Functional bonding and shielding of Profibus and Profinet - Peter Thomas

Peter Thomas – Control Specialists Ltd
Chairman PROFIBUS & PROFINET International Training Centre’s
Technical Officer PI UK
Functional Bonding and Shielding of
PROFIBUS and PROFINET

Peter Thomas – Control Specialists Ltd
Chairman PROFIBUS & PROFINET International Training Centre’s
Technical Officer PI UK
Functional Bonding and Shielding of
PROFIBUS and PROFINET
Slide 1 of 301

33
New Guideline Released – March 2018
• PI have released a guideline entitled
“Functional Bonding and Shielding of
PROFIBUS and PROFINET”
• It is available from
https://www.profibus.com/download/funct
ional-earthing-and-shielding/
• The guidelines contain 6 clear
recommendations that network designers
are advised to consider.
PI UK - MTC Coventry - October 2018 © 2018 – PI UK and Control Specialists Ltd

44© 2018 PI UK and Control Specialists Ltd
Coupling Paths
The PI guidelines are associated with
Line-Conducted (Galvanic) and
Nearby Couplings. Distant (Radiated)
emissions are outside the scope of
the guidelines.
• Line-conducted disturbances are
caused by conductive connections
between devices.
• Nearby disturbances are produced
by magnetic or electric fields
causing inductive or capacitive
coupling.
PI UK - MTC Coventry - October 2018

Assumptions
The guidelines do not specifically mention the following EMC-related activities,
but both remain valid and should be adhered to:
• Adequate separation between “sensitive” cables like PROFIBUS and PROFINET
from higher category cables, i.e. higher voltage and/or higher current.
• Good panel design from an EMC point of view, e.g. all sides rf-bonded to each
other, specific attention paid to cable routing and equipment layout,
awareness of other EMI-mitigation techniques, e.g. termination of shields on
entry/exit to panels, EMC grills for ventilation fans etc.
Note also that the guidelines are for “safe area / non-hazardous” applications
only.

EMC is a topic full of misunderstandings …….. “Earthing of cable shield”
Earthing and EMC

77PI UK - MTC Coventry - October 2018 © 2017
Earthing and EMC
• Earthing (grounding) and EMC are completely separate issues.
• We often talk about “earthing or grounding” cable shields, but the important
aspect from an EMC point of view is that the shields are connected to a
common, low impedance, reference.
• The fact that this common reference is usually connected to earth (ground) is
a safety-related matter and nothing whatsoever to do with EMC.
• However, we have to accept that the linking of the two terms is common
practice, often in published documents, even though technically it is
incorrect.

Potential Sources of Disturbances

99
R1 – Equipotential Bonding and Functional Earthing
via a Common Bonded Network (CBN)
• Whilst safety will always be paramount, the fact remains that many earth
systems are far from ideal from an EMI point of view.
• The UK 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”.

1010
• The increasing use of high-
frequency switching devices can
result in the Bonding cables carrying
currents at frequencies much higher
than 50 Hz.
• The impedance of these cables at
higher frequencies can result in the
currents flowing back to earth via
lower impedance routes not
designed to carry these currents.

1111
• 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.
• The practice of creating separate “clean” and “dirty”
earths is also considered an out of date practice
and impossible to achieve.
Type A

1212
R2 – Ensure that your low voltage distribution
system is TN-S
• TN-C systems are
considered very bad
for EMC.
• The main advantage of
TN-S over TN-C in this
respect are the
separation of Earth
and Neutral currents.

1313
R3 – The CBN should be part of a MESH-BN
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.

1414PI UK - MTC Coventry - October 2018 © 2017
R4 – Terminate the cable shields at both ends
• Confusion still remains as to whether the shields (screens) of PROFIBUS
and PROFINET cables should be terminated to a common reference
(usually ground) at one end only or at both ends.
• Electrical Interference takes two forms, electrostatic and electromagnetic.
• To protect against both, the shield MUST be terminated at both ends to a
common reference.

1515
R4 – Terminate the cable shields at 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 terminated at both ends to a
low impedance reference (ground).
• This induces current in the screen
with generates an opposing
magnetic field.

1616
Bad Screen Currents
• The terminating of the PROFIBUS (or PROFINET) shields at both ends can
lead to the unintentional participation of these screens in the equipotential
bonding system.
• It was originally recommended that an equipotential bonding cable be run as
close as possible to the PROFIBUS (or PROFINET) cable to limit the screen
current by providing an alternative path for the bonding current to flow
PROFIBUS (or PROFINET) cable
Equipotential Bonding Cable

1717
Parallel Earth Conductors (PEC’s)
• The equipotential bonding cable mentioned on the previous slide is a form of
PEC.
• However it is now generally considered to be a technique appropriate to
50/60Hz automation systems – max of 150 Hz.
• Most EMC experts will look at alternative PEC’s to overcome this limitation
before recommending a full-blown MESH-BN
• Examples of PEC’s includer the use of cable trays and structural metal work
with RF (low impedance) bonding straps between joints.
• The PI guidelines also contain alternative suggestions for improving the
equipotential bonding on an existing system – see later slides.
© 2018 – PI UK and Control Specialists LtdPI UK - MTC Coventry - October 2018

1818
R5 – Variable Speed Drives
• Variable Speed Drives (VSD’s) are a source of harmonics that can feed
back into the mains supply and also EMI that can be galvanic or
airborne.
• Whilst the manufacturers guidelines should always be followed, they
generally recommend the use of specially designed motor cables
between the VSD and motor and that the shield of that cable should be
terminated at both ends to prevent EMI “leaking out”.

1919
R5 – Variable Speed Drives
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.
There is a benefit to having these devices switch faster than they
currently do, which means more EMI.
Unless the installation caters for this reality, there is a real
possibility of intermittent network outages and production loss.

2020
A common source of high frequency bonding
currents
The four channels of the EMC INspektor
were connected as shown.

2121
currents

2222
currents
Motor Cable

2323
R6 – Be aware of the dangers of multiple grounds
on 24v dc supplies
• The ground (negative) of most 24v dc power supplies are usually
grounded as close as possible to the supply.
• Earth loops can be created by the unintentional grounding of the
negative wire to ground at other points.

2424
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?

2525
Indu-Sol Recommended Reference Values
Source: Indu-Sol GmbH

2626
EMC Measuring 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.
A spectrum analyser is the
“tool of choice” for EMC
Engineers and consultants.
However, clamps are
available to assist with
potential equalisation issues

2727
Optimum improvements to equipotential
bonding systems
Conventional equipotential bonding

2828
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.

2929
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).

3030
Impact on PI Training
• EMC is always a topic that raises many questions and there are
often misunderstandings.
• The existing Certified Training courses for PROFIBUS and
PROFINET will need to be updated to include the techniques
mentioned in the new guidelines.
• PI Instructors may need assistance in gaining sufficient knowledge
to deliver this aspect of training and be able to answer questions
about it.
• This will be handled under a periodic review of the learning
outcomes.

3131PI UK - MTC Coventry - October 2018 © 2018 – PI UK and Control Specialists Ltd
Additional Information
EN50310-2016 EMC Optimum
Equipotential Bonding
– Indu-Sol
IEC 61000-5-2 1997

3232
Thankyou
Peter Thomas
Chairman of PI Training Centres and Technical Officer – PI UK
www.linkedin.com/in/petermthomas
Control Specialists Ltd
www.controlspecialists.co.uk
Tel +44(0)1925 824003

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