The ever-increasing use of high-frequency switching devices in industrial automation can result in high-frequency currents circulating in an equipotential bonding network that was not designed for this purpose. The absence of a proven low-impedance bonding network can result in these currents taking alternative and often undesirable routes. Occasionally these will be the shields of industrial network cables such as PROFIBUS and PROFINET that in turn can lead to intermittent communications problems. This presentation will discuss the issues in more detail and will explain the relevance of the recently released PI guidelines “Functional Bonding and Shielding for PROFIBUS and PROFINET”.

1. 1
EMC for Networks
March 27 & 29 2019
Peter Thomas, Control Specialists, UK
Jos Knockaert, Ghent University – INCASE, BE

2. Emission Immunity
Source Coupling Victim
path
Three ways to solve the problem
2
What we will discuss …
Focus on drives
Jos Knockaert
Focus on networks
Peter Thomas

3. Troubleshooting EMI problems in industry have taught us:
- 90% caused by power electronics
- 10% other reasons
- 90% of the problems are due to bad grounding/reference
- 10% due to bad design, production faults and aging
EMI in industrial
networks …

4. The immunity problem
EMC from the network viewpoint

5. • PROFIBUS and PROFINET networks
are designed to operate in an
industrial automation environment.
• To achieve this, both use screened /
twisted pair cable.
• The combination of the shield and
twisted pair construction provides
the basic protection against two
forms of interference, these being
Electrostatic and Electromagnetic.
PROFIBUS and PROFINET
5

6. The purpose of cable shielding
Cable shielding has two main purposes, depending upon the cable
and its purpose.
• One is to prevent external sources of interference from
corrupting the signals in side the cable – this is called Immunity.
• The other is to prevent internal sources of interference from
propagating to the area surrounding the cable – this is called
emissions
• In both cases, the shield has to be capable of shielding against
two forms of interference, electrostatic (or capacitive) and
electromagnetic (or inductive)
6

7. Cable Installation
For the shields of PROFIBUS and PROFINET cables to perform their
role, the following installation techniques should be followed:-
• Ensure that there is adequate separation between the cable of
concern and other cable categories.
• Try to ensure that the cables run as close as possible to the
reference (parallel earth conductor)
• Connect the shield at both ends to the reference using 360
degree termination, i.e. avoid pig tailing.
• But what reference?
7

8. EMC and grounding
• From an EMC point of view, the most important aspect of cable
shielding is to ensure that both ends are connected to a
common reference.
• The fact that, in the world of industrial automation, this
reference is grounded is a safety issue and of no importance to
EMC.
8

9. The purpose of earthing
Earthing systems perform several roles:-
• To protect personnel from electrocution (Safety)
• To ensure that the 0v line of several devices are at the same
potential (functional)
• To protect buildings and equipment from lightning strikes
• To provide a common reference for EMC purposes.
9

10. 10
• Whilst star earthing (Type A) is adequate for
safety purposes, it is considered poor for
EMC purposes.
• For example, if we were to connect a
network cable, whose shield is connected to
ground at both ends, between equipment on
different limbs of the star, the cable shield
will become part of the bonding network
increasing the possibility of excessive
screen currents flowing.
• A Common Bonded Network (CBN)
overcomes these issues.
Type A
Common Bonded Network (CBN)

11. 11
IEC 61000-5-2:1997 “EMC – Earthing & Cabling”
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.
Type D – Mesh Bonded Network

12. Shielding and Electrostatic
interference
12
• In terms of PROFIBUS and PROFINET cables, Electrostatic
interference is a consequence of the cable being influenced by an
external electric field.
• The most common reason being that the cable is too close a cable
(or cables) that have a higher voltage on them.
• Adequate cable separation and connecting the shield to a reference
at one end will protect against electrostatic interference.

