Introduction to Arc Flash/Blast Electrical Hazards

Editor's Notes

• #20: NIOSH Suggested Discussion Questions: If one of the incidents in the video had happened to you, how would it have changed your life or the life of your family? Have you ever experienced an arc flash? What happened? Did you change any safety practices in your work after the incident? What were they? What are some behaviors that you can change to help prevent an arc flash incident to yourself and/or to your fellow electricians? What PPE is recommended for your task? What are some of the excuses electricians use for not wearing appropriate PPE? In, general, as electricians become more experienced, do you think their safety habits change? How? After viewing this video, which, if any, of your safety work habits would you like to change? Do you have any suggestions for your supervisor that you feel could help prevent an arc flash incident from happening to you or any of your co-workers? What suggestions do you have for your company to help prevent arc flash incidents? Arc Flash Awareness: DHHS(NIOSH) Publication No. 2007-116D Ordering Information: Copies of NIOSH documents are available from: NIOSH-Publications Dissemination 4676 Columbia Parkway Cincinnati, OH 45226-1998 Fax: 513-533-8573, Phone: 1-800-35-NIOSH Email: pubstaft@cdc.gov, website: www.cdc.gov/niosh
• #31: The NEC warning label should remind a qualified worker who intends to open the equipment for analysis or work that a serious hazard exists and that the worker should follow appropriate work practices and wear appropriate PPE for the specific hazard (a non qualified worker must not be opening the equipment).
• #32: The NEC 2002, section 110.16 requirement for labeling does not call for any quantification of the incident energy, however, some companies are choosing to provide more information on the arc flash label as shown above. The manufacturer of the equipment is normally not aware of the site specific configuration of the power system at the point where the equipment will be connected or aware of the site specific tasks that will be evaluated. The installer or the owner of the equipment must label the equipment.
• #35: Arc-flash protection is always based on the incident energy level on the person’s face and body at the working distance, not the incident energy on the hands or arms. The degree of injury in a burn depends on the percentage of a person’s skin that is burned. The head and body are a large percentage of total skin surface area and injury to these areas is much more life threatening than burns on the extremities.
• #38: Data required for this study is similar to data collected for typical short-circuit and protective device coordination studies, however it also requires information on low-voltage distribution and control equipment plus its feeders and large branch circuits. This information is used to determine the time of arc duration from the protective device time current curves.
• #39: Find the symmetrical root-mean-square (RMS) bolted fault current and X/R ratio at each point of concern for the 1.5 to 4 cycle case. This includes all locations where people could be working. It is important to include all cables. Conservatively high fault levels do not necessarily increase safety, they may reduce it. Don’t add a safety factor. Higher current can give shorter arc fault duration and the wrong energy value. Lower fault currents often persist longer than higher currents because of the characteristic shape of most protective-device time-current curves. Common causes of bolted faults: 1. Re-energizing without removing temporary grounding. 2. Incorrect connection of a parallel run of cables to a motor terminal box. Instead of connecting a-a, b-b, c-c; connections are a-b, a-b, c-c.
• #40: The arc fault current for each location where an arc flash hazard exists and the portion of that current that flows through the closest upstream protective device that will clear this fault must be determined. The arc fault current depends primarily on the bolted fault current. The bolted fault current in the protective device can be found from the short-circuit study by looking at the detailed bus fault printout or looking at the fault levels that are “one-bus-away”. It is important to separate fault contributions from all other sources that will not flow through the protective device that will clear the fault. This includes alternate feeder and downstream motor fault contributions. The arc fault currents can then be calculated. The calculated arc fault current will be lower than the bolted fault current due to arc impedance, especially for applications less than 1000 V.
• #42: There are two types of protective devices in power distribution systems. These are fuses or circuit breakers. For fuses, the manufacturer’s time-current curves may include both melting and clearing time. If so, use the clearing time. If they show only the average melt time, add to that time 15% for times up to 0.03 seconds, and 10% above 0.03 seconds to determine total clearing time. For Current Limiting Fuses (CLF), use the maximum clearing time from the fuse time current curve if fault current is not in the current limiting range. The current limit point is at the 0.01 sec point on the fuse curve. For one to two times the limit point in amps, use 0.008 seconds (1/2 cycle). For over two times the limit point, use 0.004 seconds (1/4 cycle). For circuit breakers with integral trip units, the manufacturer’s time-current curves include both the tripping time and the clearing time. The total clearing time as shown by the top band of the curve is the time value that should be used. For relay operated circuit breakers, the relay curves show only the relay operating time in the time-delay region. For relays operating in their instantaneous region, allow 16 milliseconds on 60 Hz systems for operation. The circuit breaker opening time must be added to the relay time to give a total clearing time.
• #43: Equation 1 provides smaller values of the flash boundary because actual transformer ratings and power system impedances are used. Equation must not be applied without an accurate up-to-date short circuit analysis at the point of exposure. For transformers rated below .75 MVA, multiply transformer MVA by 1.25.
• #44: For each bus, document the system voltage and the class of equipment. For example, record whether the equipment is metal-enclosed switchgear or metal-clad switchgear. IEEE 1584, Table - Classes of equipment and typical bus gaps Arc-flash protection is always based on the incident energy level on the person’s face and body at the working distance, not the incident energy on the hands or arms. The degree of injury in a burn depends on the percentage of a person’s skin that is burned. The head and body are a large percentage of total skin surface area and injury to these areas is much more life threatening than burns on the extremities.
• #45: The equations in IEEE 1584-2002 are available in spreadsheet format. These equations have been incorporated into the packages of power system software vendors.
• #47: lg is the log10 Ia is arcing current in kA K is -0.153 for open configurations, and is -0.097 for box configurations Ibf is bolted fault current for 3-ph symmetrical RMS in kA V is system voltage in kV G is the gap between conductors in mm
• #48: NFPA equations for “Open Air” and “Arc in a Box” are valid for 600 V or below.
• #49: NFPA equations for “Open Air” and “Arc in a Box” are valid for 600 V or below.
• #64: A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
• #65: A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
• #66: A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
• #67: A simplified two-category approach is found in NFPA 70E-2000, Table F-1 of Part II, Appendix F NFPA. This table assures adequate PPE for electrical workers within facilities with large and diverse electrical systems. The clothing listed in Table F-1 fulfills the minimum FR clothing requirements of NFPA 70E-2000, Tables 3-3.9.1 and 3-3.9.2 NFPA and should be used with the other PPE appropriate for the Hazard/ Risk Category that is found in of NFPA 70E-2000, Table 3-3.9.2 NFPA.
• #74: 1910.333(a)(1) "Deenergized parts." Live parts to which an employee may be exposed shall be deenergized before the employee works on or near them, unless the employer can demonstrate that deenergizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. Live parts that operate at less than 50 volts to ground need not be deenergized if there will be no increased exposure to electrical burns or to explosion due to electric arcs. Note 1: Examples of increased or additional hazards include interruption of life support equipment, deactivation of emergency alarm systems, shutdown of hazardous location ventilation equipment, or removal of illumination for an area. Note 2: Examples of work that may be performed on or near energized circuit parts because of infeasibility due to equipment design or operational limitations include testing of electric circuits that can only be performed with the circuit energized and work on circuits that form an integral part of a continuous industrial process in a chemical plant that would otherwise need to be completely shut down in order to permit work on one circuit or piece of equipment.