.5) / 100

= 29.4 kA.

Maximum arc current = Iarc * (100 + Max Tolerance %)/100
= 40 * (100 + 26.0) / 100

= 50.4 kA.

Variation of Arcing Current with Arc Gap

If the exact arc gap (or gap between conductors) was used in obtaining the arc current estimates,
then further adjustments need not be applied. However, if the gap is an average value or an
assumed value, then the possible range of arc current may need to be adjusted. Appendix A
describes in detail the effect of gap on the arc current. The gap is used in calculations in the
IEEE 1584 equations. NFPA 70E does not take the gap into account. Table 4.6 provides the
sensitivity of arc current to gap. For every mm of difference in gap the arc current value is
modified by the sensitivity amount. The voltage across an arc gap is roughly proportional to the
length of the gap. Higher voltage means higher arc power for the same arc current. Since the arc
resistance is non-linear, the resistance is not directly proportional to the gap length. Therefore
statistical approach is preferred in the evaluation of the effect of variation of arc gap length on

47


arc current. The procedure given below should be used only for small differences in arc gap, as
with most other sensitivity analyses.
Table 4.6 Sensitivity of arc current to gap for IEEE 1584 method
Voltage

Enclosure

Sensitivity (% / mm)

LV

Box

-1.0%

LV

Open

-0.7%

MV

-

Not Required

Example
The exact gap between conductors for various devices are not known. For low voltage box, it
was generally observed that the gap ranged from 25mm to 40mm with an average of 32mm.
There are two ways to deal with this. The first method is to obtain two estimates for arc current,
one for the least gap and the other for the highest gap. The second method is to adjust the IEEE
estimate for the average gap using the sensitivity shown in Table 4.6. Let us say that the arc
current for 32mm gap was found to be 40 kA.
Arc current for min. gap = Iarc * (1+sensitivity*(Min. gap – Average gap)/100)
= 40 * (1-1.0*(25-32)/100)
= 42.8 kA.
Arc current for max. gap = Iarc * (1+sensitivity*(Max. gap – Average gap)/100)
= 40 * (1-1.0*(40-32)/100)
= 36.8 kA.
If the variation in gap is small, then extensive analysis need not be carried out.
4.5.2.3

Limits for Arc Currents

After calculating the range of possible arc current, it is necessary to check whether the calculated
values are within the practical range. Check for the following:


48

Upper limit: It is not possible for the arc current to be greater than the bolted fault current
because of the additional impedance of the arc. Therefore, after applying adjustments for
random variations and for gap variations, if the upper limit of the range of arc current is
greater than the bolted fault current, then discard that value and take the bolted fault
current as the upper limit.


Example
For a bolted fault current of 50 kA at medium voltage equipment in box, the arc current was
calculated to be 49 kA using IEEE 1584 equations. To account for random variations
tolerance data from Table 4.5 was applied. This takes the upper limit of arc current to 12.3%
greater than the estimated value. Therefore, the upper limit of the arc current was calculated
to be 49*(100+12.3)/100 = 55 kA. This is higher than the bolted fault current (50 kA), and
therefore, is not possible. Take the upper limit of arc current as 50 kA.


Lower Limit: Arcs do not sustain when the current is very low. For 480-volt systems, the
industry accepted minimum level for a sustaining arcing fault current is 38% of the
available three-phase fault current21. Test data accompanying IEEE Standard 1584 shows
arc sustaining for 0.2 seconds at 0.208 kV at a current of 21% of bolted fault current.
Table 4.7 shows the minimum arc current as a percentage of bolted fault current obtained
during tests. The lower limit of arc current is not yet clear. Until further information is
obtained, it may be reasonable to use Table 4.7 as the cut-off minimum arc current as a
percentage of the bolted fault current.
Table 4.7: Adjusted minimum arc current as a percentage of bolted fault currents*.
Voltage (kV)

Min Measured Iarc % of
Ibf

0.2/0.25

21%

0.4/0.48

21%

0.6

28%

2.3

51%

4.16

64%

13.8

84%

*The adjustment is based on maximum measured values taken normalized to the bolted fault
current.
4.5.3 Calculate Branch Currents Contributing to the Arc Current
This is done using the branch current contributions to the bolted fault current obtained from
section 4.4.2. To calculate the contributing currents to the arc fault, use equation (4.1).
Ix,arc = Ix,BF * Iarc / IBF

(4.1)

Where,
Ix,arc = Current through branch x for arc fault
Ix,BF = Current through branch x for bolted fault

49


IBF = Bolted fault current.
Arc currents have been observed to be non-sinusoidal due to the non-linear nature of the arc
resistance. The harmonic contribution of different branches may vary, however, the fundamental
component can be approximated using the method describe above. It has been observed that
although the voltage waveform is highly distorted, the arc current however has low harmonic
content. Therefore the linear relation (4.1) is a reasonable approximation.
Section 4.5.2 describes taking upper and lower bounds of the range of arc current. The branch
contribution must be calculated for each case. These are later used to determine the trip time of
protective devices.

4.6 Step 6 – Determine Arcing Time
Estimate arcing time from the protective device characteristics and the contributing arc current
passing through this device for every branch that significantly contributes to the arc fault. Since
we are considering a range of arc currents instead of a single value, we need to determine the trip
time for each arc current value – the upper bound, the lower bound and the value calculated from
NFPA 70E or IEEE 1584 equations.
The trip time of a protective device is obtained from its time-current characteristics (TCC).
Information may be obtained from manufacturers in the form of TCC plots or equations. Relays
and circuit breaker trip units usually have adjustable time delay for tripping operation. The delay
time may depend upon the magnitude of the current sensed by the device. Time delays are
provided to coordinate the tripping of the relays so that maximum reliability of supply may be
maintained. Refer to literature on protective device coordination for details. Since arc flash
hazard can be minimized by reducing the duration of faults, it is beneficial to have a good
understanding of protective device coordination. Typically, for lower fault currents, the trip time
may be high due to the inverse time-current relationship of the TCC. For higher currents, the
arcing-fault current may be greater than the instantaneous pickup of the protective device, and
therefore the device may trip at the minimum response time. For fault currents near the
transition from the inverse-time curve to the instantaneous trip, a small change in arcing current
value can cause a very large difference in calculated arc energy. Refer to Appendix A for details.
Determining the trip time manually requires visual inspection of each time-current curve to
determine the operating time for a particular fault current. This also requires adjustment of the
fault currents to reflect the transformation ratio of any transformers involved. This must be done
before obtaining the trip times of the protective devices across the transformer.
EasyPower’s integrated protective device coordination program automatically determines the
arcing time for each protective device, operating condition, and arcing current level. Total
integration saves you time and resources, and ensures the most accurate solution.
Typically, for any given current, protective devices have a tolerance about the specified trip time.
Many low voltage breakers and fuses specify the upper and lower limits of the trip time for
different current values. For such cases, the time-current curve looks like a thick band instead of

50


a single line. Relays typically show a single line for the TCC curve, and specify the tolerance as
+/-x% (usually 10% to 15%) somewhere in their product literature. Some fuse curves provide
only the average melting time or the minimum melting time. Follow the guidelines provided
below for determining the trip time.


TCC with tolerance band: Take the total clearing time (upper bound of the band)
corresponding to the branch current seen by the device.



Relays with single line curve: Find within the TCC data or the product literature, the
tolerance for trip time. Add the tolerance to the trip time obtained for the TCC. Breaker
opening time must be added to this value.



Fuse TCC with total clearing time: No adjustment is required since total clearing time is what
we need.



Fuse TCC with average melting time: Obtain the tolerance from the product literature, TCC
data or the manufacturer. Add the tolerance to the average melting time obtained for the
TCC. If tolerance data is not available, make an assumption using data with similar devices.
For most purposes, a tolerance of +/-15% should suffice. IEEE 1584 suggests taking a
tolerance of 15% when average trip time is below 0.03 seconds and 10% otherwise. Some
commonly used fuse curves have been found to have a tolerance as high as 40%. If the
tolerance is known to be small, then additional computation can be ignored.



Fuse TCC with minimum melting time: Obtain the tolerance from the product literature, TCC
data or the manufacturer. Add the tolerance to the minimum melting time obtained for the
TCC. If tolerance data is not available, make an assumption using data with similar devices.
The tolerance may vary with the slope of the curve. For smaller melting times the total
clearing time may be 30% to 100% higher than the minimum melting time.



Circuit breaker clearing time: The TCC of relay or trip unit accompanying the breaker may
or may not include the breaker clearing time. If the breaker clearing time is not included in
the TCC data, find the breaker clearing time and add it to the delay of the trip unit. Breakers
typically have a maximum clearing time of 3 to 5 cycles after the trip coil is energized.

51


9 10

2

3 4 5 6 7 8 9 100

CURRENT IN AMPERES
2
3 4 5 6 7 8 9 1000 2

700

3 4 5 6 7 8 9 100000
700

500
400

500
400

3 4 5 6 7 8 9 10000 2

300

300

Fuse

200

200

100

100

70

70

50
40
30

50
40

Electronic
Trip LV
Breaker

Pickup

20

30
20

10

10
7

5
4

5
4

3

3

2

2

1

1

.7

.7

Total

.5
.4 Clearing

.5
.4

Instantaneous
Pickup

.3 Time
.2 Curve

.3
.2

.1
.07

TIME IN SECONDS

TIME IN SECONDS

7

.1
.07

Minimum

.05
.04 Time

.05
.04

Thermal
Magnetic LV
Breaker

.03 Curve
.02

.03
.02

.01
9 10

2

3 4 5 6 7 8 9 100

2

3 4 5 6 7 8 9 1000 2

3 4 5 6 7 8 9 10000 2

.01
3 4 5 6 7 8 9 100000

CURRENT IN AMPERES

Figure 4.2: Example of TCC plot with tolerance band

4.6.1 Evaluate Protective Device Performance
Special attention is required when the calculated branch currents seen by any protective device is
close to pickup current of the device. If the branch current is lower than the pickup then the
device will not trip. Typically, protective devices are coordinated such that the downstream
device trips before upstream device for the same current (or the equivalent current converted to
the same voltage base). However in the absence of proper coordination, if the downstream
device does not trip at a given fault current, then the upstream device may trip. Therefore, it is
necessary to identify which device will interrupt the arc fault. Selective coordination of devices
should be maximized by adjustment of trip unit settings, if possible. This will not only improve
the continuity of supply but will also provide the opportunity to lower arc flash hazard by
reducing the arcing time (although selectivity and arc flash reduction are often conflicting goals).
The arcing fault currents close to pickup current of instantaneous trip function should be
examined closely. If the calculated fault current value is within the tolerance band of the pickup,
52


then there is a likelihood of the device not tripping at the expected instantaneous value. In such
cases, if the time-overcurrent function exists in the device then the trip time for the timeovercurrent function should be taken for arc flash calculations.
It is also important to realize that any changes to the protective device settings can have a major
influence on the arc energy. If device or setting changes are made, the arc flash calculations
must be re-checked and appropriate changes made if necessary.
4.6.2 Trip time for multiple feeds
When a bus is fed from multiple sources, as shown in Figure 4.3, a fault at the bus may cause a
series of breaker operations. The actual fault current will change as the breakers open, since the
sources of power will be sequentially removed from the faulted bus. Since the current seen by
the relays will change over time, further calculations are required to determine the actual trip
time for each breaker. We cannot simply obtain the trip time corresponding to a single branch
current by looking at the TCC data. Protective devices with time-overcurrent functions typically
operate like an integrating device. That means, the overcurrent or its function is integrated or
"added" over time until the sum reaches a predetermined trip value. This is when the relay trips.
For details on how a relay or fuse integrates the function of current, refer to literature on
operation of protective devices.

UTIL-1

UTIL-1

GEN-1 BUS

GEN-1 BUSOPEN

2.706

4.
05
8

1.353

GEN2-BUSOPEN

1.608

GEN2-BUS
1.566

1.353

0.
01
6

7.
20
8

2.389

0.
02
7

2.389

1.353

UTIL-BUS

3.212

0.824

UTIL-BUS

GEN-1

GEN-1

GEN-2

(a)

GEN-2

(b)
UTIL-1

Connection Status

GEN2-BUSOPEN

7.208

(b) Generators
disconnected

4.058

(c) Only one feed

3.
99
8

0.
04
6

GEN-1 BUSOPEN

OPEN

Bolted Fault
Current (kA) at 5
cycles

(a) All Connected

3.998

0.000

UTIL-BUS

3.998

GEN-1

(c)

GEN-2

53


Figure 4.3: Example showing multiple source fed bus fault and series of operations with
fault current changing with breaker operation.

4.7 Step 7 – Determine Incident Energy
Estimate the incident energy for the equipment at the given working distances. The equations for
determining the arc flash incident energy are provided in Chapter 3. The incident energy is a
function of the arc current, arcing time, the enclosure type and the distance from the arc. IEEE
1584 also includes the gap between electrodes as a parameter. NFPA 70E Annex B method uses
the bolted fault current rather than the arc currents.
In steps 5 and 6, various scenarios were considered. Using the selected method, calculate the
incident energy for each scenario. Make sure the following cases are evaluated:
1. Arc current based on IEEE-1584 standard and its associated trip time.
2. Lower bound of arc current due to random variations, and its associated trip time.
3. Upper bound of arc current due to random variations and its associated trip time.
4. Multiple feed scenarios:
a. Evaluate incident energy for each type of possible connections as noted in Step 4.
b. Evaluate incident energy for arc current changing through series of breaker operations, as
described in step 6. An example is presented below.
Example:
In the example shown in Figure 4.3(a), a 3-phase fault at GEN2-BUS results in a 7.2 kA bolted
fault current. NFPA 70E-2003 ROP Annex C method is used for this medium voltage arc in
box. Working distance of 18 inches is assumed. The contributing branch currents are shown in
Table 4.8. The branch currents I1, I2 and I3 respectively are GEN2 current, current from GEN-1
BUS to GEN-2 BUS and current from UTIL-BUS. The generators are disconnected in 0.1
second. This reduces the bolted fault current to 4.05 kA. Next, the GEN-1 BUS is disconnected
from GEN-2 BUS at 0.2 seconds. The bolted fault current drops further to 3.99 kA. The last
breaker trips at 0.3 seconds. In this example definite time delay functions have been used to
obtain the trip time, only for the sake of simplicity. In many cases the delays are of inverse-time
function, and the arcing time may be longer as the fault current reduces with sources being
removed from the faulted bus.
Table 4.8: Branch currents and trip times used for example shown in Figure 4.3.
Connection
Status

54

Bolted
Fault
Current

Arc
Current
(kA)

Branch Currents
(kA)

Trip Time from
Start of Fault
(s)


(kA) at 5
cycles

I1

T1

T2

T3

1.60 2.38 3.21 0.2

-

-

0.4

-

All
Connected

7.208

7.20

Generators
disconnected

4.058

4.05

0

Only one
feed

3.998

3.99

0

I2

I3

1.35 2.70
0

3.99

0.5

The total incident energy needs to be calculated by adding the incident energies for each
sequence of operation as the sources are removed from the fault.

Table 4.9: Total incident energy for example shown in Figure 4.3.
Connection Status

Arc
Current
(kA)

Duration
(s)

Incident
Energy
(cal/cm2)

All Connected

7.20

0.2

2.6

Generators disconnected

4.05

0.2

1.4

Only one feed

3.99

0.1

0.7

Total

4.7

Alternatively, we could take a simpler and more conservative approach of using the highest arc
current and the total arcing time. If we take the highest arc current, 7.20 kA, and the total fault
duration of 0.5 seconds, the calculated incident energy will be 6.4 cal/cm2. For practical
purpose, this small difference will not matter. However, the difference may be significant when
the protective devices have inverse-time characteristics.
4.7.1 Tolerance of Calculated Incident Energy
The selected method may have tolerance about the calculated value of incident energy because of
the random nature of arcs. Different calculation methods will generally yield different results.
For IEEE 1584 equations use Table 4.10 to find a more conservative estimate that accounts for
the randomness of arcs. This table is based on further analysis of test data accompanying IEEE
Standard 1584, and is described in greater detail in Appendix A. The maximum tolerance should
be added to the calculated incident energy. This procedure is suggested to minimize risk to

55


workers since test data has been found to have greater incident energy than that yielded by IEEE
1584 formula.
Table 4.10: Tolerances for IEEE 1584 incident energy estimates.
Voltage/ Type of Enclosure

Maximum Tolerance (% of Calculated Incident Energy)
For adjusted arc current a

For IEEE 1584 arc current b

Low voltage arc in open air

66%

85%

Low voltage arc in box

63%

64%

Medium voltage arc in open
air

93%

54%

Medium voltage arc in box

50%

52%

a. This is using the arc current after adjusting for random variations (upper and lower bounds); b. This is using the
arc current from IEEE 1584 formula.

