11. Screen currents and why you should measure them - Peter Thomas

Peter Thomas – Control Specialists Ltd
Chairman PROFIBUS & PROFINET International Training Centre’s
Screen currents and why you
should measure them.

2PROFIBUS UK 2PROFIBUS UK – September 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 up to 150Hz, the use of high frequency switching
devices such as variable speed drives requires a different approach.
PROFIBUS (or PROFINET) cable
Equipotential Bonding Cable
X

What’s the issue?
EMI is rapidly increasing in modern industry due to the increasing
use of high-efficiency switching power conversion in DC power
supplies, motor drives, HVAC and lighting.
Unless the installation caters for this reality, there is a real
possibility of intermittent network outages and production loss.

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.

13PROFIBUS UK 13PI UK – September 2017 © 2017
bonding currents

14PROFIBUS UK 14PROFIBUS UK – September 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?

Indu-Sol Recommended Reference Values
Source: Indu-Sol GmbH

Tools
The leakage current clamp EMCheck LSMZ I is
capable of measuring 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 Functional
Earthing & Shielding of PROFIBUS
and PROFINET - PI
Coming Soon !

24PROFIBUS UK 24Editable name of event © 2017
EMI Training Course – November 2017
A one-day training course providing all the
necessary information on well-proven practical
techniques for reducing EMI in PROFIBUS and
PROFINET systems, now and in the future.
Wednesday 22nd November 2017
Endress + Hauser (Manchester)

Thankyou
Peter Thomas
Chairman of PI Training Centres
www.linkedin.com/in/petermthomas
Control Specialists Ltd
www.controlspecialists.co.uk
https://controlspecialists.co.uk/wp-content/uploads/2017/08/16012_Indu_Sol_EMC_brochure_2017.pdf

11. Screen currents and why you should measure them - Peter Thomas

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11. Screen currents and why you should measure them - Peter Thomas