Why you should measure shield currents in PROFIBUS and PROFINET networks - Peter Thomas

Peter Thomas – Control Specialists Ltd
Screen currents and why you
should measure them.

2PROFIBUS UK 2PROFIBUS UK – March 2017 © 2017
Traditional Fieldbus and Network Analysis
• Tools for the analysis of PROFIBUS and PROFINET networks have been
around for many years.
• They tend to concentrate on the logical (telegrams, events etc.) and, in
the case of PROFIBUS, the physical (electrical waveforms) analysis of
the networks.
• It has recently become apparent that situations can arise where
traditional network analysis tools show the networks to be operating
within specification yet the networks unexplainably continue to
intermittently fail with no obvious reason.
• Permanent network monitors can often help identify the cause but there
are occasions where this is not the case.
• Recent work involving the measuring of currents flowing in the cable
screens has shed new light on the situation.

Typical causes of Network Failure
EMC
35.5%
21.3%
14.9%
13.5%
11.3%
3.5%
Source – Vortex Report 2016 – Indu-Sol GmbH
Software &
Device related
Cable-related
Other
Excessive Cable length
Connectors

Screen Grounding - One end or Both ends?
• Confusion remains as to whether the screen of PROFIBUS and PROFINET
cables should be grounded at one end only or at both ends.
• Electromagnetic Interference takes two forms, electrostatic and
electromagnetic.
• Electrostatic interference is voltage-related and associated with
proximity to high voltage cables.
• Adequate cable separation and the grounding the screen at one end
protects PROFIBUS (and PROFINET) cables against electrostatic
interference.
PROFIBUS (or PROFINET) Cable
Power Cable

Screen Grounding - One end or Both ends?
• Electromagnetic interference is associated with proximity to current-
carrying cables which generate a magnetic field.
• Any data cables within this field are susceptible to interference.
• To protect against electromagnetic interference, the screen of the
PROFIBUS (or PROFINET) cable must be grounded at both ends to a low
impedance earth “Ze”
• This induces current in the screen with generates an opposing magnetic
field – this is known as “active” shielding.
Ze

Screen Grounding – Unbalanced Systems
• The idea that grounding a screen at both ends causes earth loops is
really only an issue on unbalanced signal transmission, i.e. where one of
the wires acts as a 0v reference.
• To prevent earth loops forming, unbalanced systems like this should only
have their screens grounded at one end.
-
+
-
+

Screen Grounding – Balanced Systems
• PROFIBUS and PROFINET networks used balanced transmission, i.e.
both wires carry data and neither is referenced to ground.
• Balanced systems like this can have the screens connected at both ends
without adversely affecting the signals.
B
A
B
A
Small earth currents have no effect

Screen Currents – Good Currents
• Screen currents can be categorised as “good” currents and “bad”
currents.
• Good currents are those currents that flow in a screen to generate an
opposing magnetic field to any localised electromagnetic interference.
• Spatially separating the cables will limit the amount of screen current
that needs to flow in the screen.
Image courtesy of Indu-Sol GmbHGood Screen Currents

Bad Screen Currents
• Bad currents are currents flowing because the grounding of the
PROFIBUS (or PROFINET) screens at both ends have led to the
unintentional participation of these screens in the equipotential bonding
system.
• It was originally assumed that running an equipotential bonding cable as
close as possible to the PROFIBUS (or PROFINET) cable would limit the
screen current by providing an alternative path for the bonding current
to flow
• Whilst this is valid for 50Hz systems, the use of high frequency
switching devices such as variable speed drives makes this technique out
of date.
PROFIBUS (or PROFINET) cable
Equipotential Bonding Cable
X

What’s the issue?
“There is widespread lack of awareness and attention with
regard to another major cause of excessive basic loads on
existing bonding systems (e.g. cable shielding), namely,
incorrect or insufficiently dimensioned return current paths”.
Source - Indu-Sol GmbH