Table 4.10 shows incident energy tolerances for two different kinds of arc currents. It can be
seen that there is not much difference in incident energy for arc in box whether the exact IEEE
1584 arc current or the arc currents adjusted for random variations is used. The choice of arcing
current significantly affects only the arcing time, which in turn affects the incident energy. From
the table it can be observed that the calculated incident energy is higher than maximum measured
values for low voltage open air. However, for other cases, the estimate may be much lower than
the maximum measured values.
Example
The incident energy for low voltage in box was calculated to be 10 cal/cm2 using the IEEE 1584
equations. The adjusted arc current was used in this estimate. What is the maximum possible
incident energy assuming random behavior of arcs?
From Table 4.10, the maximum tolerance is 63% of calculated value.
Maximum possible value of incident energy with +63% tolerance:
=10 * (100 + 63)/100 = 16.3 cal/cm2.

4.8 Step 8 – Determine Hazard/Risk Category
Hazard/risk category (HRC) is specified as a number representing the level of danger, which
depends upon the incident energy. Category 0 represents little or no risk, whereas category 4 is
the most dangerous. Table 4.11 provides the classification guide for the risk category number.
Refer to NFPA 70E (2004) for classification and updates.

56


Table 4.11: Hazard/risk classification as per NFPA 70E (2004)
Category

Energy Level

0

< 2 cal/cm2

1

5 cal/cm2

2

8 cal/cm2

3

25 cal/cm2

4

40 cal/cm2

Workers should prepare according to the risk category before commencing work or inspection
near exposed, live conductors. Documentation and warning labels are also required. Although
the incident energy itself may provide a more accurate picture of the risk, the scale of 0 to 4 for
the risk category may convey more meaningful information to most workers. Employers are
required to perform a complete hazard assessment prior to commencing any work near exposed
conductors.

4.9 Step 9 – Determine Flash Protection Boundary
The flash protection boundary is the distance at which persons exposed to arc flash, without
appropriate PPE, will obtain second degree burns that are curable. The flash protection boundary
is a function of the arc flash incident energy. The higher the arc flash energy, the farther away
the boundary will be. Calculate the flash protection boundary using the equation prescribed by
the standard being followed. Use the highest incident energy calculated in step 7, after
accounting for all the system connections and variations due to randomness of arcs. Use the
equation (4.2) to determine the flash protection boundary. This is applicable for both IEEE Std
1584 and proposed NFPA 70E (2004).
 E 

EB 
 

D B  D


1
x

where,
DB = distance of the boundary from the arcing point (see note)
D = working distance (see note)
E = maximum incident energy at working distance in cal/cm2
EB = incident energy at boundary, usually 1.2 cal/cm2 for arcing time > 0.1s.
x = distance exponent factor (see Table 4.12)
Note: Distances DB and D must both be in the same units. They can be expressed in inches or
mm.

57


Table 4.12: Distance exponent "x"
Enclosure Type

IEEE 1584

NFPA 70E –
2000

Open air (0 – 0.6
kV)

2

1.9593

Open air (> 0.6 kV)

2

2

Switchgear

2

1.473

MCC and Panels

NFPA 70E – 2003

1.641

Cable

2

Box (0 – 0.6 kV)

1.4738

Box (< 1 kV)

1.6

Box (> 5 kV)

0.77

Example
The incident energy for a low voltage switchgear at a working distance of 18 inches was found to
be 25 cal/cm2 using the proposed NFPA 70E (2004) method. What is the flash protection
boundary for arcing time greater than 0.1s?
Flash protection boundary is:
DB = 18* (25 / 1.2)1/1.6 = 120 inches.

4.10 Step 10 – Document The Arc Flash Hazard Assessment
The arc flash hazard assessment should be documented in detailed reports, one-line diagrams and
on the equipment. Provide as much detail as possible. Documentation has the following
advantages:
1. Easy for workers to access the necessary details and drawings. This is necessary for
safety planning.
2. Compliance with OSHA and NFPA.
3. Easy to implement changes in assessment when power system changes are made or when
the standards are revised.
4. In case of arc flash related injuries, investigation is facilitated by documents. Lack of
assessment documents may result in penalty to the company.
EasyPower software provides a detailed data repository and self documents the required short
circuit, protective device coordination, and arc flash analyses for all system modes of operation.

58


A single source program to maintain compliance with the many aspects of NFPA-70E arc flash
requirements can greatly simplify a safety program.
4.10.1 Documentation in Reports
The assessment report should include the following details:
1. Name of person performing the assessment.
2. Date of assessment.
3. All data collected and used in the assessment, including protective device settings.
4. Assumptions used in the absence of data.
5. Method of hazard assessment used – the standard and the revision year.
6. If software was used, the name of the software and the version.
7. The results – incident energy, hazard/risk category and flash protection boundary for
every equipment.
8. If various modes of operation are possible, document assessment for each mode.
The assessment report should be available to all concerned persons. Some of these may be:
1. Safety coordinator.
2. Safety division/department.
3. Foremen and electricians.
4. Electrical engineer.
5. Affiliated contractors.
4.10.2 Documentation in One-Line Diagrams
Figure 4.4 shows an example EasyPower one-line diagram. This is the LV part of a substation
showing the results of arc flash hazard assessment. The computation and drawing was performed
by the commercial integrated software EasyPower. Four circuit breakers are located in the same
switchgear lineup. A person working on the switchgear should inspect the drawing for the arc
flash incident energy levels and the hazard category on all the exposed conductor parts. In this
example, the line side of the main breaker of the panel would produce the highest incident
energy if arc flash were to occur. The arc energy released would depend on the upstream
protective devices, relay R-TX-2 and breaker main breaker. Arrows should be placed to indicate
the side of breaker (line side versus load side) for which arc flash values are noted in the
diagram. This would provide the workers the knowledge of risk at each part of the panel.
59


The following steps are recommended for a practical documentation of arc flash data on one-line
diagrams:
1. Place the arc flash hazard assessment results on every equipment that poses a risk.
2. Specify the flash protection boundary, also known as arc flash boundary (AFB).
3. Specify the incident energy at the estimated working distance in the standard unit, for
example in calories per cm2. Specify the estimated working distance as well. Workers
should check whether the working distance will be maintained while working on live
equipment. If closer working distances are required, then it may be necessary to revise the
assessment to reflect true working condition. The closer the distance the more the incident
energy, and higher the risk.
4. Specify the hazard risk category at the estimated working distance.
5. For breakers and fuses, specify the values for both the line side and the load side. Remember
that a fault on the load side of the protective device would be interrupted by that device itself.
However, should a fault occur at the line side of the protective device, then the fault would
be interrupted by the upstream protective device. This would normally have higher incident
energy because of longer tripping time. Therefore it is important to evaluate and document
arc flash energies for the line side of protective device, and communicate this with workers.
6. Use arrows to provide a clear indication of the load side and line side on the equipment if the
arc flash values are different.
7. In any equipment, if different parts have different incident energy values, the worst case
should be highlighted or mentioned first in the list.
8. Mention the protective device that limits the incident energy. If that device is at some
distance, provide information on its location. It is also suggested that the settings be
documented.

60


BH-TX-2
MF

TX-2

R-TX-2

Line Side of BL-1
147" AFB
22.7 cal/cm2 @ 18"
#4@18"
Protected by Relay R-TX-2
and Breaker BH-TX-2
116" AFB
18.7 cal/cm2 @ 18"
#3@18"

BUS-4
M-1

MCC-1
78" AFB
10.5 cal/cm2 @ 18"
#3@18"

27" AFB
2.8 cal/cm2 @ 18"
#1@18"

M-1

MCC-1
78" AFB
10.5 cal/cm2 @ 18"
#3@18"

MCC-2
78" AFB
10.5 cal/cm2 @ 18"
#3@18"

MCC-2
78" AFB
10.5 cal/cm2 @ 18"
#3@18"

Figure 4.4: Example of arc flash assessment results
on EasyPower one-line diagram
9. If there are various possible sources or interconnections, clearly mention in the one-line,
which source is connected and/or which breaker is open or closed. Workers should first
determine if the assumed condition in the diagram reflects the condition of the power system
at the time of work. If the system conditions are different from those for which the
assessment was performed, it is necessary to revise the evaluation.
4.10.3 Documentation on Equipment
Three types of documentation are recommended for arc flash hazard assessment results placed
on the equipment:
1. Warning labels with arc flash values: Permanent stickers with a warning sign of adequate
size. The label should be located in a place that is easily visible and readable from some
distance. The flash protection boundary and its units, the incident energy at the estimated
working distance and its corresponding risk category number must be clearly printed on the
label. Additional information that is useful for future revisions are the date of assessment,
the method of calculation, and the software name and version. Warning labels should also be
placed just outside the flash protection boundary, so that workers may see it and prepare
accordingly before they enter the hazardous area.

61


Figure 4.5: Example of arc flash warning label printed from EasyPower
2. Arc flash assessment results, in the form of a table and a small one-line diagram as
described in the previous section, should be placed on the equipment at a spot which
workers can easily access.
3. Large multi-section equipment may be labeled at various sections of the equipment. This
facilitates hazard communications. Different sections may have different potential arc
flash energies. If the same label is to be used on all sections, the highest possible incident
energy must be specified. For example, a large transformer can have higher incident
energy on the low voltage terminals than on the high voltage terminals. If the incident
energy differences are high, different labels can be placed. This avoids workers having to
wear extra PPE while working on terminals with less incident energy.
NEC, NFPA and IEEE do not specify the details of the information to be placed on arc flash
warning labels. The labels may be more or less detailed than the one shown in Fig. 4.5. The
label can be as simple as “Warning – Arc Flash Hazard”. It is up to the facilities management to
decide on detail desired. Arc flash calculations provide the maximum expected incident
energies. There are a number of tasks that can be done around electrical equipment that does not
require the maximum PPE to be worn. Referring to NFPA 70E-2000 Table 3.3.9-1 the task of
reading meters with the door closed is classified as risk category 0 (zero). While NFPA lists
racking in a breaker as risk category 3, if calculations give the maximum PPE as category 4, then
risk category 4 should override NFPA risk class 3. Table 4.13 below is a summary of the NFPA
task table.

62

Table 4.13: Summary of NFPA 70E Task and Risk Categories.
“Max. Calculated” is shown for the highest risk values used in the NFPA table. For actual risk category
refer to NFPA tables.

Energized Equipment

0

Breaker or switch
operating with
door/covers open

1

Removing bolted covers

Max Calculated

Racking in/out breakers

Max Calculated

Reading meters with
doors/panels closed

0

Work on energized parts

Max Calculated

Breaker or switch
operating with
doors/covers closed

2

Breaker or switch
operating with
door/covers open

Max Calculated

Removing bolted covers

Max Calculated

Racking in/out breakers

Max Calculated

Reading meters with
doors/panels closed

0

Work on energized parts

1-15 kV switchgear,
motor starters

Risk Category

Breaker or switch
operating with
door/covers closed
Panel board, MCC, LV
switchgear

Task

Max Calculated

18

Conrad St. Pierre, A Practical Guide to Short-Circuit Calculations, Electrical Power Consultants, LLC,
2001.
19

"IEEE Red Book" - IEEE Recommended Practice for Electric Power Distribution for Industrial Plants,
ANSI/IEEE Std 141-1986, IEEE, 1986.
20

William D. Stevenson, Jr., Elements of Power System Analysis, McGraw-Hill.

21

NFPA 70E — May 2003 ROP, "Standard for Electrical Safety Requirements for Employee Workplaces",
2003 Edition, page 57.

63

Chapter 5. Limiting Arc Exposure

5 Limiting Arc Exposure
Exposure to arc flash can be limited in three ways:
1. Avoiding arc flash accidents.
2. Reducing the level of arc energy released.
3. The proper use of personal protective equipment (PPE).
Arc flash accidents can be reduced by following procedures correctly, use of proper tools, good
preventive maintenance, planning and coordination of work, as well as skill development and
practical experience. Also important is the mental and physical conditions of the workers such
that the dropping of tools, accidental touching, etc., are avoided. Taking care of the causes of arc
flash is the principal strategy for avoiding exposure.
Accidents may occur despite precautions taken to avoid them. In such cases, it is always better if
the incident energy is low and the worker is prepared for the worst by using appropriate PPE.
This chapter discusses the first two methods. PPE is discussed in chapter 6.

5.1 Avoiding Arc Flash Accidents
Arc flash can be avoided by understanding its causes and taking steps to minimize them. The
various causes of arc flash discussed in Chapter 1 are summarized below. The mitigation
measures are described in the following sections.
Summary of causes:


Dust, impurities, and corrosion at contact surfaces producing heat, loosening contact and
creating sparks.



Sparks produced during racking of breakers, replacement of fuses, breakers/fuses closing
into faulted lines.



Failure of insulating materials.



Snapping of leads at connections due to force – human, rodents or birds.



Accidental touching and dropping of tools, nuts-bolts, or metal parts.

5.1.1 Preventive Maintenance
Preventive maintenance practices exist in most companies that require high reliability of supply
or process continuity. Preventive maintenance also provides for a safer workplace. Enhance
maintenance procedures when carrying out inspections, preventive maintenance, or even
breakdown maintenance by including procedures that address arc flash hazards. This reduces the

65


overall cost of implementing an arc flash program. Include the following in maintenance
practices.
1. Rodents and birds entering panels and switchgear are not uncommon. These can lead to
short-circuits and eventually arc flash22. This risk can be prevented by closing all open areas
of equipment with wire net or sealant so that rodents and birds cannot enter.
2. Use corrosion resistant terminals. Corrosion can lead to snapping of small wires, which in
turn may create sparks and fumes when the tip of the wire hits the metal enclosure or another
phase conductor. Check for corroded terminals and parts regularly if the electrical equipment
is at a chemical plant or near a marine atmosphere. Electrical contact grease is typically used
in joints and terminations. This will reduce corrosion.
3. Check for loose connections and overheated terminals. Impurities at the terminal connectors
or dust can create additional contact resistance, heating the terminals. A sign of such case is
discoloring of the nearby insulation. Heating of cable insulation can damage the insulation another cause of flashover. Infra-red thermography can provide valuable data on poor
connections and overheated electrical conductors or terminations.
4. Insulate exposed metal parts if possible. If heat dissipation is not really needed from the
exposed metal part, and insulating it with some insulating tape, sleeve or cover is not a
problem, then it is better to do so, rather than to let them be exposed. Insulation prevents
arcing. For example, if a worker drops an uninsulated spanner, which touches bare bus bars
of two phases, a short circuit will occur. However, this will not happen if the spanner or the
bus bar is insulated.
5. Make sure relays and breakers operate properly. Failure could lead to prolonged exposure to
arc flash, which could result in death. Routine inspection and testing of relays are carried out
in companies with good maintenance practices. The frequency of relay tests may be a couple
of years up to five years, depending upon the manufacturer's suggestions and the policy of
the company.
6. Pitting of contacts takes place when fuses are operated. Replace contacts of the fuse holder
or the fuse holder itself when excessive pitting is noticed.
7. When a fuse melts, make sure that the fault has cleared before installing a new fuse. Closing
on to a fault can produce sparks that could lead to arc flash.
8. Wire harness for control and instrumentation should be kept in proper condition. It is not
uncommon for these wires to become bundled and messy over time. Occurrence of arc blasts
is possible while opening covers of such switchgear/MCC.
9. Check for excessive moisture or water/ice on insulating surfaces of equipment. This may
cause flashover, specially on high voltage equipment.

66


5.1.2 Working on Live Equipment
1. It is ALWAYS preferable to work on de-energized equipment, regardless of the hazard risk
category (HRC). When work on live equipment is unavoidable, then justification and written
authorization is required. Incorporate this precautionary measure into the work procedure.
2. Use insulated tools. Dropping of tools can cause momentary faults, sparks and arcs.
Insulated tools can help reduce this type of accident.
3. Torque control: When using spanners, wrenches or screwdrivers to fasten or loosen a
connection use appropriate torque. When excessive force is required, it is not uncommon to
lose control. Slipping of screws or nuts and bolts may cause accidental touching. Corroded
or heated fasteners can be difficult to loosen. Work off line if loosening is difficult.
4. Do not use paint, cleaning chemicals, spray, etc., on live exposed metal parts. The fumes or
spray may be conductive and it may reduce the insulting property of air and allow an arc to
strike through. Spraying directly on live conductor can also provide a conducting path that
will result in electric shock.

