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.

2 PROFIBUS UK 2 PROFIBUS 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 GmbH Good 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



The four channels of the EMC INspektor

were connected as shown.

13 PROFIBUS UK 13 PI UK – September 2017 © 2017


14 PROFIBUS UK 14 PROFIBUS 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

EN50310-2016 EMC Optimum

Equipotential Bonding

– Indu-Sol

Recommendations for the Functional

Earthing & Shielding of PROFIBUS

and PROFINET - PI

Coming Soon !

24 PROFIBUS UK 24 Editable 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





Engineering

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