13. Shielding and Electromagnetic
interference
13
• In terms of PROFIBUS and PROFINET cables, Electromagnetic
interference is a consequence of the cable being influenced by an
external magnetic field.
• The most common reason being that the cable is too close a cable
(or cables) that have a currents flowing in them.
• Adequate cable separation and connecting the shield to a reference
at both ends will protect against Electromagnetic interference.

14. Shielding and Electromagnetic
interference
• The induced currents flowing in the cable shield as a result of
magnetic induction are essential in ensuring that the cable
shield can protect against magnetic fields.
• Currents can only flow in a shield if both ends are connected
to a common reference, i.e. we need a circuit.
• At low frequencies, the protection is achieved by the
magnetic field generated by the induced current which, in
theory, cancels the incoming magnetic field.
• At high frequencies, the protection is achieved by making use
of the skin effect.
14

15. PROFINET / PROFIBUS
and “Earth Loops”
• PROFINET, and its predecessor
PROFIBUS, are balanced forms of
networks.
• One of the benefits of this is that
none of the pairs are connected to
ground which means that “earth
loops” are less of an issue.
• However the recommendation to
“ground” the cable shield at both
ends can only be followed if the two
ends are at similar potentials.
15

16. The emission problem
EMC from the drive viewpoint

17. “High frequency” problems
Rising egde PWM (dv/dt)

19. Losses in power electronic switches
Control signal

20. Losses in Power Switches
20
M
IT
VT

21. Basic drive:

22. Basic drive:
2.475 2.48 2.485 2.49
x 10
-5
-100
0
100
200
300
400
500
600
t [s]
[V]
dv/dt > 5 kV/us

23. Rectangular signal with finit rise/fall time
A
x(t)
tTτ
A
𝒓 𝒇
τ

24. Spectrum (Consider tr = tf)
Het spectrum lineaire schaalHet spectrum
1
𝜋 𝜏
- 0 dB/dec
- 20 dB/dec
1
𝜋 𝑡

25. What does it mean for your industrial installation?
Simulation of the PWM-spectrum (6 kHz switching frequency, 100 ns risetime)
2 - 10kHz
3MHz
Conclusion: new drives cause larger EMI problems than old drives, so better precautions are
needed

26. Influence of switching frequency
20 log 2𝐴𝐷
f
- 0dB/dec - 20dB/dec
- 40dB/dec
1
𝜋. 𝐷. 𝑇
1
𝜋. 𝐷.
𝑇
2
1
𝜋. 𝑡
- 6dB
Conclusion: never use higher switching frequencies than needed

27. Current can be split into differential mode (DM) and common mode (CM)
components:
Caused by:
- unbalance
- common mode source

28. Common mode causes EMI-problems:
I1 = Id + Ic
I2 = Id + Ic
Id
Id
Ic
Ic
2
2
21
21
II
I
II
I
c
d




Radiated emission at
distance d:
Conductor 1
Conductor 2
Combined field DM CM
Principle of shielding: return path (grounding) close to feeding path.

29. v
Filter capacitor
What does it mean for your industrial installation?

30. What does it mean for your industrial installation?
Connect the shielded motor cable at both sides to reference in a proper way
Do not us pigtails Do not interrupt the shield
Use cable
glands
Use metallic
fasteners

31. OK Jos, but I disconnected the shield of my motor cable and my
communication problem was solved?
Residual current device
Disconnecting the PE
solves the problem?

32. What does it mean for your industrial installation?

33. Some pictures
- What is wrong?

34. Some pictures
Flat braid wire

35. Some pictures
- What is wrong?

36. 10
4
10
5
10
6
10
7
20
30
40
50
60
70
80
90
100
110
f [Hz]
V[dBµV]
36
Some pictures
- What is wrong?

37. 37
Peter Thomas, Control Specialists Ltd, UK
peter.thomas@controlspecialists.co.uk
Jos Knockaert, Ghent University – INCASE, BE -
Jos.Knockaert@UGent.be
THANKYOU