5.2 Reducing Incident Energy on Worker
The incident energy exposure can be reduced by system design or operating procedures. Given
below are several ways to reduce the energy on an existing system.
1. Reduce the fault level
2. Reduce the exposure time
3. Remote operation
4. Remote racking
5.2.1 Reducing the Level of Fault
Fault level can be reduced in the following ways:
1. Change system configurations to reduce available fault current/smaller kVA
transformers.
2. Current limiting fuses/breakers.
3. Current limiting reactors.
5.2.1.1

System Configuration

Reducing the fault level depends on the existing system configuration. Double-ended load
centers with a normally closed tie (Figure 5.1) is a prime example where the fault level can be
reduced by either opening the tie or one incoming breaker. The fault current will be reduced by
approximately 50% and the incident fault energy will also be reduced, although not necessarily
in the same proportion. If the bus has two sources or a source and a normally closed tie as
67


0.
48

N.C. TIE

MAIN-2

kV

MAIN-1

0.
48

kV

shown in Figure 5.2, opening one of the sources (or tie) will reduce the fault level while
maintenance is done on the equipment. For both situations, the loading and relay setting should
be checked to make sure that the opening of a breaker does not overload the other source.

Figure 5.1: Double-End Load Center Configuration

Source 1

Source 2

N.C.
Feeder 1

Feeder 2

Feeder 3

Tie

Figure 5.2: Dual Sources
5.2.1.2

Current Limiting Fuses/Breakers

Current limiting fuses/breakers introduce additional resistance within the fuse element while the
fuse is melting. This limits the fault current. Fault currents within the current-limiting region of
the fuse are cleared quickly, usually within half a cycle. Since the incident arc energy is
proportional to arcing time, current limiting fuses/breakers limit the arc energy.
5.2.1.3

Current Limiting Reactors

Current limiting reactors introduce additional impedance in the system and are used to limit the
fault current. This not only reduces damages caused by faults but also allows the use of circuit
breakers with smaller duty. Limiting the fault current can also increase the fault clearing time if
the fault current happens to lie in the inverse time delay characteristics of the protective relays.
68


Therefore, protective device coordination analysis is also required when selecting current
limiting reactors.
5.2.2 Reducing Arcing Time
Arcing time can be reduced in several ways. Some changes in the system of settings may be
required for this purpose. Some strategies outlined in this section are as follows.
1. Reducing safety margin for relay and breaker operation with improved solid state trip
devices.
2. Bus differential protection to combine selectivity with instantaneous operation.
3. Temporary instantaneous trip setting during work.
4. Retrofit time-overcurrent relays with delayed instantaneous trip device if needed.
5. Optical sensor to trip breaker in the event of arc flash.
6. Use smaller fuse size if possible; smaller current limiting fuses may clear faster. Fuses
will generally be much faster than breakers at high fault currents – even ignoring currentlimiting effect this can greatly reduce arc energy.
7. Protective device coordination study to balance improving reliability with reducing arc
flash hazard.
5.2.2.1

Reducing Breaker Response Time Safety Margin

Incident energy increases with time and fault current. Reducing either or both lowers the
incident energy due to an arcing fault. Faster acting relays and trip devices can reduce the arcing
time to some degree. In this regard, a protective relaying review may be performed in order to
see if they can be lowered in time and pick-up. If a protective device study was done a number
of years ago when electro-mechanical relays were the norm, 0.4 second margin between relay
was common. This allowed for breaker operating time, over-travel, and a time safety margin.
Breaker times are now commonly 5 cycles rather than the 8 cycles of older breakers.
Microprocessor relays are now being used, for which the over travel has essentially been
eliminated. The repeatability of the microprocessor relay is better than that of the electromechanical relay. Therefore, the safety margin can be reduced. The end result is that the relay
coordination margins can be 0.2 to 0.25 seconds instead of 0.4 seconds. This is a 25%-35%
reduction in arc energy exposure.
5.2.2.2

Bus Differential Protection

A major improvement in clearing time would result if an instantaneous relay could be used
instead of time-delay relays. Feeders on both low and high voltage systems are likely to have
instantaneous settings and therefore, would limit the arc exposure time. However, a fault on the
main bus is often cleared by time-delay relays for selectivity reasons. In high voltage systems,

69


using bus-differential relaying reduces the arc exposure time to a minimum. The arc energy
exposure can easily be 5 to 30 times less than that of the delayed clearing time.
5.2.2.3

Temporary Instantaneous Setting

Replacement low-voltage trip devices from Satin, Joslyn, Carriere and possibly others, have an
instantaneous unit that that can be turned on or off. This has a high advantage on the incoming
main breakers. In many cases, for coordination purposes, the instantaneous is not set and fault
clearing times are delayed for selectivity. A main breaker clearing time with a load center tie
and feeder breakers could easily have a short time setting of 0.4 seconds. If the instantaneous
trip could easily be enabled while work is being preformed lower fault currents could be tripped
and cleared in less than 0.04 seconds. The incident energy exposure is reduced to 10% of its
previous value. During maintenance, full selectivity of devices may be lost, but the reduction in
arc flash exposure makes it worthwhile. The temporary instantaneous setting should be disabled
and the original protective setting should be restored for normal operations after the work is
completed. Separate instantaneous trip devices with increased protection can also be added to
shunt trip or transfer trip for added protection during work procedures.
5.2.2.4

Retrofit Instantaneous Trip Device

If bus-differential relaying is not possible then the main relay can be retrofitted with an
instantaneous protective device and a safety control switch. As shown in Figure 5.3, a selector
switch can be used to place the instantaneous in service when maintenance is being done.
Normally the instantaneous protection would not be functional due to the open contact of the
selector switch. However, when work is being done on the energized equipment, the safety
switch would be turned ‘ON’ and thereby limiting the arc exposure time to the worker should an
arcing fault accident occur. The delayed fault clearing time could be in the range of 0.4 to 2.0
seconds on the main breaker instead of 0.1 second. The delayed trip time greatly increases the
arc exposure time and amount of radiation a worker would receive if the arc blast pressure were
not enough to propel the worker away from the fault. The time-selective protection system
would be eliminated for duration of the work in the interest of safety. The selector switch should
be lockable in the maintenance position. Ideally, positive feedback from the trip unit would be
used for an indicating light associated with the switch to confirm the setting change was in
effect.
Many medium voltage multifunction relays have provisions for different protective settings for
various operating modes. For example, one group of setting is used for normal operation; a
second group of settings is used for emergency mode. Another group setting could be for
maintenance where the tripping and current pick-up settings are reduced and set as instantaneous.
Again, these temporary settings could result in the loss of selectivity with a gain in human
protection.

70


51

1200/5

Relay without
Instantaneous
Setting

MAIN BREAKER
SWITCHGEAR BUS

FEEDER 1 BREAKER

FEEDER 2 BREAKER
50
51

1200/5

1200/5

Relay with
Instantaneous
Setting

50
51

Relay with
Instantaneous
Setting

Main Breaker Control Circuit + DC Bus
Safety On/Off
Switch
Time Overcurrent
Relay Trip Contact

Lt

Inst. Overcurrent
Trip Contact

To DC Bus

To Main Breaker
Trip Circuit

Figure 5.3: Schematic to Control Arc Exposure on Relayed Breakers

Figure 5.4 shows the possible time current curves of a load center. The relay operating times for
both the 100% and 85% fault currents are shown in Table 5.1. If an instantaneous trip were set
on the main breaker at 3 times the long time pick-up, a fault on the bus would be cleared in
approximately 0.05 seconds with the incident energy being approximately 1.0 cal/cm2 instead of
the 4.4 and 5.3 cal/cm2 as shown in Table 1.

71


2

CURRENT IN AMPERES X 100 AT 480 VOLTS
3 4 5 6 7 8 9 10
2 3 4 5 6 7 8 9 100
2 3 4 5 6 7 8 9 1000 2

3 4 5 6 7 8 9 10000
1000

700

700

500
400
300

500
400
300

4160V
250E

200

200

100

1000KVA

70
50
40
30
20

TIME IN SECONDS

10
7
5
4
3

480V

20

600A

85% Transf Amp

1

.1
.07

10

5
4
3
2

1

Transf Amps

.7

.2

.

7

2

.5
.4
.3

50
40
30

1600A
1600A
GE RMS-9
Sensor = 1600
Plug = 1600
Cur Set = 1
(1600A)
LT Band = 1
STPU = 2 (3200A)
ST Delay = Min

100
70

600A
GE RMS-9
Sensor = 800
Plug = 600
Cur Set = 1 (600A)
LT Band = 1
STPU = 4 (2400A)
ST Delay = Min
Inst = 10 (6000A)

.7

250E
S&C
SMU
SMU-40
250E

.5
.4
.3
.2

.1
.07

.05
.04
.03

.05
.04
.03

.02

.02

.01
.5 .6 .8 1

2

3 4 5 6 7 8 9 10

2

3 4 5 6 7 8 9 100

2

3 4 5 6 7 8 9 1000 2


Figure 5.4: Load center time current curves

72

TIME IN SECONDS

.5 .6 .8 1
1000

.01
3 4 5 6 7 8 9 10000


Table 5.1: Clearing times and Incident Energy for a 1000-kVA load Center

100% Calculated Fault Current
Clearing Device

Main
Feeder
Fuse
Clearing Device

Main
Feeder
Fuse
5.2.2.5

kA at 480V
Operating Time
17.3
0.21
23.2
0.05
17.3
0.42
85% Calculated Fault Current
kA at 480V
Operating Time
14.3
0.30
19.7
0.05
14.3
1.0

Cal/cm2
4.4
1.4
8.8
Cal/cm2
5.3
1.2
17.6

Optical Sensor Trip

ABB has an “Arc Guard System TVOC” which has a light sensor to detect an electrical arc flash.
It can be activated by light only or light input supervised with an overcurrent detector. Its output
is used to trip a breaker and has an operating time of 10 milliseconds. If auxiliary tripping relays
are needed to trip several breakers at once, then the auxiliary relay time needs to be factored in to
the total clearing time. Placement of the detector and its control wiring could be critical. These
should be placed close enough to detect an arc but not be damaged by the initial arc rendering the
protection useless.
5.2.2.6

Fuse Size and Speed

Fuse sizes could be reviewed to determine if smaller fuses can be used. Smaller fuses reduce the
exposure time. This can be significant when the arcing current or 85% of arcing current is not in
the current limiting range of the fuse. Referring to Fig. 4, while the 250E fuse satisfies the NEC,
a smaller 175E fuse would also satisfy the NEC. The smaller fuse would operate quicker and
reduce the arc energy exposure, should the main breaker fail or should a fault occur between the
transformer and main breaker. Speed of fuses are selected to coordinate with other protective
devices and the over-current capacity of equipment being protected. A disadvantage of lowering
the fuse size is the possibility of fuses not being able to discern a temporary fault from a
persistent fault. A temporary fault, such as those found in overhead distribution lines, exist for a
few cycles. Some fuses are selected such that they allow temporary faults but interrupt persistent
faults. If the fuse size is lowered with the intent of reducing arc flash hazard, then the fuse may
melt upon temporary faults, thus reducing the reliability of supply.
Operating fuses can create sparks and may lead to arc flash accidents. Fuses should not be
temporarily lowered just for the purpose of working on live line.
5.2.2.7

Temporary Relay Settings

It has to be recognized that the act of changing protective settings on electrical equipment could
place the workman in jeopardy. While the protective devices are at low voltage a spontaneous
73


fault could occur in the switchgear at this time. Most relay resetting are done with a keypad and
not with screwdrivers, the chance of a fault at this time is extremely low.
Review protective devices to see if they can be lowered in time and pick-up. Due to reliability
reasons using temporary settings is usually not a preferred practice. Tampering with settings of
protective devices is prohibited. However, if a qualified person, for instance the engineer, can
temporarily provide the alternate settings during the work period, then the incident energy can be
reduced by lowering the trip time.
5.2.2.8

Protective Device Coordination Study

A protective device coordination study is carried out to improve system reliability. This study
can be done on a regular basis, perhaps every few years or whenever there are changes in the
system. Such studies could also include as one of its goals, the reduction of incident energy from
arc flash. The engineer performing the study should simultaneously evaluate the arc flash hazard,
and seek to minimize the hazard by keeping the arcing time as low as possible.
5.2.3 Remote Operation and Racking

Placing distance between electrical conductors and the worker greatly reduces the arc incident
energy and the arc blast force. The reduction is not linear. For example, a worker twice as far as
another worker from the arc will receive 25 to 50% less energy than the closer worker. New
high voltage equipment can be ordered with the breaker “Open” and “Close” switches remote
from the breaker unit. These could be placed on a non-breaker unit, in a separate control panel,
or in a remote room. Older switchgears can be retrofitted with remote control switches.
New microprocessor-relays can be programmed to supervise manually the closing of a breaker
using a “punch and run” time, that allows the operator 3 to 10 seconds after initiating a “close” to
evacuate the vicinity before the breaker is actually closed.
While fully electrically operated low voltage breakers are available they are not the norm. Low
voltage breakers that are fully electrically operated would be useful for remotely located control
switches. As the insurance companies and OSHA begin to demand better arc flash safety
measures, fully operated electrically low voltage breakers may become more common.
Placing a breaker in or out of a switchgear cubicle exposes the worker to a possible arc flash
hazard. While the breaker’s mechanical indicator may note that the breaker is fully open, there
have been cases where it was not open due to contact or indicator failure. Placing a breaker in a
cubicle when it is not in the fully open condition can result in an arc. While the distance from
live conductors to the worker can be over an arm's length away, the arc gases can flow around
the breaker and result in burns. For breakers that are being withdrawn from a cubicle, check the
following three items before withdrawal – the mechanical indicator shows the breaker open, the
breaker indicating lights show the breaker open, and the ammeter shows all three phases with
zero current.
Using a longer operating arm to rack in the breaker can provide the needed distance. Remotely
controlled breaker racking mechanisms are available for some breakers as part of the new

74


equipment or as retrofits.
Placing a barrier such as a closed door or a portable shield as shown in Figure 5.7 would limit the
arc flash exposure. While the shield as shown would help remove direct arc burns, radiant
energy burns are still possible and PPE is still needed. With a shield the surface area is
increased, therefore making the force exerted by the arc blast more of a concern.

Switchgear
Cubicle

High Impact
Plastic Shield
with Arm

Breaker

Figure
5.7: Using
a shield
when
racking in
a circuit
breaker

Although
not a way
to reduce
arc
incident
energy, it is good practice to use the buddy system. In the event an incident should happen, help
can be summoned quickly if a second person is around.
22

Some of the authors inspected a substation of an irrigation pump in Eastern Oregon in January, 2003.
The switchgear had been damaged by arc-flash. It was observed that both rodents and birds had
inhabited the MCC/panel. The leads to the primary side of the potential transformer had snapped and
touched the metal enclosure creating sparks. The arc traveled from the mains side of the 4.16 kV copper
bus bar towards the remote end and melted the bus bar butts and the steel sheet cover. The recloser at
the utility substation tripped several times.

75

Chapter 6. Personal Protective Equipment

6 Personal Protective Equipment
Personal protective equipment (PPE) is required by various standards such as NFPA and OSHA
to protect workers from hazards in the workplace. The type of PPE required depends upon the
hazard that has been assessed and documented. In the case of arc flash hazard, the main purpose
of PPE is to reduce burn injury to worker to a level of curable burn.
Personal protective equipment may, or may not, provide adequate protection in the case of arc
flash exposure. It is important that workers understand the use, care and limitations. Employers
should ensure that the workers have adequate understanding and training on the use of PPE.
Workers must not treat PPE as a substitute for common sense and safe work practices.
The most common and industry accepted PPE that protects the body is flame resistant (FR)
clothing. Flame resistance is the characteristic of a cloth that causes it not to burn in air. This is
achieved by treating the cloth fiber such as cotton with flame retardant chemicals. Synthetic FR
clothing is also widely used.
Purpose of flame resistant fabric:

1. Resistance to flame and self-extinguishing.
2. Provides thermal insulation to the body from heat radiation.

6.1 Standards on Personal Protective Equipment
Some of the standards on personal protective equipment are briefly outlined in the following
sections. Note that these are only some of the main standards and not all of them. It is important
that PPE selection and training be carried out in the guidance of an experienced safety
professional that is aware of all the applicable standards.

77


6.1.1 OSHA
Table 6.1: Various OSHA standards on personal protective equipment
1910.132(a),
1926.95(a)

Application
What?: Protection shall be provided for eyes, face, head and extremities.
When?: Whenever it is necessary by reason of hazards of processes or
environment, chemical hazards, radiological hazards, or mechanical irritants in
a manner capable of causing injury or impairment in the function of any part of
the body through absorption, inhalation or physical contact.
How?: Protective clothing, respiratory devices, protective shields and barriers.

