6. 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

22
New Guideline Released – March 2018
• After many months of discussion, PI have
released a guideline entitled “Functional
Bonding and Shielding of PROFIBUS and
PROFINET”
• It is available free of charge to members of PI
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 - Glasgow Seminar - April 2018 © 2018 – PI UK and Control Specialists Ltd

33
R1 – Equipotential Bonding and Functional Earthing
via a Common Bonded Network (CBN)
• The increasing use of high-
frequency switching devices can
result in the Earthing and Bonding
cables carrying currents at
frequencies much higher than 50 Hz.
• The inductance 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.

44
R1 – Equipotential Bonding and Functional Earthing
via a Common Bonded Network (CBN)
• 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 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earths is also considered an out of date practice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.

55
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.

66
R3 – The CBN should be part of a MESH-BN
BS 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.

77PI UK - Glasgow Seminar - April 2018 © 2017
R4 – Terminate the cable shields at both ends
• Confusion still remains as to whether the shields of PROFIBUS and
PROFINET cables should be terminated to a common reference (usually
ground) at one end only or at both ends.
• Electromagnetic Interference takes two forms, electrostatic and
electromagnetic.
• To protect against both, the shield MUST be terminated at both ends to a
common reference.
• The use of a MESH-BN will ensure that the voltage difference at either end
of the shield is low.

88
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 “Ze”
• This induces current in the screen with generates an opposing magnetic field
Ze

99
R5 – Variable Speed Drives
• Variable Speed Drives (VSD’s) are a source of harmonics that can feed
back into the mains supply and 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 invertor and motor cable and that the shield should be
terminated at both ends to prevent EMI “leaking out”.

1010
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.

1111
A common source of high frequency bonding
currents
The four channels of the EMC INspektor
were connected as shown.

1212
A common source of high frequency bonding
currents

1313
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.

1414
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?

1515
Indu-Sol Recommended Reference Values
Source: Indu-Sol GmbH

1616
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.

1717
Optimum improvements to equipotential
bonding systems
Conventional equipotential bonding

1818
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.

1919
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).

2020
Need help or want to know more ?
Training Courses
• Certified PROFIBUS Training
• Certified PROFINET Training
• AS-i Training Courses
• IO-Link Training Courses
• CAN Bus Training Courses
• EMC for installations
Site-based Support
• PROFIBUS
• PROFINET
• AS-I
• CAN

2121PI UK - Glasgow Seminar - April 2018 © 2018 – PI UK and Control Specialists Ltd
Additional Information
EN50310-2016EMC Optimum
Equipotential Bonding
– Indu-Sol
https://controlspecialists.co.uk/wp-content/uploads/2017/08/16012_Indu_Sol_EMC_brochure_2017.pdf

2222
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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