A common source of high frequency
bonding currents
Ever increasing switching speeds and the accompanying higher frequencies
have resulted in a hugely growing impact of stray capacities and the
generation of higher-frequency vagrant currents (for example, in the line
leading from a phase to the PE conductor). Particularly in case of
asymmetric motor cables, this results in an induction on the PE (Ipe)
within the cables. This can be as much as 5% of the phase current!
Image courtesy of Indu-Sol GmbH

bonding currents
The four channels of the EMC
INspektor were connected as shown.

bonding currents

14PROFIBUS UK 14PROFIBUS UK – March 2017
Equipotential Bonding Recommendations
• Whilst safety will always be paramount, the fact remains that many
earth systems are far from ideal from an EMI point of view.
• The Requirements for Electrical Installations (BS7671) state that a
potential of <50v ac can exist between exposed metal parts in a
potential equalisation system.
• However several IT and telecommunication standards stipulate a much
lower value of <1v ac, with others also stating that the impedance of
the earth should be low at the “frequencies of concern”.
© 2017

Star Earthing (Type A)
• Whilst star earthing (Type A) is adequate for
safety purposes, it is considered poor for
Electro Magnetic Compatibility (EMC) purposes.
• For example, if we were to connect a network
cable between equipment on different limbs of
the star, the cable will become part of the
bonding network increasing the possibility of
excessive screen currents flowing.
• The practice of creating separate “clean” and
“dirty” earths is also considered an out of date
practice and impossible to achieve.

Meshed BN (Type-D)
EN 50310 specifically recommends the use of a
Type D - MESH-BN which requires that all metallic
parts within a building be bonded together to
provide an electrically continuous earthing network
with low impedance and shall include:-
• Cabinets, frames and racks.
• Conductive pathway systems.
• Cable screens
• Bonding mats.
This shall be achieved by a combination of
• Additional bonding conductors.
• Improvement of finishing and fastening methods
for existing bonding conductors.

Questions to be answered
1. How much current should be allowed to flow in the screens of
PROFIBUS (or PROFINET) cables?
2. How low does the earth loop impedance need to be to allow active
shielding to work effectively at typical automation frequencies?
3. What about currents / impedances associated with bonding cables
and motor to invertor cables?
4. What tools are available to measure these?
5. How can I improve an existing equipotential system to provide an
improved bonding system from an EMC point of view?

Recommended Reference Values
Source: Indu-Sol GmbH

Recommended Tools
The leakage current clamp EMCheck LSMZ I is
capable of measuring low (50/60 Hz) and high
(5KHz) frequency currents (mA and A) in the
functional earth circuit and the shields of network
cables.
The mesh resistance measuring clamp EMCheck®
MWMZ II is suited for providing evidence of a low
impedance potential equalisation according to EN
50310.

Optimum improvements to equipotential
bonding systems
Conventional equipotential bonding

Optimum improvements to equipotential
bonding systems - EmClots
EmClots terminals
are fastened by
means of an M 6x9
screw
connection to e.g.
trays or other
conductive system
components.
The terminals have a
female thread for
easy installation.
Sizes up to 16 mm²
have a continuous
female thread.

Partial improvement to equipotential
bonding systems – FE Cable
• Improvement 1 - In the event of heavy
exposure to electromagnetic fields, FE
cable acts like an additional shield. It
relieves the actual shield of excessive
shield currents, and of their negative
impact on signal quality and on the
functional safety of devices.
• Improvement 2 - Conventional bonding
systems (BN) generally have a star-
shaped structure (type A) and are
therefore unsuitable for the purpose
of functional bonding (FE). The
shielding relief conductor has the
function of improving the bonding
system (improved type A).

Additional Information
EN50310-2016EMC Optimum
Equipotential
Bonding – Indu-Sol
Recommendations for the
grounding & shielding of PROFIBUS
and PROFINET networks - PI
Coming Soon !

Thankyou
Peter Thomas
Chairman of PI Training Centres
www.linkedin.com/in/petermthomas
Control Specialists Ltd
www.controlspecialists.co.uk
Authorised solutions partner of Indu-Sol GmbH
http://www.controlspecialists.co.uk/downloads_pdf/4666_Indu_Sol_EMC_brochure_2017.pdf

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