1910.132(b),
1926.95(b)

Employee-owned equipment: The owner shall be responsible to assure its
adequacy, including proper maintenance and sanitation.

1910.132(c),
1926.95(c)

Design: PPE shall be of safe design and construction.

1910.132(d)

Hazard assessment and equipment selection:
(1) Employer shall assess the workplace to determine if hazards are present or
likely to be present, which necessitate the use of PPE. If hazard exists or
may arise, the employer shall:
 Select, and have each affected employee use the appropriate PPE as per
the hazard assessment.
 Communicate selection decisions to each affected employee.
 Select PPE that properly fits each affected employee.
(2) Documentation of hazard assessment:
 written certification,
 identification of workplace evaluated,
 person certifying the evaluation,
 date(s) of hazard assessment.

1910.132(e)
1910.132(f)

Training: When PPE is necessary, what PPE is necessary, how to properly use
the PPE, and how to care, maintain and dispose the PPE. Each affected
employee shall demonstrate an understanding of the training. Retraining may
be required depending upon changes in workplace or PPE. The required
training shall be certified and documented.

1926.100(a)

Employees working in areas where there is a possible danger of head injury
from impact, or from falling or flying objects, or from electrical shock and
burns, shall be protected by protective helmets.

1926.100(b)

Helmets for the protection of employees against impact and penetration of
falling and flying objects shall meet the specifications contained in American
National Standards Institute, Z89.1-1969, Safety Requirements for Industrial
Head Protection.

1926.100(c)

78

Defective and damaged equipment shall not be used.

Helmets for the head protection of employees exposed to high voltage
electrical shock and burns shall meet the specifications contained in American
National Standards Institute, Z89.2-1971.


6.1.2 NFPA 70E
Proposed NFPA 70E (2003 ROP) Article 220.2(B)(3): Protective Clothing and Personal
Protective Equipment for Application with a Flash Hazard Analysis. The employer is required
to carry out an evaluation of arc flash hazard, and document the incident energy, in calories per
square centimeter, that a worker may be exposed to arc heat on the face and chest. The worker is
required to wear FR clothing and PPE adequate to protect the body from injury from the
calculated exposure to heat from arc.
Proposed NFPA 70E (2003 ROP) Article 220.6: Workers must use adequate PPE on various
parts of the body suitable for the work to be performed. Various standard pertaining to care,
testing and use of PPE are outlined in this section. Please refer to the standards for details. Some
of the main requirements are as follows:


All employees within the flash protection boundary are required to wear PPE.



PPE should cover all other clothing that can be ignited.



PPE should not restrict visibility and movement.



Non-conductive protective headwear is required when in contact with live parts or when
there is a possibility of electrical explosion. The face, neck and chin must be protected.



Eye protection is required.



FR clothing should be worn when the estimated incident energy at the body may cause a
second degree (curable) burn (1.2 cal/cm2 for arc time greater than 0.1 second or 1.5
cal/cm2 for arc time 0.1 seconds or less).



Leather or FR gloves are required to protect the hand.



If incident energy exceeds 4 cal/cm2, heavy duty boots are required to protect the feet.

6.1.3 ASTM

American Society for Testing and Materials (ASTM) develops standards that specify the quality
of various materials including safety materials such as PPE. The following standards are
applicable to arc flash hazard protection equipment.
ASTM F1506: Standard Performance Specification for Textile Materials for Wearing Apparel
for Use by Electrical Workers Exposed to Momentary Arc and Related Thermal Hazards, 2002.
This standard specifies the requirements for flame resistant clothing. There are three basic
requirements in this standard:

79


a) The fabric under test must self-extinguish in less than 2 seconds after the ignition source
has been removed.
b) Char length for ASTM Test Method D6413 must be less than 6 inches. A fabric specimen
of 12 inch length is hung vertically in an enclosed space and the bottom is exposed to a
methane flame for 12 seconds. The length of the fabric destroyed by flame is the char
length. This test is also known as the standard vertical flame test.
c) Apart from meeting these pass/fail tests, the fabric is also tested for the Arc Thermal
Performance Value (ATPV) as per ASTMF1959. Manufacturers are required to report
the test results to the end users of the material as an Arc rating on a garment label.
Any fabric that meets the ASTM F1506 complies with OSHA 1910.269. This performance
specification does not cover coated fabrics commonly used in rainwear.
ASTM F1959: Standard Test Method for Determining Arc Thermal Performance (Value) of
Textile Materials for Clothing by Electric Arc and Related Thermal Hazards. This test
determines how much incident energy is blocked by the fabric before the wearer of the protective
clothing may get a second degree burn. The amount of energy blocked is reported as Arc
Thermal Heat Performance Value (ATPV). If the fabric breaks open the value is also called the
Breakopen Energy Threshold.
ASTM F1891: Standard Specification for Arc and Flame Resistant Rainwear. See ASTM F1506
for the three basic requirements.

Any fabric that meets the ASTM F1506 complies with OSHA 1910.269.
ASTM F1449: Standard Guide for Care and Maintenance of Flame, Thermally and Arc
Resistant Clothing. This guide provides recommendations for the care and maintenance of
clothing that is flame, thermal and arc resistant. The standard focuses on the industrial
laundering process and also identifies inspection criteria that are significant to the performance
of clothing.

6.2 Fire Resistant Clothing
6.2.1 Factors Affecting Protection Level From Arc
Material: Untreated natural fabrics may continue to burn until the fabric is totally consumed.
Synthetic fabrics that are not flame resistant will burn with melting and dripping and may cause
severe burns to the skin. Flame resistant fabric will be charred by arc flash heat, but will not
continue to burn after the arcing ceases. The burning of garments may cause greater injury to the
skin than direct exposure to the heat from the arc.
Weight: The weight of FR fabric is specified in weight per unit area (ounces/square yard or
g/m2). Higher weights provide more thermal insulation.

80


Layers: Multiple layers of clothing retain air space between the layers, thus providing greater
thermal insulation than a single layer. Single, thick clothing provide less physical comfort,
whereas multiple layers allow flexibility. Comfort and flexibility are important in avoiding
accidents while working on live equipment.
6.2.2 Care of FR Clothing
Laundering: Obtain complete instructions on care of FR clothing from the manufacturer. Some
cleaning chemicals such as chlorine bleach may affect the finish, reduce the fabric strength and
remove the color of the cloth. Some manufacturers claim that the flame resistance property is not
affected by the bleach23. Follow laundering instructions provided by the manufacturer.
Contamination: Grease, oil, or other flammable materials catch fire easily and will continue to
burn even after the arc ceases. Therefore FR clothing contaminated with these substances should
not be used. Care should be taken at work to avoid contaminating FR clothing from such
materials.
Storage: The clothing should be stored in a safe condition so that it is reliable.
6.2.3 Useful Life of PPE

The useful life of a PPE may depend on various factors such as the material with which it is
made, the severity of work activity and the abrasion resistance characteristics of the PPE. Obtain
information from the manufacturer to determine the useful life.
The useful life of a PPE is normally stated following some assumptions. It must be remembered
that if the actual conditions are different from these assumptions, then the stated expected life
may not be applicable. It is best if the PPE user obtains from the manufacturer, the useful life of
the PPE for the intended use.

Table 6.2: Examples of expected useful life of FR Clothing

Fabric

Industrial
Launderings

Expected Service Life
(months)

INDURA 100% Cotton24

36-50*

18-24#

INDURA Ultra Soft 88%
Cotton 12% High Tenacity
Nylon

60-80*

28-38#

*The manufacturer clearly states that the launderings cited in the table are based on market experience for these types of fabrics
and relate to average expected wear life. These estimates do not take into account work activities leading to extreme wear and
exposure to thermal sources of high heat and long duration.

The useful life for any PPE suggested by a manufacturer may be
applicable for normal wear and tear. OSHA requires the worker to carry
out a visual inspection of the PPE prior to its use. The PPE should not be
used if it appears to have deteriorated, even though the PPE may not have
reached its expected useful life.

#The
expected

81

service life is estimated with the assumption that clothing is industrially laundered every other week.

6.2.4 Selection of PPE

PPE should be selected according to the needs of the worker and the nature of work performed.
Some of the factors are discussed below.
Comfort: It is vital that the worker is not uncomfortable. Otherwise there could be a risk of
accidents occurring. Comfort is important both physically and mentally. PPE for high incident
energy (hazard/risk category #4 or greater) may have thick and heavy clothing, headgear and
gloves. The comfort level may differ from one individual to another. It is necessary to ensure
that each worker feels as comfortable as possible, wearing the PPE. Different workers may find
different materials more comfortable than others. It may be beneficial to let the workers try out
the PPE to make sure that it is satisfactory in terms of comfort. It may take some time before a
worker adjusts to new PPE. Therefore, it is recommended that the worker practice wearing the
PPE before working on live exposed equipment. This also ensures that the PPE does not
interfere with the task.
Fit: A loose fitting PPE provides more thermal insulation through the air trapped inside.
However, it should not be too tight or too loose so as to interfere with the task.
Layers: As mentioned in the previous section, multiple layers provide additional air insulation
and greater degree of protection. Multi-layer FR clothing is also more comfortable than a single
layer of thick and heavy clothing.
Materials: Choice of fabric material can affect both comfort and weight. There are different
types of treated cotton and synthetic fabric available from various manufacturers. For multilayer clothing, the workers may choose to have untreated flammable fabric such as cotton or
wool for inner garments at lower incident energies.
Abrasion Resistance: Some FR clothing is available with high abrasion resistance quality.
Employees who do heavy duty work should use this kind of PPE. Clothing without such quality
can be easily damaged, and may fail to adequately protect the worker from an arc flash.

82


6.2.5 Types of FR Clothing

(a)

(b)

(c)

Figure 6.1: FR clothing (a) jacket; (b) bib overall; (c) complete flash suit (courtesy W.H.
Salisbury & Co.)
Vest/Undergarment: These can be worn underneath shirts, jackets or pants. They provide an
extra layer of protection. Multi-layered clothing is more flexible, easy to work with and has
trapped air to provide additional thermal insulation. Combination of vest/undergarment with a
shirt increases the total arc rating.
Shirt/Pant: FR shirts and pants can be used for incident energy of 4.0 cal/cm2 or below. These
can be multi-layered for higher arc rating.
Bib Overall: Bib overalls worn with a shirt provides higher protection to the chest area than a
shirt worn with a pant. See Figure 6.1 (b).
Coverall: Coveralls are equivalent to shirt and pant.
Jacket: These are usually multi-layered and are like multi-layered shirts. See Figure 6.1 (a).
Hood: The hood is part of the headgear, has face protection and has FR fabric covering the
head, ears, neck and shoulders.

83


6.3 Other PPE

Figure 6.2: Headgears (courtesy W.H. Salisbury & Co.)
Headgear: The headgear consists of a non-conductive helmet and hood that covers the head,
ears, neck and shoulders. It also has a face shield and chin cover. See Figure 6.2. The face
shield absorbs some of the incident energy. However, it should not impair visibility. Safety
glasses should be worn underneath the headgear.
Gloves: Gloves provide insulation from both electricity and heat. A combination of rubber
(worn inside) and leather (worn outside) materials is typically used. The gloves should be long
enough to cover the sleeves.
Table 8.1 - Voltage Classification of Gloves

Glove Voltage Classification

Proof Test kV

Class 00

500

2.5

Class 0

1,000

5.0

Class 1

7,500

10

Class 2

17,000

20

Class 3

26,500

30

Class 4

84

Maximum Working Voltage

36,000

40


Figure 6.3: Gloves (rubber and leather) and boots (courtesy W.H. Salisbury & Co.)
Boots: Heavy duty shoes25 or boots should be worn where incident energies are higher than 4
cal/cm2.
Hot Stick: Hot sticks are use to operate fuses and switches. These provide insulation from the
high voltage parts. They also allow the worker to maintain increased working distance, so that
the incident energy is less.
Arc Suppression Blanket: This provides a barrier from arc flash.
Ear Muffs: Arc blast can cause severe ear injuries. Ear muffs should be worn to provide sound
insulation and reduce the impact.
Mechanical Barriers: As mentioned in the previous chapter, mechanical barriers can provide
protection from thermal radiation as well as from blast pressure. They can be used for racking
breakers, but are not suitable for most other work.
23

Indura, Nomex.

24

Westex Inc., Product Literature: INDURA Ultra-Soft Flame Resistance Fabrics, September 2002.

25

Proposed NFPA 70E- May 2003 ROP, page 45.

85

Chapter 7. Arc-flash Hazard Program

7 Arc Flash Hazard Program
7.1 Electrical Safety Program
Ray Jones26, et al, describe in their book, Electrical Safety in the Workplace: “A safety program
is an organized effort to reduce injuries. Any electrical safety program should be a subset of an
overall site safety program.” For these programs to be effective, it is necessary for safety
professionals and technical professionals to collaborate. Much of this chapter follows their book.
Additional discussions are provided so that arc flash hazard related topics will be addressed in
greater detail.

7.2 Training
Training must provide people the knowledge and understanding of the existence, nature, causes
and methods to prevent electrical hazards. The training should also include the selection and use
of appropriate PPE.
As part of regular electrical safety training it would be beneficial to include the following arc
flash related topics. Special arc flash hazard sessions are recommended for introductory training
exercises.
7.2.1 Awareness

b. Existence of the arc flash hazard: Arc flash accidents are not as common as electrical shock.
Therefore, many are not aware of the hazard. Trainers and managers need to place adequate
effort in trying to convince the workers that arc flash hazard is indeed something to take
seriously.
c. Causes: Knowing the causes helps immensely in avoiding the hazard.
d. Nature of arcs: This can relate to the degree of potential damage and possible ways to reduce
hazard.
e. Possible injuries/damage: Findings and statistics from various studies and reported incidents
reveal the gravity of the hazard.
f. Historical cases: Literature on arc flash incidents can be found in many documents. Review
of these cases is illuminating and convincing and is likely to influence workers to consider
taking measures to avoid arc flash injuries.
Chapter 1 provides a brief introduction on the first four topics. IEEE Standard 1584 and
numerous papers on electrical safety provide examples of historical cases. The awareness
training should not be limited to electrical worker only. Since it is the responsibility of the

87


employer to provide training, PPE and other means of minimizing the hazard for the electrical
worker, managers or the responsible persons should be included in the training.
7.2.2 Standards and Codes

Standards and codes not only provide information on what employers and workers are required
to do, but also suggest solutions/methodology. In following the standards and codes,
“compliance” should not be the only motive. The following provide the standards and codes.
Some of the related topics are summarized in Chapters 3 and 6.
a. NFPA 70E
b. OSHA
7.2.3 Understanding of Arc Flash Quantities

Workers are expected to read signs/labels, drawings and tables to understand the degree of
hazard a worker may be exposed to. Some of these pertain to the rating of the required PPE or
protection boundaries. It is important that workers understand the following quantities and their
units. An understanding of the physical significance of these quantities is helpful.
a. Flash protection boundary
b. Working distance
c. Incident energy
d. Hazard/risk category
e. ATPV of PPE.
7.2.4 PPE

Chapter 6 provides some information on PPE. More detailed information can be obtained from
PPE vendors. Use of PPE can restrict visibility and movement, cause discomfort, and slow down
the work. Practice is recommended with PPE before working on energized equipment. The
following topics should be included in training on PPE.
a. Selection of PPE
b. Information/Labels on PPE
c. Training with new PPE
d. Inspection, care & maintenance
e. Useful life & disposal
f. Documentation of use and maintenance of PPE
g. Limitations and potential risks using PPE
h. Limitation of PPE & degree of protection provided

88


7.2.5 Reading and Following Warning Signs and Labels

Warning signs and labels are part of any safety program. When new labels or signs are placed,
workers need to be able to understand the meaning, and follow instructions precisely.
7.2.6 Methods to Reduce Risk While Working on Live Exposed Parts

Chapter 5 provides some methods of reducing risk. Many procedures can be developed in-house
to suit the system and type of work the workers may need to perform. Practice on de-energized
systems will greatly augment the knowledge, and make the work safer. New procedures are
introduced during the initial stages of the arc flash program with the help of experienced workers
and safety professionals. Formulation of new procedures as an ongoing process is expected.
Clear communication between different workers who are part of procedure formulation expedites
resolving safety issues.
7.2.7 Arc Flash Hazard Assessment

Arc flash hazard assessment should be carried out by skilled and experienced professionals. Inhouse engineers can be provided with the necessary training. Chapter 4 provides practical steps
to performing an assessment. It is necessary for the engineer to apply short circuit analysis and
protective device coordination, along with arc flash energy calculations. Engineers should be
aware of the limitations of the standards or methods they employ and also have a good
understanding of how these limitations can be overcome. It should be noted that only some of
the practical issues are discussed in this book because of time limitations. AFH programs,
calculations, and procedures are new to the industry and changing at a rapid pace. Participation
in seminars and forums, reading of publications, and peer discussions are recommended.
7.2.8 Documentation

Documentation is described in greater detail later in this chapter. Workers should be able to
document any changes performed in the power system, update the single-line diagrams, and also
make note of any discrepancy between the actual equipment/system interconnection and the
single-line diagrams. Workers should also understand the consequences of the drawings not
truly reflecting the actual system condition. Illustrations can be provided as to how such
circumstances may lead to selection of wrong PPE or wrong procedures.

