Case Study – PROFIBUS Network, Waveforms, Network screen currents, EMC measurement and Network screen currents Control Specialists Ltd. were asked to provide site support on a 6-year old PROFIBUS installation. The installation consisted of several separate PROFIBUS networks all of which had been suffering from intermittent network failures since the network had been commissioned. The engineering staff of the client already had a portable PROFIBUS analyser and a permanent monitor but had been unable to identify the cause of the problems.

1. PROFIBUS / PROFINET
shield currents
- and why
you should measure them
Peter Thomas
Control Specialists Ltd
Chairman of the
PROFIBUS & PROFINET International (PI)
Training Centres

2. Case Study – UK PROFIBUS Support Visit
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
Control Specialists Ltd were asked to provide site support on a 6-year old
PROFIBUS installation.
The installation consisted of several separate PROFIBUS networks all of which
had been suffering from intermittent network failures since the network had been
commissioned.
The engineering staff of the client already had a portable PROFIBUS analyser and
a permanent monitor but had been unable to identify the cause of the problems.

3. Case Study – PROFIBUS Network
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
• The network can be seen to consist of a main PROFIBUS backbone, connecting
the PLC to 5 Hubs, each of which have 5 independent segments going to switch
room and plant-based PROFIBUS slaves.
• The network was running at 1.5MBits/second and was correctly terminated. The
engineering staff of the client had estimated that the length of the main
backbone was around 160m. The permanent monitor had been connected to the
monitoring port of HUB 04.

4. Case Study – PROFIBUS Waveforms
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
In July 2015, this network was particularly troublesome and resulted in the engineering
staff having to spend more time than usual on the network.
Control Specialists were contacted for assistance, initially being asked to comment
upon the data derived from the permanent monitor. The image above shows several
oscilloscope traces for nodes downstream of the Hubs at the time of a fault.

5. Case Study – PROFIBUS Waveforms
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
This was particularly interesting because the waveforms were from the permanent
monitor on the backbone which, by definition, is showing a “cleaned-up” version of
the waveforms downstream of the hubs.
This suggested that the fault lay on the backbone as opposed to anything
downstream of the hubs.

6. Case Study – PROFIBUS Waveforms
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
When a portable analyser was eventually connected, the reported fault had cleared
but came back intermittently with no warning.
Following on from a presentation given by Indu-Sol GmbH about the effect of
potential equalisation currents in PROFIBUS networks, it was decided to measure
the currents flowing in the screens of the PROFIBUS cables.

7. Case Study – Network screen currents
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
If the currents flowing through the screens of cables are >> 40mA, they may
indicate installation-related problems.
The current in the screen will be due to one of two main reasons:-
1. Compensating currents due to inductive coupling.
2. Potential Equalisation currents.
Compensating currents are “good” currents and can be reduced below 40mA by
existing cable segregation techniques. Potential Equalisation currents are “bad”
currents and should be investigated further.
Investigation in this particular instance indicated that the likelihood was the fact
that the cable between Hub 3 and Hub 4 came within 0.3m of a large power
cable for part of its length. However, further investigation on the site showed
evidence of several instances of bad practice from an EMC point of view so a
second visit was arranged to look at currents flowing in the PE cables.

8. Shielding (Network Cables)
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
• Despite the recommendation of connecting cable shields (screens) to earth
at both ends, it is still common practice to follow the old rule of earthing at
one end only.
• Earthing the screen at one end only on a balanced system like PROFIBUS
or PROFINET only protects against electrostatic (capacitive) interference.
• In order to protect against electromagnetic (inductive) interference, a
counter-balancing current must flow in the screen.
• But the earth loop must be a low impedance.
• Specific limits for screen currents and loop impedances will be required. We
currently use >0mA and <=40mA with an impedance of <0.6 @ 2KHz

9. Case Study – EMC measurement tools
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
Control Specialists Ltd wondered if galvanic EMC-related issues might be the
cause of the problems.
To assist in determining this, an LSMZ I clamp meter and an MWMZ II
impedance clamp meter were used. The latter injects a 2.05 KHz signal into the
screen and calculates the impedance at that frequency.
LSMZ I Current Clamp MWMZ II Impedance Clamp

10. Case Study – Measurements
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
The diagram below shows the measurements taken for each cable on the
backbone using the LSMZ I and MWMZ II clamps. The following limits were
applied:-
• Screen Currents: > 0mA and <=40mA
• Screen Impedance: < 0.6 @ 2KHz
It was interesting to note that the large currents (> 40mA) flowing between hub 3
and 4 were also where a significant number of faults had occurred.
Note: PLC – EARTH = PROFIBUS cable between PLC and floor-mounted earthing bar.

11. Case Study – Network screen currents
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
Investigation in this particular instance indicated that the likelihood was the fact
that the cable between Hub 3 and Hub 4 came within 0.3m of a large power
cable for part of its length.
However, further investigation on the site showed evidence of several instances
of bad practice from an EMC point of view so a second visit was arranged to
look at currents flowing in the PE cables.

12. EMC & Equipotential Bonding – The current situation
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
• The increasing use of high-frequency switching devices (e.g.
invertors) increases the likelihood of industrial networks like
PROFIBUS and PROFINET being affected by EMC-related issues.
• These EMC issues will not always be airborne, many will be
galvanic as a result of inadequate equipotential bonding.
• Conventional bus analysers are of little use in identifying galvanic-
driven EMC issues.
• The lack of awareness of EMC on the factory floor is staggering,
with many competent engineers quoting and following outdated
practices.
• There is no obvious single source of guidance regarding EMC.
• There is a real need for the promotion of an increased awareness
of EMC within industry.

