Why you should measure shield currents in PROFIBUS and PROFINET networks - Peter Thomas

Peter Thomas – Control Specialists Ltd

Screen currents and why you should measure them.

2PROFIBUS UK 2PROFIBUS UK – March 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.

BA

BA

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 GmbHGood 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 50Hz systems, the use of high frequency switching devices such as variable speed drives makes this technique out of date.

PROFIBUS (or PROFINET) cable

Equipotential Bonding Cable

X


What’s the issue?

“There is widespread lack of awareness and attention with regard to another major cause of excessive basic loads on existing bonding systems (e.g. cable shielding), namely, incorrect or insufficiently dimensioned return current paths”.

Source - Indu-Sol GmbH


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.



14PROFIBUS UK 14PROFIBUS UK – March 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?


Recommended Reference Values

Source: Indu-Sol GmbH


Recommended Tools

The leakage current clamp EMCheck LSMZ I is capable of measuring low (50/60 Hz) and high (5KHz) frequency 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 screwconnection 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

EN50310-2016EMC Optimum Equipotential

Bonding – Indu-Sol

Recommendations for the grounding & shielding of PROFIBUS

and PROFINET networks - PI

Coming Soon !


Thankyou

Peter ThomasChairman of PI Training Centres

www.linkedin.com/in/petermthomas

Control Specialists Ltdwww.controlspecialists.co.uk

Authorised solutions partner of Indu-Sol GmbHhttp://www.controlspecialists.co.uk/downloads_pdf/4666_Indu_Sol_EMC_brochure_2017.pdf

Engineering

Why you should measure shield currents in PROFIBUS and PROFINET networks - Peter Thomas