7.3 Safety Audit
Safety audits are performed on a regular basis to evaluate various aspects of a safety program.
Audit intervals should not exceed one year27. The audit should be performed by experienced
safety professionals.

89


7.3.1 Purpose of Safety Audit


Evaluate the effectiveness of safety program.



Evaluate the performance of workers with regard to safety procedures.



Evaluate the status of safety related activities such as documentation, training,
communications, etc.



Recommend actions for improvement.

7.3.2 Arc Flash Hazard and Safety Audit

As part of the regular safety audit, the following arc flash hazard related points should be
examined.
1. System changes: Since the last audit, were there any changes in the power system? If "yes":
a. Was the change properly documented? Were drawings updated?
b. Were any new arc flash calculations performed?
c. Were new warning labels installed at the affected locations?
2. PPE: Do the employees have the PPE needed for the highest level of arc flash exposure in the
facility? What is the condition of PPE?
3. Do workers follow arc flash hazard warning labels, signs and instructions regarding flash
protection boundary and appropriate PPE?
4. Are arc flash hazard procedures followed correctly as they are documented?
5. Do the existing procedures provide protection adequately?
6. Do workers have adequate knowledge and training in arc flash hazard?
7. Have protective device settings been tampered with or modified from the intended settings?
Are the fuse sizes the same as those specified by the engineer?
8. If any equipment requires special operation/maintenance procedures for safety reasons, is the
information readily available to workers? What is their understanding of the special
procedures?
9. Do workers have the tools and equipment for working safely? (Insulated tools, shields, hot
sticks, etc.)
10. Review of accidents and near misses.

90


7.3.3 Methods of Obtaining Information for Audit

1. Interview with safety coordinators, responsible persons, trainers.
2. Inspection of records, documents, labels, signs, drawings pertaining to arc flash hazard.
3. Interview with electrical workers.
4. Observation of work procedures.
5. Inspection of PPE, safety tools and equipment.

7.4 Safety Meetings
Safety meetings (or job briefings) are usually carried out before working on energized
equipment. This provides a review of the following:


Work to be performed.



Potential hazards.



Individual responsibilities.



Specific procedures that require attention.

Regular safety meetings are also conducted in companies in which workers are continually
exposed to hazards. Monday morning meetings are held briefly to talk about safety. Accidents
and near misses are discussed. Safety meetings are instrumental in disseminating information,
bringing new issues to attention, and discussing possible solutions. The Monday morning
meetings also provide the workers an opportunity to focus on safety matters after returning to
work from their weekend. Workers should be encouraged to discuss openly and share their
ideas. For example, if a worker drops a tool on exposed live equipment, it should be discussed,
since this event could have led to arc flash. This discussion could lead to reasoning why the tool
was dropped, and whether a better method or tool would have avoided the incident. From these
insights, the procedures for arc flash hazards can be enhanced.

7.5 Documentation
1. Document all data that was used for the arc flash hazard assessment. This is useful for
implementing changes and future assessment.
2. Prepare a report of the assessment identifying the type, name/ID, incident energy at working
distances, flash protection boundary, hazard/risk category, and other pertinent information
such as voltage, available fault current, protective device description and its trip time, arc gap
and arc current.
3. Prepare electrical drawings that contain the results of the arc flash analysis. Electrical
drawings a usually referred to for planning out maintenance jobs.
91


4. Circulate documents to all concerned people.
5. Install easily visible warning signs at the door, fence, etc. of the location where the hazard
exists, with information on the hazard, the arc flash boundary and the requirement of PPE
within the area.
6. Install warning labels on the equipment at some easily visible location near the exposed live
part with information on hazard/risk category, estimated incident energy at working
distances, requirement of PPE, and the flash protection boundary.
7. If the equipment has a cover that needs to be removed before working on exposed live
conductors, install similar warning labels inside so that the worker may see it after removing
the cover.
8. Document and report the installation of all warning signs and labels.
9. Document the safety audit and use it for continual improvement.
10. Document arc flash related accidents and near misses. OSHA requires documentation and
reporting of all injuries that result in loss of workday. Documentation is suggested for near
misses as well, with the hope that prevention can be achieved more effectively.

7.6 Personal Protective Equipment
The following steps need to be taken regarding PPE. See Chapter 6 for details.
1. Select PPE based on arc flash hazard assessment.
2. Provide information/labels on PPE on thermal rating.
3. Train with new PPE.
4. Provide regular inspection, care and maintenance of PPE.
5. Document use and maintenance of PPE.
6. Dispose PPE after useful life has exceeded.

26

Ray Jones & Jane Jones, "Electrical Safety in the Workplace", page 147, National Fire Protection
Association, 2000.
27

See endnote 26.

92

Appendix A. Uncertainties in Arc-flash Hazard Analysis

A. Uncertainties in Arc Flash Hazard Analysis
and Methods to Deal with Them
The random nature of arcs has made it impossible to predict exactly the arc currents and
its associated incident energies. This section considers some statistical methods of
evaluating the randomness based on measured data. Other areas of uncertainties are also
explored and some simple methods to deal with these uncertainties have been proposed.
The calculation methods proposed in various standards provide specific values of arcing
current, incident energy and flash protection boundary. The statistical methods suggested
in this section consider a range of values about the calculated value given the uncertainty
in the behavior of arcing faults.
Various standards and studies have indicated that there are numerous factors such as
ambient temperature, humidity, pressure, surroundings, etc. that may affect actual arc
fault characteristics other than those factors included in the equations provided in the
standards. Approximations and assumptions in the data collection process prior to
computation may also result in variation of actual arc characteristics from the expected.
Although it is possible to adopt highly conservative calculation methods, caution has
been placed with workers wearing excess personal protective equipment (PPE). It is
believed that heavy and thick PPE could lead to difficulty in work, and therefore would
increase the chances of accidents and the possibility of arc flash hazards. In order to
provide "adequate" protection to the workers without being too conservative, it is
necessary for the engineer carrying out the arc flash hazard assessment to understand the
risks due to uncertainty and to be able to use statistical probability based on measured
data.

Variation of Arc Current from Estimate
Random Variations

Table A.1 shows the variation of measured arc current from the IEEE 1584 estimate of
arc current for low voltages. For the test data conditions, the arc current was calculated
using the proposed formula. These values were compared against the actual measured arc
current. The measured values of arc current were found to range between -37.5% and
57.2% of the estimated values. Bare in mind that the equations presented in IEEE 1584
for estimating arcing current are empirical equations based on regression analysis. This
equation is a best-fit curve with an R-square of 98.3%. R-square is a measure of the
equation fit to the data; 100% is perfect28. Therefore, after calculating the arc current
from the equation it is necessary to consider the upper and lower limits of the arc current
due to the random nature of arcs. The upper and lower limits would render the arc
current as a possible range rather than a definite calculated value. Applying the variation
limits presented in Table A.1 may not be the practical approach. It is customary in
engineering practices to define probable ranges based on statistical analysis.

95


Table A.1: Variation of measured data about arc current estimated with IEEE 1584
equation for low voltage (includes both open and box)

Percent Variation from Estimate (%)
Voltage (kV)

Minimum

Maximum

0.2 / 0.24

-10 %

+57.2 %

0.4 / 0.48

-37.5 %

+23.4 %

0.6

-27.6 %

+27.4%

Distribution of Measured Arc Current about IEEE 1584 Estimate
for Low Voltage Arc

70
Frequency (Count)

60
50
40
30
20
10
0
-40% -30% -20% -10% 0% 10% 20% 30% 40% 50% 60%
Percent Variation

Figure A.1: Distribution of measured arc currents about IEEE 1584 equations for
low voltage arc currents from bolted fault currents.

Figure A.1 shows the distribution of measured arc currents as deviation from the IEEE
1584 estimate for low voltage, in percentages of estimated arc current. This histogram is
the result of further analysis of the data published and used by IEEE for the equations in
1584. For 166 arc tests, the mean deviation from estimated arc current was 2% and the
standard deviation of the variation from estimated arc current was found to be 15.2% of
estimated arc current. The distribution has a skewness of 0.236 and a kurtosis of 1.553.
Although the skewness and kurtosis suggest that the data is slightly off normal
distribution, it may be assumed that with more data samples, the random nature of arc
will follow normal distribution. Therefore, further statistical analysis will follow the
assumption of normal distribution.
Table A.2 presents limits for low-voltage arcing current for various confidence levels.
For a confidence level of 95%, the arc current can have any value between -23.0% and
+27.1% of calculated arc current. The confidence level is a measure of probability or
likelihood. A 95% confidence level implies that out of 100 random samples, 95 samples
96


of observations will be in the specified range, and 5 samples will be above or below the
specified range. Therefore, there is 95% probability that a measured arc current at low
voltage may have a value between –23.0% to +27.1% of the estimated arc current. The
higher the confidence level, the greater the range. It is up to the engineer to select the
confidence level based on how conservative the arc flash evaluation needs to be. A
confidence level of 99% may appear to be too conservative for practical purpose. It is
suggested that 95% be used. As a rule of thumb, the arc current can be considered to be
within the range of +/-25% of the calculated value from IEEE 1584 equations for systems
with voltages less than a 1000V.
Table A.2: Minimum and maximum likely deviations in low-voltage arc currents
from the calculated arc current for various confidence levels.

Minimum Maximum
Mean Standard Confidence
Arc
Arc
Variation Deviation Level
Current Current
95%
15.2%

27.1%

99%

-33.4%

37.4%

90%

-17.5%

21.5%

68%

2.0%

-23.0%

-5.1%

9.1%

Table A.3: Variation of arc currents from estimate for various confidence levels for
2.4 to 15 kV

Arc in

Percent
Variation from
Estimate (%)
Min

Standard
Deviatio
n

Confidenc
e Level

Probable Arc
Current Range

-4.2%

7.6%

0.1%

8.9%

2.5%

12.4%

99%

-22.8%

18.4%

-13.6%

9.1%

-4.0%

4.2%

99%

-5.7%

5.9%

90%

Open

-2.2%

-16.8%

95%

21.1%

Max

90%

-38.9%

Max

95.0%
Box

Max

Mean
Variatio
n

-3.1%

3.3%

For medium voltages, the variation of arc current from the calculated value may be less
than for low voltages, as shown in Table A.3. Arc in open air appears to be more
predictable than arc in box. For a 95% confidence level take a tolerance of approximately
+/-14% from the calculated arc current in box. For arc in open air on medium voltages,
the variation of +/-4% is small enough to be ignored.
There are two reasons for considering the deviation in arc current from the estimate:
a) The incident energy is calculated from the arc current.
97


b) The arc current affects the clearing time of a protective device. As described in
the previous chapter, the incident energy is proportional to the arc time.
Gap Between Electrodes

For low voltages, the gap between electrodes affects magnitude of the arc current. The
gap (G) is part of the IEEE 1584 equation (1, 30, 31) for estimating the arc current. Using
this equation, the graph shown in Figure A.2 was plotted for arcing current as a function
of electrode gaps for various voltage levels, bolted fault current and enclosure type. For
all conditions, the arcing current reduces with increased gap. For medium voltage
systems the gap distance is not an issue for typical switchgear and cable spacing.
The arc voltage is roughly proportional to the arc gap length29. Although the arc
resistance is expected to increase with gap length, it is not a linear relationship. The arc
current decreases with arc resistance. However, since resistance is highly non-linear, it is
preferable to use the empirically derived method of evaluating the effect of gap on arc
current.
0.2kV, 87kA, Box
50.0

0.4kV, 103kA, Box
0.6kV, 36kA, Open

40.0
Arc Current (kA)

45.0

0.48kV, 52kA, Box

35.0
30.0
25.0
20.0
15.0
10.0
5.0
0.0
0

10

20
30
40
Gap Betw een Electrodes (m m )

50

60

Figure A.2: Effect of gap between electrodes on calculated arc current for low
voltages (<1000V) using IEEE 1584 equation.

The phase conductor spacing required by NEMA for various voltages is presented in
Table A.4. Similarly, IEEE 1584 guide mentions typical gaps. When we assume a certain
arc gap distance for low voltages, either based on NEMA or some typical value, we must
be aware of the fact that any variation in the actual electrode gap could lead to variation
in actual arc current. Therefore, the incident energy to which a worker may be exposed
may vary. The gap between exposed conductors may vary from equipment to equipment.
Also, at the terminal of a given equipment, the gap between conductors may vary
depending upon the shape and layout. For conductors that are not exactly parallel
(spatially), the minimum and maximum gap may be of interest. Arcs travel away from
98


the power source30. Therefore, it may be sufficient to record the gap between the tip of
the exposed conductor away from the source. For these reasons, it may be very difficult
for the engineer to determine the exact electrode gap for every equipment, especially if
the equipment is live. Although it can be safely assumed that the NEMA gap will be
maintained for all manufactured equipment, the same cannot be guaranteed about the
field installations of conductors, tap-offs and terminations, or repair jobs. If the exact
gaps are known, it is always better to use the data. In case the gaps tend to vary with
equipment, and the engineer may have an idea of the approximate variation from typical
values, then analysis can be carried out for various scenarios such as typical, minimum,
and maximum gaps.
Table A.4: NEMA gap between conductors for various voltage levels

System kV, L-L

Equipment Type

NEMA Gap
between
conductors

0.1 - 1.0

Switchgear
MCC/Panel
Open Air
Conductor
Switchgear
MCC/Panel
Open Air
Conductor
Switchgear
MCC/Panel
Open Air
Conductor

32
25
32
13
102
102
102
13
153
153
153
13

1.0 - 5.0

5.0 - 15

Gap Unit
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm
mm

Considering multiple scenarios for electrode gaps may be somewhat time consuming,
although more reliable. A quick method of examining the effect of variation in electrode
gap is the sensitivity analysis. Figure A.2 shows the relation between electrode gap and
arc current to be almost linear. Table A.5 shows the sensitivity of arc current to electrode
gap for various voltages, bolted fault currents and enclosure type. The sensitivity is the
percent variation in arc current for 1 mm variation in electrode gap.
The sensitivity varies with the system voltage and the available fault current. However,
for practical purpose the average sensitivity of –1% can be used. The following example
illustrates how sensitivity analysis can be used to quickly determine the possible range of
arc current due to variation in arc gap.

99


Table A.5: Sensitivity of arc current to electrode gap for various conditions

Enclosure

Voltage L-L Bolted Fault Sensitivity
(kV)
Current (kA) (% / mm)

Box
Box
Open
Box

0.208
0.400
0.610
0.485

36
53
88
103
Average

-1.0%
-1.3%
-0.7%
-1.0%
-1.0%

Example:

For a system with line-line voltage of 0.6 kV, assumed electrode gap of 32 mm, and
bolted fault current of 22.6 kA, the arc current in box was estimated to be 15.7 kA using
IEEE 1584 equation. It was also observed that the electrode gaps varied approximately
from 25mm to 40mm at different terminals and connections. What is the likely arcing
current?
Case 1: Gap, G = 25mm.
Variation in gap, g = 25 – 32 = - 7mm.
Assume sensitivity of -1%/mm.
Variation in arc current = - 7 * (-1%) = 7%
Maximum arc current = 15.7 * (1 + 0.07) = 16.8 kA.
Case 2: Gap, G = 40mm.
Variation in gap, g = 40 – 32 = 8mm.
Assume sensitivity of -1%/mm.
Variation in arc current = 8 * (-1%) = -8%
Minimum arc current = 15.7 * (1 - 0.08) = 14.5 kA.
The calculated arc current can have values anywhere between 16.8 kA and 14.5 kA.
Further variations can be considered due to randomness of arcs. For a confidence level of
95% the arc current can be within the limit of +/- 25% of the estimated value.
Lower limit of arc current = 14.5 * (1 – 0.25) = 10.8 kA.
Upper limit of arc current = 16.8 * (1 + 0.25) = 21.0 kA.

100


NOTE: It is not possible for the arc current to be higher than the bolted fault current.
This condition is not violated for an upper limit in arc current of 21 kA. In cases where
the estimated arc current exceeds the bolted fault current, take the arc current as equal to
the bolted fault current. For very low arcing currents, it is beneficial to check whether the
arc current is likely to be self-sustaining under the given system parameters. For 480 volt
systems, the industry accepted minimum level for a sustaining arcing fault current is 38%
of the available three phase fault current31.