13. International / European Guidelines
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
IEC 61000-5-2 1997
covers guidelines for the
earthing and cabling of
electrical and electronic
systems and installations
aimed at ensuring
electromagnetic
compatibility among
electrical and electronic
apparatus or systems.
EN 50310 (VDE
0800-2-310) specifies
requirements and
recommendations for
connections (bonds)
to earthing networks
in buildings in which
information
technology (IT)
equipment is intended
to be installed.
EN 60204-1 2006
provides requirements
and recommendations
relating to the electrical
equipment of machines
so as to promote the
safety of persons and
property, consistency of
control response and
ease of maintenance.

14. Supply Distribution Systems
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))

15. Supply Distribution Systems (TN-C-S)
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))

16. Distribution Systems (EN 50310)
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))

17. Equipotential Bonding
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
EN 50310 - Multiple bonds, including those
using building structures, should be used
rather than a single bond since this reduces
the impedance (inductance) of the resulting
bond.

18. Type A Earth Bonding Network – Star
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
A recommended, but not ideal, improvement can be
made used bonding conductors as shown.
Note - It is important to ensure that the screens of
network cables are not inadvertently used as
bonding conductors. This can be easier said than
done.
A conventional Type A Star network whilst
considered good for electrical safety is poor for
EMC at all frequencies.

19. Type D Earth Bonding Network – MESH BN
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
IEC 61000-5-2 and EN 50310 specifically recommend
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.
The lower diagram shows multiple MESH-BN’s connected
together to create a Common Bonded Network (CBN).
Here, all Protective and Functional earthing is combined
(CPFE).
Note – specific recommendations will be required for
improving existing installations.

20. Power Supply Earthing
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
Sections 6 and 9.4.3.1 of IEC
60204-1 address the earthing of
Protective Extra-Low Voltage
(PELV) supplies (ac and dc). Here
the need to earth at the point of
supply is stipulated.
Consumers
Multiple earthing of dc supply
lines can make the automation
devices connected to them
susceptible to unintended
consequences.

21. Case Study – Other EMC measurements
The LSMZI and MWMZII were also used to measure the
currents and calculate the impedances of the following:-
• Potential equalisation installation.
• Variable Speed Drive cable screens
Overall a structured, and documented, approach had to
be taken so that a clearer picture of the issues could be
obtained.
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))

22. Case Study – Variable Speed Drives
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
The four channels of the
EMC INspektor were
connected as shown.

23. Variable Speed Drives
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
The UK trade associations GAMBICA
and REMA have produced a technical
guide to meet a demand for an
authoritative guide on best practice for
the installation of Power Drive Systems.
It is the result of a study that takes note
of well-established fundamental theory,
technical papers, and the results of
specific investigations.

24. Motor Cables (GAMBICA/REMA)
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
• It is strongly recommended that the motor cable be screened. This is
essential in order to meet many of the relevant EMC standards, and is
recommended in any case to reduce the risk of interference to other equipment.
• The screen will have to carry relatively high amplitude current pulses, therefore
its RF impedance and connection method at each end are critical.
• Effective forms of screen are: Copper braid, Copper tape, Close wound
galvanised steel wire armour, Aluminium tape armour, Steel braid and solid
copper cladding
• To be effective, the screen must have good electrical continuity along the
cable length.
• The braided or helical construction must be such that current can easily pass
from turn to turn along the length.
• The screen coverage must be as close as possible to 100%, and the
resistance must be low, especially at high frequencies.

25. Variable Speed Drives (Motor Cable)
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
The PE of the cable between the drive and motor can carry surprisingly
high currents.
This can be significantly reduced
using symmetric cables.

26. Case Study – Measurements
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))

27. Variable Speed Drives
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
It would be timely to wake up industry to this problem, and start to deal
with it, because the next generations of VSDs will be smaller and less
costly, and run cooler, because of the use of newly-developed types of
power switching devices which can operate at 10 to 100 times faster
than current devices.
These will cause truly horrendous problems if the present incorrect
installation techniques are allowed to continue.
UK EMC Expert and Author of books on EMC

28. PI Recommendations
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
Coming Soon !
New Guidelines

29. Conclusions
• EMC and its effects on control systems are nothing new. However
misunderstandings and the lack of awareness on all sides make it
essential that PI take a leading role in promoting awareness and
good practice in this respect.
• Industrial Network Engineers clearly need to know more than just
configuration, protocols and network-based fault-finding
techniques. They also need a knowledge of EMC, equipotential
bonding techniques, low voltage distribution systems and the
standards that apply to them.
• The installation and operational qualification phase of a new
installation should incorporate tests to establish conformance to
these standards. A structured approach will be required.
• It will not be easy to include the necessary knowledge within the
Certified Installer training but what is there needs to be updated.
• The PI recommendations for grounding are a good start but there
is still work to do.
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))

30. Thankyou
Control Specialists Ltd – PROFIBUS and PROFINET Shield Currents (Nov 2016))
Clear
Recommendations
Clear Guidelines Better Training
Peter Thomas
Control Specialists Ltd
peter.thomas@controlspecialists.co.uk
www.linkedin.com/in/petermthomas
Presentation available at http://www.slideshare.net/ProfibusUK