Variation of Incident Energy from Estimate
Random Variations

The measured incident energy may deviate more widely than the arc current from their
respective calculated values. Figure A.3 shows the deviation of measured incident energy
about the calculated incident energy using the IEEE 1584 equations for low voltages.
First, the incident energy was calculated for various test conditions. Then it was
compared with the measured values. The deviation here is the difference in percent of
calculated values. For low voltage arcs in open air, the measured incident energies during
various tests were found to be lower than the calculated incident energy. The smallest
negative deviation is -25%. It can be concluded that highest possible incident energy is
about 75% of the calculated incident energy when using the IEEE 1584 equations for low
voltage arcs in open air. For LV arc in box, it can be seen from Figure A.3 that the
maximum deviation is very high. The maximum deviation was found to be about 62%.
Therefore, if the calculated incident energy for a case is 10 cal/cm2, there is a possibility
that the actual incident energy may be as high as 10*1.62 or 16.2 cal/cm2.
Although the nature of arcs is highly unpredictable, it is observed that arcs in open air, for
both low voltage and medium voltage, are more predictable than those in box. This can
be concluded from the shape of the histogram (or frequency distribution). The plots for
arcs in air have higher peaks and narrower bases (bottoms) than do the curves for arcs in
box.
The deviation of measured incident energy for medium voltages is shown in Figure A.4.
The maximum deviations are 46% for open air and 49% for box. Therefore if the
incident energy for a case of medium voltage in open air is calculated to be 10 cal/cm2
the actual energy may be as high as 10*1.46 or 14.6 cal/cm2.

101


60%

Low Voltage
LV Open Air

50%
Frequency (%)

LV Box

40%
30%
20%
10%
0%

-100%

-50%
0%
50%
Deviation from Estimated Incident Energy (%)

100%

Figure A.3: Frequency distribution of deviation of measured incident energy from
calculated value for <1.0-kV using IEEE 1584 equations.
60%

Medium Voltage
MV Open Air

50%
Frequency (%)

MV Box
40%
30%
20%
10%
0%

-100%

-50%

0%

50%

100%

Deviation from Estimated Incident Energy (%)

Figure A.4: Frequency distribution of deviation of measured incident energy from
calculated value for 1.0 to 15-kV using IEEE 1584 equations.

For the sake of simplicity, the discussions in this section have been based upon univariate analysis. In reality, the variations can be dependent upon multiple factors. To see
the effect of bolted fault current on the deviation let us look at Figure A.5. It can be
observed from this plot that the deviations tend to decrease with increasing bolted fault
current. In Figure A.5, trend lines are drawn for the maximum incident energies. For
higher bolted fault currents, the number of observations are small, and therefore it is not
possible to guarantee that the actual incident energy will not be higher than the predicted
value after accounting for deviations. It is learned at the time of writing of this article
that further tests on arc flash are being planned. It is hoped that more conclusive

102


information will be available in the future. In the mean time, with the available data, the
following procedures can be adopted as a rule of thumb.
The medium voltage trend line in Figure A.5 shows almost constant deviation with
respect to bolted fault current. Therefore, we can assume that the maximum possible
incident energy is 49% higher than the estimate provided by the IEEE 1584 equations.
For low voltage, the maximum possible deviations of incident energy from the calculated
values are about 60% at 10 kA IBF, 40% at 40 kA, and 25% at 60 kA, 10% at 80 kA and
0% at 100 kA. Further breakdown is possible with various enclosure types – in air or in
box, but is not discussed in this text. Readers are encouraged to obtain the test data from
IEEE and compare the results. This chapter deals with applying the statistical adjustments
on calculated results without changing the calculation methods proposed in the standards.
Another approach with similar results is to vary the calculation factors (Cf) in the IEEE
1584 equations to obtain more reasonable results directly.
Deviation of Measured Incident Energy about Estimated
100%

LV Measured

80%

HV Measured

60%

Linear (Maximum
MV Mesaured)
Linear (Maximum
LV Measured)

Deviation IE (%)

40%
20%
0%
-20%
-40%
-60%
-80%
-100%
0

20

40

60

80

100

120

Bolted Fault Current

Figure A.5: Deviation of measured incident energy from calculated value for various
bolted fault currents using IEEE 1584 equations.
Arc Gap

The arc voltage varies with the arc gap length. A higher arc voltage means a higher arc
power for the same arc current. Therefore the incident energy can be expected to
increase with arc gap length, as long as the arc current is not sharply reduced.

103


In the IEEE 1584 equations, the incident energy is also a function of the gap between
electrodes. As mentioned in the previous section on arc current, the actual gap may vary
from the NEMA gaps or assumed gaps due to various reasons. Further adjustments can
be made to account for the variation in incident energy as a result of variation in gap
between electrodes. Figure A.6 shows the effect of arc gap on the IEEE estimate for
incident energy.
For low voltage, the sensitivity of calculated incident energy is about 0.3% per mm
deviation in arc gap. For a 10mm deviation in gap, the incident energy can be expected to
be 3% higher, which is a rather small increment and can be neglected. In most cases the
variation in gap is not likely to be substantial, and therefore the effect of variation in gap
on the incident energy can be ignored.

Multiple of Incident Energy

2.5
2

1.5
1

0.5
0
0

50

100

150

200

250

300

350

Gap (m m )

Figure A.6: Effect arc gap on incident energy using IEEE 1584 equations.

In obtaining the incident energy based on IEEE equations for the test data, the measured
values of the arc current were used instead of the calculated arc currents. This was done
because we will be using the various adjusted values of arc current rather than just the
calculated arc current. This will cover the entire possible range of arc currents and also
provide more conservative and reliable results in the evaluation of incident energy.

Variations in Arcing Time
Arcing time depends upon the trip characteristics of the upstream protective device. The
trip characteristics themselves can have a some degree of randomness about the specified
trip time value. For instance, a relay may have tolerance of –15% to +15% about the
curve provided in the time-current characteristics (TCC). Tolerances are measured by the
manufacturer and the information is usually provided with the device data documents.
The incident energy is directly proportional to the arcing time. For the purpose of safety
it is better to use the maximum trip time, since the workers need to be prepared for the

104


worst possible condition. Persons carrying out the arc flash hazard assessment should
read the protective device data documents carefully to ensure that the maximum trip time
has been taken for the calculations and not the average, median or minimum trip times.
Some manufacturers publish fuse TCC for just the minimum melting time of the fuse. In
such cases, it is necessary to contact the manufacturer and obtain the total clearing time
curve or apply a tolerance to estimate the total clearing time. The tolerance may be
provided in the fuse data document or it can be assumed to be similar to the tolerance of
other similar fuses.
.2 .3 .4.5.6 .8 1
700
500
400
300
200
100
70
50
40
30

2 3 4 5 6789 10 2 3 4 5 6789 100 2 3 4 5 6789 1000 2 3

BL-1
Cutler Hammer Series C
MDLB
Frame = 800(700-800AT)
Trip = 800
Inst = HIGH (6400A)

700
500
400
300
200

Estimated Arc
Current, 7kA

100
70
50
40
30

10
7
5
4
3

10
7
5
4
3

2

2

Possible Trip times:
a) Thermal Trip
b) Magnetic Trip

1
.7
.5
.4
.3

TIME IN SECONDS

20

TIME IN SECONDS

20

1
.7
.5
.4
.3

.2

.2

.1
.07
.05
.04
.03

.1
.07
.05
.04
.03

.02

.02
.2 .3 .4.5.6 .8 1

2 3 4 5 6789 10 2 3 4 5 6789 100 2 3 4 5 6789 1000 2 3

Figure A.7: Trip time at instantaneous pickup current for thermo magnetic
breaker.

Figure A.7 shows the TCC of a thermal magnetic molded case circuit breaker (MCCB).
The estimated arc current of 7 kA shown here as an example, is equal to the pickup of the
magnetic trip. It is likely that the magnetic trip will activate and stop the arcing current at

105


or below 0.04 seconds. However, there is also a possibility that the MCCB may not trip at
that value pickup value. In such a case the thermal unit will trip the breaker at a much
higher time, at 6 seconds. This is a great difference in trip time and hence will result in
great difference in estimated incident energy. Close attention is required in obtaining the
trip times at arc current values close to pickup values.
Figure A.8 shows the incident energies for various values of arc current for a typical solid
state trip device using the IEEE equations. At the IEEE estimate of arc current, the
calculated incident energy is 2.5 cal/cm2. It was mentioned in the previous section that
the actual arc current could be as low as 75% of the IEEE estimate. At 75% of the IEEE
estimate, the trip is much higher and therefore the high incident energy of 9 cal/cm2. The
sharp change in the incident energy at about 82% of the IEEE estimate of arc current is
due to the pickup of short time delay unit of the device, similar to the observation made
for the MCCB in Figure A.7.

Incident Energy (Cal/sq. cm)

12
10

9 cal/cm2

8
6
4

2.5 cal/cm2

75% of
IEEE
Estimate

2

IEEE
Estimate Iarc

0
50

60

70

80

90

100

Percent of Estimated Arcing Current (%)

Figure A.8: Incident energies for various arc current values for typical solid state
trip device.

Arc Protection Boundary
The arc protection boundary can be worked backwards using the adjusted maximum
possible incident energy. This would mean adjusting the normalized incident energy (En).
The uncertainties mentioned previously in this chapter affects the boundary in a similar
way.

106


Theoretical Formulas
Ralph Lee32 proposed the arc flash boundary in terms of the bolted fault MVA. This
assumes the maximum possible arc power, which is half the total available fault MVA or
bolted fault MVA. Calculation of the incident energy is based upon this formula.
DB 

2.65 * 1.732 * V * Ibf * t

where
DB =
V =
Ibf =
t =

distance of the boundary from the arcing point (feet)
system voltage L-L (kV)
bolted fault current (kA).

(A.1)

This formula is applicable when definite time trip function is used to interrupt the fault.
A definite time trip function is a fixed time delay, and is independent of the fault current
passing through the protective device. Instantaneous trips are also approximately fixed
time in most devices. If the trip time is independent of the fault current, then making the
assumption that the arc current may have a value that will yield the maximum arc power
is justified. However, this formula needs to be modified if the trip time is a function of
the fault current. Inverse type relays, fuses, thermal trip units and solid state trip units
with I2T time delays have current dependent trip time. Assessment for inverse time
functions can be approached using the same circuit assumptions with which the above
equation (A.1) was derived.
Equivalent Circuit Model

Iarc

V

Xs

Rs

Vs

Ra

Figure A.9: Equivalent circuit diagram with Thevenin source and impedance, and
arc resistance.

The Thevenin equivalent circuit for the arc fault is shown in Figure A.9. Here, V is the
system voltage at the point of fault, Ra is the equivalent arc resistance, Xs and Rs are
components of the Thevenin impedance Zs, and Iarc is the arc current. When the arc
resistance is zero (a hypothetical case), the arc current is equal to the bolted fault current.
No power is dissipated through the arc. As the arc resistance increases, the arc current
decreases. The arc power reaches a maximum when the arc current is approximately 0.7

107


per unit of the bolted fault current. This holds true only if the X/R ratio of the system is
very high (Rs is negligible). If the X/R ratio is low, then the maximum power transfer
occurs when arc current ratio (Iarc/IBF) is less than 0.7, and the maximum arc power is less
than 0.5 times the bolted fault MVA. A plot of arc power as a function of arc current is
shown in Figure A.10. The arc power and arc current have been normalized in this plot.
1

Parc / Parc[max]

0.8
0.6
0.4
0.2
0
0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

Ia / IBF (pu)

Figure A.10: Plot of arc power as function of arc current. (Arc current is expressed
in per unit of the bolted fault current and the arc power is expressed in per unit of
the maximum arc power.)

We can substitute the bolted fault MVA term (1.732 * V * IBF) in equation (A.1) by
2*Parc, since the maximum arc power is released when Parc = 0.5 * MVABF. The
equation now becomes:
DB 

(A.2)

2.65 * 2 * Parc * t

For inverse type trip functions, both the arc power and the arcing time are dependent on
the arc current. Since the total energy released by the arc, Earc, is equal to Parc* t, this too
is a function of the arc current. We are now interested in finding the maximum arc
energy released as allowed by the protective device.
For an inverse square time-current function, the trip curve may be expressed as:
t K /I

2

(A.3)

From Figure A.9, the arc power may be derived as:
  V
2
Parc  Iarc *  

  Iarc


108

2


  Xs2





 Rs 




(A.4)


Assuming the current seen by the trip device is equal to the arc current, we can express
the total arc energy as allowed to be released by the inverse-square time-current trip
device:
E arc  Parc


* t  K*



 V

I
 arc



2


  Xs2



 Rs 

(A.5)




1

0.8

0.6

0.4

0.2

0
0.30

Parc /Parc[max] (pu)
I^2T Time (s)
Earc I^2T
0.40

0.50

0.60

0.70

0.80

0.90

1.00

Iarc / IBF (pu)

Figure A.11: Plot of arc power, trip time and arc energy released as function of arc
current. All values in the plot are normalized: Parc is in per unit of maximum arc energy
(0.5*MVABF); trip time for inverse square curve (I^2T) is 0.1 seconds for Iarc = IBF; Earc
is normalized Parc times arcing time.

The plot of trip time and arc energy released as a function of arc current is shown in
Figure A.11. This shows that the arc current at which the arc power is maximum (70% of
bolted fault current in this case) does not yield the maximum arc energy. Because of the
inverse-square time-current trip device, the highest energy released is for the lowest arc
current. For the example shown in Figure A.11, the arc energy released when the arc
current is 30% of bolted fault current is about five times greater than that when the arc
current is 70% of bolted fault current.
From (A.2) and (A.5), we get the arc flash boundary as a function of the arc current when
inverse-square trip function is used.
DB 


2.65 * 2 * K * 




 V

I
 arc

2


  Xs2




 Rs 




(A.6)

109


Summary


Engineers carrying out arc flash hazard assessment need to be aware of the various
uncertainties in the nature of arcs and other factors affecting the evaluation process.



The test result upon which IEEE equations are based is available through IEEE.
Statistical analysis based on this data can provide insight into the deviation of
possible outcomes from the estimated values.



Probability based deviations can provide likely ranges of arc current or incident
energy for a given confidence level. Using these deviations, adjustment can be made
for the IEEE estimates to obtain more reasonable estimates.



Uncertainties in arc gaps and trip time can cause the actual outcome to differ from the
estimate. The highest calculated cal/cm² should be considered to provide the worker
maximum safety.



For equations based on theoretical formulas such equation A.1, modification is
required if the trip time varies inversely with the arc current. If the arc current is not
close to 70% of the bolted fault current, then the condition of maximum power
transfer will not hold true.

28

IEEE Standards 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations, IEEE
Industry Applications Society, September 23, 2002. (SH95023)

29

A. P. Strom, “Long 60-Cycle Arcs in Air”, AIEE Transactions, Vol. 65, pages 113-117, March
1946.

30

Lawrence E. Fisher, "Resistance of Low-Voltage AC Arcs", IEEE Transactions on Industry and
General Applications, Vol. IGA-6, No. 6, November/December 1970.
31

NFPA 70E — May 2003 ROP, "Standard for Electrical Safety Requirements for Employee
Workplaces", 2003 Edition, page 57.
32

Ralph Lee, “The other Electrical Hazard: Electric Arc Blast Burns”, IEEE Transactions on
Industry Applications, Vol. IA-18, No. 3, May/June 1982.

110

Appendix B. Implementing The New Arc-flash Standards

B. Implementing The New Arc Flash Standards
The nature of explosive equipment failures and the rate of serious burn injuries in the electrical
industry have been studied for many years. Detailed investigation into the arc flash phenomena
by many researchers has led the NFPA to adopt arc flash guidelines in NFPA-70E (2000) for
work on or near energized electrical equipment. The 2002 National Electric Code also adopted
Arc flash Hazard labeling requirements. In September 2002, the IEEE-1584 “IEEE Guide For
Performing Arc Flash Hazard Calculations” was released, providing the detailed equations for
determining arc flash energies. Proposed NFPA-70E (2004) scheduled for release in January of
2004 enhances the original 70E guidelines and adopts IEEE-1584 as one of the methods for
determining arc flash energies.
Arc flash hazard studies require knowledge of both the electrical power system in a facility and
the systems electrical protection. Arc flash studies can be considered a continuation of the short
circuit and coordination aspects of a power system since the results for each are required to
assess flash hazards. The effort required to perform an arc hazard assessment is greatly reduced
with close integration between the short circuit, protective device coordination, and the arc flash
software.
This paper provides a guideline for performing an arc hazard assessment using power system
analysis software. All references and examples in this paper refer to EasyPower software. In
order to minimize space requirements, it will assume the user has the power system modeled in
EasyPower, and has performed a short circuit and protective device coordination study.

Step 1. Data Collection And System Modeling
The greatest single effort in performing an arc flash study is data collection. For a system with
up-to-date one-line diagrams, data collection can take from 25-40 percent of the study effort.
The main difference between an arc hazard assessment and other studies is that you may need to
model the system in more detail, increasing the data collection time and study effort. The results
given from the software printout requires engineering judgment based on physical equipment
design. If the equipment has the potential to be worked on while energized, it should be
assessed. This includes branch circuits in data centers, panels and switchboards being served by
smaller (<500 kVA) transformers at 480 volts. Panels and switchboards rated below 240 volts
can be ignored if the service transformer is less than 125 kVA. See Figure B.1.
In years past, it was common practice for some engineers to exclude cable impedances and
sometimes equipment resistance in the system model to ensure the highest possible short circuit
values when calculating withstand duties for equipment. This is not recommended for several
reasons. First, EasyPower can accurately model all equipment types in detail, so there are no
reasons for minimized models or built in safety factors. Secondly, conductor impedances and
X/R ratios should be modeled for all equipment in order to obtain realistic short circuit values.
When assessing arc flash hazards, higher short circuit currents may actually be non-conservative
as far as PPE level is concerned due to fast clearing times. Higher PPE can result at lower fault

111


levels because of the inverse characteristics of many protective devices. Depending on the
maximum fault level to provide the maximum PPE may result in decreased worker safety.
It should be noted that the study results will only be as good as the system model. Every effort
should be made to model the actual equipment as found in the field.

Figure B.1: Example showing bus excluded from arc flash assessment

Step 2. System Operating Modes
For plants with simple radial service from the utility, only one mode of operation typically exists
– normal. However, for larger plants, there may be multiple modes of operation. These may
include:


Multiple utility sources that are switched in or out.



Multiple generator sources that are operated in parallel or isolated depending on the
system configuration.



Emergency operating conditions. This may be with only small backup generators.



Maintenance conditions where short circuit currents are low and trip time high.



Parallel feeds to Switchgear or MCC’s.

112




Tie breakers which can be operated open or closed.



Large motors or process sections not in operation.

UTIL-2

TX-1
30 / 44.8 MVA
115 - 13.8 kV
9%

11
5

11
5

SWITCHGEAR-A

13
.8

kV

kV

TX-2
30 / 44.8 MVA
115 - 13.8 kV
9%

SWITCHGEAR-B

13
.8

UTIL-1

kV

kV

What is important is that each one of these conditions may change the level of short circuit
current, which in turn changes the clearing time of the protective devices. These changes can
have a significant impact on the arc flash hazard and the PPE requirements for each piece of
equipment.

Tie Breaker Closed
LOAD-A
10 MW
8 MVAR

Both Utility Sources On-Line

LOAD-B
12 MW
7 MVAR

Figure B.2: Closed tie breaker increases available short circuit current.

In Figure B.2, an example system with maximum available short circuit current is shown. Both
utility sources are online and the switchgear tiebreaker is closed.
In Figure B.3, an example system with minimum short circuit current is shown. Both utility
sources are online and the switchgear tiebreaker is open, reducing the available short circuit
current on each bus. The examples above consider a double-ended utility tie system, but the
application applies to either low voltage systems with tie breakers or to emergency generation
providing stand alone power or working in parallel with the normal system.

113

5

11

5

UTIL-2

11

UTIL-1

kV

kV


TX-1
30 / 44.8 MVA
115 - 13.8 kV
9%

kV
.8
13

SWITCHGEAR-B

OPEN

13

SWITCHGEAR-A

.8

kV

TX-2
30 / 44.8 MVA
115 - 13.8 kV
9%

Tie Breaker Open
Both Utility Sources On-Line

LOAD-A

LOAD-B

10 MW
8 MVAR

12 MW
7 MVAR

Figure B.3: Open tie breaker reduces available fault current.
3

4

5 6 7 8 9 10

2

3

4

5 6 7 8 9 100

2

3

4

5 6 7 8 9 1000

40

40

30

30

20

20

10
9
8
7
6
5

10
9
8
7
6
5

4

4

3

3

2

2

1
.9
.8
.7
.6
.5

1
.9
.8
.7
.6
.5

4000A

.4

TIME IN SECONDS

100
90
80
70
60
50

TIME IN SECONDS

100
90
80
70
60
50

.4

.3

.3

.2

.2

14000A
.1
.09

.1
.09
3

4

5 6 7 8 9 10

2

3

4

5 6 7 8 9 100

2

3

4

5 6 7 8 9 1000


Figure B.4 Extremely inverse relay TCC.

114


The time current curve (TCC) of Figure B.4 shows an extremely inverse relay characteristic with
the trip time increasing as the current decreases. Decreased short circuit current (opening a tie
breaker, removing generation, etc.) can cause longer trip times and may increase incident
energies and the resulting arc flash hazard.
In summary, arc flash assessment should include each operating mode for the power system to
insure correct incident energies are calculated for all system conditions.

Step 3. Working Distance and Threshold Boundaries
Before running the actual analysis portion of the study, the user should determine the parameters
on which the study will be based. These include; working distances, units of measurement,
threshold boundaries, and the calculation standard or the “equations” for the analysis.
Working Distance

The arc flash incident energy and associated protection requirements are based on potential burns
to the person’s chest or face, not the hands or arms. The degree of injury depends on the
percentage of the person’s skin that is burned and the critical nature of the burn. Obviously, the
head and chest areas are more critical to survival than fingers or arms.
Appropriate working distances for most operations can be estimated by placing your elbow at
your side and extending your hand to the equipment. A typical average for this distance is 18
inches. By extending the arm to the full out position, this can be increased to 24-28 inches for
most people, but out-stretched arms are not a typical working distance. See Figure B.5.

Figure B.5: Working distance

EasyPower provides up to five (5) commonly used working distances for each voltage level.
This allows the user to develop a safety program where distances can be modified for a specific
operation or maintenance function, allowing easy standardization of clothing levels and safety
benefits. For certain types of work practices like hot stick operation, or when the energized bus
is set back from the worker, greater distances may be used to correctly model the reduction in
incident energy potential.
115


Unit of Measure

Working distances and arc flash boundaries are calculated and displayed in various units of
measure including; inches, feet, mm, or meters. Select the appropriate unit that will be easily
recognized and adhered to by workers. Critical safety programs such as arc flash hazards should
not confuse workers with units of measure. Example: For US markets most workers are more
familiar with inches and feet than mm and meters. The opposite would be true for facilities in
Europe.

Figure B.6: Selecting various arc flash assessment options in EasyPower.
Arc Flash Boundary

The arc flash boundary is defined as the distance from the arc source where the onset of second
degree burns can occur. This is typically defined by medical researchers as 1.2 cal/cm2 or 5.0
Joules/cm2. Some research indicates that up to 1.5 cal/cm2 can be used for exposure less than 6
cycles (0.1 seconds).
EasyPower provides the user with options based on clearing times less than 0.1 seconds and
clearing times greater than 0.1 seconds. EasyPower automatically determines the operating time
from the system protection characteristics, or from user defined times.

The arc-flash boundary incident energy must be set at the minimum energy
level in which a second-degree burn could occur. Do not increase the level from
those shown in the dialog box. Reduced values may be used based on your
safety or insurance requirements.

116


Calculation Standard

EasyPower provides four calculation standards, NFPA-70 (2000), proposed NFPA-70E (2004),
IEEE-1584 and EasyPower’s enhanced version of IEEE-1584. EasyPower’s extensive research
has corrected some of the potential inconsistencies in the 1584 standard which may lead to nonconservative results. The enhanced version uses the tolerance bands described in Appendix A
rather than the single 85% of bolted fault current in IEEE-1584. We recommend that the
enhanced version of 1584 be used or NFPA-70E (2004) to ensure more realistic results.
Threshold Incident Energy

Incident energy is defined as the amount of energy impressed on a surface area at a specific
distance away from the source during an electrical arc event. It is sometimes called surface
energy density and is directly related to the distance from the arc. Incident energy is measured in
joules per centimeter squared (J/cm2) or in calories per centimeter squared (cal/cm2).
EasyPower provides a threshold incident energy level for different voltage ranges. If the
incident energy level of a particular device is above the threshold, the device will be highlighted
on the one-line as an immediate danger. (Detailed user reports are also provided.) See Figure
B.7.
Electrical workers and safety managers can use this threshold to immediately identify areas
where current personal protective equipment (PPE) standards will not provide the required safety
margins.
For this 480 volt system, the incident energy threshold was set at 4.0-cal/cm2 or a PPE of 1 as
defined in NFPA-70E (2004). All protective devices with let through energies above this value
are highlighted red indicating danger. Notice that for work on this switchgear, a minimum PPE
of 3 is required for all work except on the load side of breaker BL-3. Table B.1 from the
proposed NFPA-70E (2004) lists PPE requirements in relation to incident energy.

TX-2

BUS-4
BL-2
67.1" AFB
10.4 cal / cm² @ 18"
#3 @ 18"

BL-1
99.6" AFB
18.7 cal / cm² @ 18"
#3 @ 18"
BL-3
31.5" AFB
2.7 cal / cm² @ 18"
#1 @ 18"

BL-4
67.1" AFB
10.4 cal / cm² @ 18
#3 @ 18"

M-1

Figure B.7: Red breakers indicating incident energy exceeds user specified threshold value.
117


Table B.1: Proposed NFPA-70E 2004 PPE Requirements

Risk
Category

Min. Arc Rating
of PPE

PPE Requirements

Class #0

0-2 cal/cm2

Untreated cotton

Class #1

2

Flame Resistant (FR) shirt and FR pants

2

2-4 cal/cm

Class #2

4-8 cal/cm

Cotton underwear plus FR shirt and FR
pants

Class #3

8-25 cal/cm2

pants plus FR Coverall

Class #4

25-40 cal/cm2

pants plus multi-layer flash suit

Step 4. Protective Device Coordination Study
After the system model is built and the operating modes are determined, the following
procedures are used to determine arcing fault incident energies.
Determine bolted fault (short circuit) currents at each bus in the system.
a) Use calculated currents to perform a protective device coordination study and develop
system relay and direct acting trip (DAT) settings. Settings are typically determined by
plotting protective devices on time current curves (TCC’s).
b) Determine arcing fault currents at each bus in the system using IEEE-1584 or NFPA-70E
equations. Note that different equations or multipliers are used for voltages <1.0kV,
1.0kV<kV<15.0kV, open air, inside box, and various system parameters.
c) Apply arcing currents and breaker/relay trip times to each device to determine arc hazard
incident energies, arc flash boundaries, working distances, and PPE requirements.
The steps shown are required for performing the calculations with power analysis software as
well as by hand. Depending on the system size (number of buses) performing this procedure can
be extremely time consuming or nearly impossible without software tools. Only software based
tools that provide true, seamless integration of short circuit, protective device coordination and
arc flash hazard analysis can provide accurate information for better worker protection and
reduced productivity losses due to over specification of gear. EasyPower’s inherent oneline/analysis integration eliminates the separate steps required by other programs and integrates
the short circuit, protective device, and arc hazard functions, thereby greatly reducing the time
and effort to perform the analysis.
Protective Device Coordination Using EasyPower

Using EasyPower, the process will be broken down into two steps for clarification purposes.

118


a) System wide protective device coordination.
b) Arc flash calculations.
While this guide does not provide the details for performing a protective device coordination
study, it should be stressed that this study is the cornerstone to providing accurate arc flash
calculations. Accurate protective device clearing times are essential for providing correct
incident energy calculations and the resulting AF boundaries.
Accurate protective device clearing times are essential for providing correct incident
energy calculations and the resulting AF boundaries.

Whil
e arc
flash calculations can be performed using standard operating times/characteristics of breakers
and relays, this method does not ensure conservative results and may compromise safety.
Several examples showing this reasoning are provided below.

In Figure B.8, the substation secondary main breaker provides selective coordination using either
setting. However, the arc flash incident energy is increased from 11 cal/cm to 29 cal/cm for the
higher short time delay setting. This increases the PPE requirement from 3 to 4 significantly
increasing costs and the probability workers may try to bypass the higher PPE clothing
requirements. This scenario is common to plants where an accurate protective device
coordination study has never been performed, or when workers unfamiliar with protection
system requirements make changes to protective device settings.

119

.8

1

2

3

4

5 6 7 8 9 10

900
800
700
600
500

2

3

4

5 6 7 8 9 100

2

3

4

5 6 7 8 9 1000

3

4

5 6 7 89
900
800
700
600
500

0.161

400
300

400
300

TX-2
1000 / 1288 kVA
6%

200

20.009

100
90
80
70
60
50

TX-2
1 / 1.288 MVA
13.8 - 0.48 kV
6%

40

BL-1

30

BUS-4

20

100
90
80
70
60
50
40

26
.0
24

M-1
250HP
IND
Full Voltage

200

TIME IN SECONDS

2

TX-2
FLA

30
20

1.614

BL-3

10
9
8
7
6
5

10
9
8
7
6
5

M-1

4
3
2

1
.9
.8
.7
.6
.5

.3
.2

1
.9
.8
.7
.6
.5

Improper Setting

.4

~BL-5
20000A

.3

Proper Setting

BL-3
GE MVT-9
Sensor = 800
Plug = 800
Cur Set = 0.5 (400A)
LT Band = 1
Inst = 4 (3200A)

.1
.09
.08
.07
.06
.05

3
2

BL-1
GE MVT-Plus
Sensor = 1600
Plug = 1600
Cur Set = 0.8 (1280A)
LT Band = 1
STPU = 4 (5120A)
ST Delay = Min

.4

4

250 HP
IND

~BL-5
GE MVT-9
Sensor = 1600
Plug = 1600
Cur Set = 0.8 (1280A)
LT Band = 1
STPU = 4 (5120A)
ST Delay = Max

TIME IN SECONDS

.6

TX-2
1000 / 1288 kVA
INRUSH

.2

.1
.09
.08
.07
.06
.05

BL-1
20009A

.04

.04

.03

.03

.02

.02

BL-3
31391A
.6

.8

1

2

3

4

5 6 7 8 9 10

2

3

4

5 6 7 8 9 100

2

3

4

5 6 7 8 9 1000

2

3

4

5 6 7 89


Figure B.8: Typical example of improper trip delay setting.

In this next example, Figure B.9, the secondary main breaker is properly set except the I2t
function is left in. This raises the arc flash incident energy from 11.0 cal/cm2 to 16 cal/cm2. If
increased arcing impedance is modeled, reducing the arcing current to 80%, the incident energy
is raised to over 20 cal/cm2. This increase in energy can result in an increased cost of personal
protective equipment and ongoing worker productivity losses associated with the increased PPE
requirements.

120

.8

1

2

3

4

5 6 7 8 9 10

2

3

4

5 6 7 8 9 100

2

3

4

5 6 7 8 9 1000

2

3

4

TX-2
FLA

0.136

1000
900
800
700
600
500

.5 .6

400

5 6 7 8 9 10000
1000
900
800
700
600
500
400

300

300

200

200

TX-2
750 / 966 kVA
8%

11.264

M-1
250HP
IND
Full Voltage

40

100
90
80
70
60
50
40

BL-1

30

BUS-4

17
.2
79

100
90
80
70
60
50

TX-2
0.75 / 0.966 MVA
13.8 - 0.48 kV
8%

30

20

20

10
9
8
7
6
5

1.614

10
9
8
7
6
5

M-1

4

4

250 HP
IND

3

3

2

2

BL-1
GE MVT-Plus
Sensor = 1600
Plug = 1600
Cur Set = 0.8 (1280A)
LT Band = 1
STPU = 4 (5120A)
ST Delay = Min

1
.9
.8
.7
.6
.5
.4
.3

1
.9
.8
.7
.6
.5
.4
.3

BL-3
GE MVT-9
Sensor = 800
Plug = 800
Cur Set = 0.5 (400A)
LT Band = 1
Inst = 4 (3200A)

.2

.1
.09
.08
.07
.06
.05

TIME IN SECONDS

TIME IN SECONDS

BL-3

.2

TX-2
750 / 966 kVA
INRUSH

BL-1
11264A

.1
.09
.08
.07
.06
.05

.04

.04

.03

.03

BL-3
20026A

.02

.02

.01
.5 .6

.8

1

2

3

4

5 6 7 8 9 10

2

3

4

5 6 7 8 9 100

2

3

4

5 6 7 8 9 1000

2

3

4

.01
5 6 7 8 9 10000


Figure B.9: Typical example of short time I2T trip curve used in place of definite time delay
(flat)

In medium and high voltage systems, it is quite common to find relay settings that are set far
above proper protective boundaries. This is especially true when new systems have been added
to older systems, or when system studies have not been updated on a regular basis.

121

6 7 8 9 10

2

3

4

5 6 7 8 9 100

2

3

TX-3
FLA

4

5 6 7 8 9 1000

2

3

4

5 6 7 8 9 10000

2

3

4

OPTION

0.600

900
800
700
600
500
400

5 6 7 8 9 100000
900
800
700
600
500
400

300

300

TX-3
5 MVA
13.8 - 2.4 kV
6%

M-3
1000HP
IND
Full Voltage

10
9
8
7
6
5
4
3

40

REFINER

30
20

400/5

R-8
GE IAC 53
51/50
Very Inverse
CT Ratio = 1200/5
Tap = 6 (1440A)
Time Dial = 3.1

MF

10
9
8
7
6
5

M-3
1000 HP
IND

TX-3
5000 kVA
6%

4
3

2

1
.9
.8
.7
.6
.5

2

R-13
Multilin 469
51/50 SC
Standard Overload
CT Ratio = 400/5
Overload = 1.01 (253A)
Time Dial = 2
SC Pickup = 7.4 (2960A)

Old Induction Disk
Unit

TIME IN SECONDS

TIME IN SECONDS

20

51

1200/5

OPTION
Multilin SR745
51/50
Extremely Inverse
CT Ratio = 400/5
Tap = 6 (2400A)
Time Dial = 0.7
Instantaneous = 20 (8000A)
Delay = 0.113

75
3

30

18
.

40

200

100
90
80
70
60
50

14.590

100
90
80
70
60
50

1.302

200

1
.9
.8
.7
.6
.5

.4

.4

New Solid State with
Inst Delay

.3
.2

.3
.2

TX-3
5000 kVA
INRUSH

.1
.09
.08
.07
.06
.05

.1
.09
.08
.07
.06
.05

R-8
14669A

.04

.04

.03

.03

R-13
26417A

.02

6 7 8 9 10

2

3

4

5 6 7 8 9 100

2

3

4

5 6 7 8 9 1000

2

3

4

5 6 7 8 9 10000

.02

2

3

4

5 6 7 8 9 100000


Figure B.10: Relay trip time compared with solid state device.

Figure B.10 shows an older style induction disk relay providing protection to a 2400 volt MCC
line-up. This unit must be set above the motor protective relays for selective coordination but
low enough to provide proper protection. A standard instantaneous unit cannot be used without
tripping the entire lineup for a motor fault. The tap and time dial setting shown is a good
compromise and typical of many systems. The unit will clear a bus fault in approximately 0.5
seconds (30 cycles). The arc flash incident energy is over 30 cal/cm2 and requires a PPE of 4.
Using a new solid-state relay with delayed instantaneous setting for selective coordination, the
incident energy is lowered to 10 cal/cm2, greatly enhancing worker safety.

122


As can be seen, proper protective device settings can greatly enhance worker safety and system
reliability. Performing an arc flash assessment without first providing proper protection settings
can significantly impact the assessment.
Arc Flash Calculations Using EasyPower

In the previous sections, we have provided the basis for setting up the system model for proper
arc flash calculations. In this section we will provide the details for actually performing the arc
flash study and understanding the results, as well as, some tricks of the trade.
Arc flash calculations are performed in EasyPower’s ShortCircuit focus. EasyPower’s
SmartClick interface allows the users to simply double click any bus for instantaneous results, to
fault selective buses, or to “Fault All” buses.
In the example below (Figure B.11) select the ArcFlash button
Double click on Bus-4. The results appear on the one-line.

on the EasyPower toolbar.

TX-2

BUS-4
BL-2
67.1" AFB
10.4 cal / cm² @ 18"
#3 @ 18"

BL-1
99.6" AFB
18.7 cal / cm² @ 18"
#3 @ 18"
BL-3
31.5" AFB
2.7 cal / cm² @ 18"
#1 @ 18"

BL-4
67.1" AFB
10.4 cal / cm² @ 18
#3 @ 18"

M-1

Figure B.11: Observing arc flash results by double clicking on a bus.

Each protective device displays the required arc flash boundary, let through energy in cal/cm2,
and PPE requirement at a user specified working distance.
The values displayed on the one-line are based on the let through energy of the
protective device, i.e. the energy on the load side of the device, not the line side.

Note
: The values displayed are based on the let through energy of the protective device, i.e. the
energy on the load side of the device, not the line side. This important safety aspect must be
understood when applying arc flash results. When working on the line side of a protective
device, i.e. the incoming terminals, breaker stabs, or incoming bus work the incident energy on
the line side must be found from the let through energy of the upstream device, not the device
you are working on. For example, when working on the primary stabs of breaker BL-2, the
incident energy available to the worker is found from the first upstream device protecting BL-2.

123


This is the let through energy of the secondary main device BL-1, which is 18.7 cal/cm2. If the
worker is working on the load side stabs of BL-2, the let through energy is controlled by BL-2.
In EasyPower these results are associated with that breaker, in this case, 10.4 cal/cm2.
Figure B.12 below shows the same system, but with a primary fuse protecting the buswork from
the TX-2 secondary terminal through the primary or line side bus stabs of breaker BL-1. Work
in this area will require a PPE level 4 requirement and be subject to a let through energy of 30.8
cal/cm2.
139.9" AFB
30.8 cal / cm² @ 18"
#4 @ 18"
TX-2

BUS-4
BL-2
67.1" AFB
10.4 cal / cm² @ 18"
#3 @ 18"

BL-1
99.6" AFB
18.7 cal / cm² @ 18"
#3 @ 18"
BL-3
31.5" AFB
2.7 cal / cm² @ 18"
#1 @ 18"

BL-4
67.1" AFB
10.4 cal / cm² @ 18"
#3 @ 18"

M-1

Figure B.12: Primary side fuse protects line side of breaker BL-1 on BUS-4 but has higher
incident energy.

Whe
n
displ
ayin
g the
results graphically, EasyPower provides the user with a clear picture of line side and load side let
through energies, as well as, a visual indication of problem areas and correct clothing
compliance. This information can be posted in the electrical room providing workers with a
clear picture of the system and the hazards that may not be as easily apparent with just stick on
labels. With the click of a mouse you can change system parameters and compare different
operating scenarios. This provides valuable training information that helps engineers and
electricians understand how system changes impact arc flash hazard ratings.
When laying out your safety plan, keep in mind that you will always be working on
either the line side (upstream) or load side (downstream) of a protective device.

For most large studies, however, it is typically more efficient to display results in spreadsheet
form and print the “Arc flash” hazard warning labels for each device. To perform this operation,
simply go to Tools ShortCircuit Options  ArcFlash Tab, and check  ArcFlash
Spreadsheet in the “Create Report” section of the tab in the EasyPower menu. See Figure B.6.

124


Now instead of double clicking on the bus to initiate the fault, select Fault All from the toolbar,
and then from the menu choose Window  ArcFlash Hazard Report. A spreadsheet similar
to Figure B.13 below will tile in the foreground of the window.

Figure B.13: Arc flash results reported in spreadsheet

The EasyPower ArcFlash spreadsheet provides all the data used in the calculations to determine
AF Boundary, Incident energy, and PPE requirements for each protective device in the system.
This data can be applied directly to comply with NEC 2002 and NFPA-70E by simply clicking
on File Print Labels in the menu.
Before you print labels it is recommended that you refer to STEP-2, and review your modes of
operation. It is highly recommended that you save your different operating modes in
EasyPower’s Scenario Manager. This will allow you to refer to each case without affecting the
base case system as you make changes and fine-tune your arc flash assessment.

Summary
1) Run base case arc flash calculations.
2) Switch to different operating modes as defined in Scenario Manager.
3) Run arc flash calculations for each operating mode to determine highest arc hazard.
4) Compare the highest incident energies from the base case and scenarios. Take the case
with the highest values (there may be multiple cases for different parts of the system) and
modify the arcing current to reflect a high impedance arcing current. This will lower the
arcing current, which may cause longer trip times and result in higher incident energies.
See STEP-2, and Figure B.6. Note: A good starting place is 80% of the calculated arcing
current. Going much lower than this may result in current values that cannot be
realistically maintained.
5) Compare the incident energies of the case selected in task 4 above with the high
impedance values of the same case. Print labels.
The following labels can be printed on plastic stock through most laser printers, or via
commercially available label printers. EasyPower provides direct output to selected label printers
so you will avoid hours of data conversion routines.

125


Figure B.14: Warning label that can be printed directly from EasyPower ArcFlash
program.

Economic Benefits
The economic benefits of performing arc flash assessments using dedicated power system
analysis software becomes readily apparent when the alternative is to use a spreadsheet
calculator like those provided in IEEE-1584. Arc hazard assessments using a spreadsheet
calculator requires the following tasks:
1) Transfer data from the short circuit program to the spreadsheet calculator. This includes
short circuit calculations, bus names, and bus voltages.
2) Determine the arc gap for each calculation or equipment in the spreadsheet.
3) Determine the trip time for each device or bus in the spreadsheet. There are usually
multiple trip times required for each bus.
4) Run the calculation.
5) Apply NFPA-70 PPE requirements to each calculation.
6) Spreadsheet calculations DO NOT provide for a device-by-device analysis, unless the
users accounts for each device in the system.
7) Perform the calculation for a change in tie breaker status or generation (mode of
operation).
8) Take the highest results (case) and re-run using a higher impedance arcing fault to insure
accurate results.
126


As can be seen, the man hours required to perform an arc flash assessment can be cost
prohibitive using a spreadsheet calculator. When applied to large systems, such as those in the
petrochemical or the pulp and paper industries, it becomes almost impossible.
Another
consideration is the potential for errors when applying the hand calculations, trip time look-ups,
and spreadsheet work.
EasyPower’s complete integration of short circuit, protective device coordination, and arc flash
can be exponential as compared to the use of an IEEE-1584 spreadsheet calculator. EasyPower
simplifies the process, reduces human error and provides a basis from which system changes and
modifications can be modeled and the study results updated immediately without the extensive
work and risk of error associated with a spreadsheet. EasyPower also helps with safety program
requirements for accurate documentation as it provides reports that become a key part of a
corporate arc flash hazard safety program. The EasyPower ArcFlash program will also be kept
up-to-date with the latest industry standards, helping to ensure the most accurate results.

Conclusion
This guide presents the basic steps for performing an arc flash hazard assessment using power
analysis software. Users performing arc flash assessments should be aware that reduced short
circuit currents could increase arc incident energies for some cases. They should also fully
understand the arc let through energies as applied to protective devices, before assigning arc
flash boundaries and incident energy ratings to equipment.
Power analysis software that provides complete one-line/analysis integration eliminates the
separate steps required by other programs and integrates the short circuit, protective device, and
arc hazard functions. This greatly reduces the time and effort to perform the analysis.

127

Index

Index
“qualified” persons, 32
3-phase faults, 43
all possible connections, 42
American Society for Testing and
Materials (ASTM), 79
arc blast pressure, 37
arc current, 29, 45, 97
arc flash analysis, 25
arc flash boundary, 118
Arc flash hazard, 1
arc flash hazard assessment, 39
arc flash hazard program, 15
arc flash hazard report, 127
arc gap, 128
arc resistance, 50
Arc Thermal Performance Value (APTV),
80
arc-flash hazard program, 12
arcing fault incident energies, 120
arcing faults
hazards of, 3
arcing time, 30, 50, 106
reducing, 69
assessing existing safety program, 17
available fault current, 8
available short circuit current, 115
average melting time, 51
awareness, 87
base case, 127
blast shrapnel, 3
bolted fault current, 29, 44, 120
boundaries, 7
box configuration, 29
branch current contribution, 49
breaker response time, 69
burn injury costs, 4
bus differential protection, 70
cable, 31
calculated arc current, 47
calculation factor, 30
calculation standard, 117, 119
care of FR clothing, 81
circuit breaker clearing time, 51
clearing time, 115, 118
clothing compliance, 126
comfort, 82
, 50

commercial integrated software, 10, 24
contributing branch currents, 45
coordination study, 120
corporate arc flash hazard safety program, 129
cost estimates, 28
current limiting fuses, 68
current limiting reactors, 69
data centers, 113
data collection, 113
direct acting trip (DAT), 120
distance exponent, 30
documentation, 12, 59, 91, 129
EasyPower, 121, 125
economic benefits, 128
electric arc, 1
emergency operating conditions, 44, 115
enhanced version of IEEE-1584, 119
equipment data, 39, 40
existing practices review, 16
exposed live parts, 32
fabric material, 82
fault buses
all, 125
selective, 125
fault duration, 3
fire resistant (FR), 7
fit, 82
flame resistant, 80
flame resistant (FR), 77
flash protection boundary, 7, 31, 58
fuse size, 73
gap between conductors, 30, 39, 47
gloves, 84
grounding type, 39
Hand calculations, 24
hand-calculations, 10
hazard assessment, 12
hazard risk category classifications, 10
Hazard Risk Category Classifications, 34
Hazard/risk category, 57
headgear, 84
hearing loss, 4
high voltage systems, 123
hot sticks, 85
human resources, 19
identification of hazard, 40
IEEE 1584, 29, 97
IEEE 1584-2002, 6
IEEE-1584, 45, 113, 119, 128
impedances

129


cable, 113
conductor, 114
incident energy, 7, 30, 54, 119
threshold, 119
induction disk relay, 124
inside box, 120
instantaneous trip, 50
kurtosis, 98
label printers, 128
labeling, 113
layers, 82
let through energy, 125
limited approach boundary, 7, 32
limits for arc currents, 48
line side, 125
line-to-ground faults, 43
load side, 125
MCC, 31
MCCs, 3
medium voltage systems, 123
minimum melting time, 51
modes of operation
multiple, 114
molten metal, 3
Multi-layered, 83
multiple utility sources, 44, 114
name-plate, 40
National Electric Code, 113
NEC, 5
NFPA, 6, 77
NFPA 70E, 23, 31, 39
NFPA 70E Tables, 34
NFPA-70 (2000), 119
NFPA-70E (2003), 113, 119
non-conservative, 114
normal distribution, 98
normalized incident energy, 30
Objectives, 21
one-line diagram, 43
one-line diagrams, 25, 113
open air, 31, 120
open configuration, 29
operating scenarios, 126
optical sensor, 73
OSHA, 6, 77
overprotection, 26
panels, 31, 113
parallel feeds, 42
percentage impedance, 35

130

personal protective equipment (PPE), 8, 77, 92,
119
PPE requirements, 119
increased, 123
preventative maintenance, 65
probability of survival, 4
prohibited approach boundary, 7
protective device characteristics, 39, 50
protective device coordination, 25, 74
protective device coordination study, 120
protective devices, 45
radial service, 114
Ralph Lee, 109
random nature of arcs, 97
random variation, 45
random variations, 97
range of arc current, 45
remote operation, 74
resistance
equipment, 113
resource persons, 22
restricted approach boundary, 7, 32
risk category, 8
safety audit, 13, 89
safety meetings, 91
Scenario Manager, 127
second degree burn, 31
threshold, 32
second degree burns, 118
second-degree burn, 31
selective coordination, 124
short circuit, 114
short circuit analysis, 24, 39
short circuit study, 43
ShortCircuit, 125
skewness, 98
SmartClick, 125
solid-state relay, 124
spreadsheet calculator, 10, 24, 128
spreadsheet form, 127
standard deviation, 98
switchboards, 113
switchgear, 31
system reliability, 125
system studies, 123
system voltage, 29
temporary instantaneous setting, 70
temporary relay settings, 74
theoretical maximum arc power, 32
Thevenin equivalent, 109
three-phase faults, 29
threshold boundaries, 117
threshold incident energy, 119

Index

tie breakers, 44, 115
time current curve (TCC), 117
time current curves (TCC’s), 120
time-current characteristics, 50
tolerance, 51
total arc rating, 83
total clearing time, 51
training, 13, 87
transformers, 45, 113
type of enclosure, 39
Typical PPE, 8

upstream protective device, 8, 106
useful life of PPE, 81

units of measurement, 117
upstream device, 126

X/R ratios, 114

visual indication, 126
warning labels, 127
warning signs, 89
worker productivity losses, 123
worker safety, 125
working distance, 30, 39
working distances, 117

131

Practical solutions guide to arc flash hazards

More Related Content

Similar to Practical solutions guide to arc flash hazards (20)

More from Deal Point (20)

Recently uploaded (20)

Practical solutions guide to arc flash hazards