Profibus and Profinet shield currents - Peter Thomas

PROFIBUS / PROFINETshield currents

- and why you should measure them

Peter ThomasControl Specialists Ltd

Chairman of the PROFIBUS & PROFINET International (PI)

Training Centres

Case Study UK PROFIBUS Support Visit

Control Specialists Ltd PROFIBUS and PROFINET Shie ld Currents (Nov 2016))

Control Specialists Ltd were asked to provide site support on a 6-year oldPROFIBUS installation.

The installation consisted of several separate PROFIBUS networks all of whichhad been suffering from intermittent network failures since the network had beencommissioned.

The engineering staff of the client already had a portable PROFIBUS analyser anda permanent monitor but had been unable to identify the cause of the problems.

Case Study PROFIBUS Network


The network can be seen to consist of a main PROFIBUS backbone, connecting the PLC to 5 Hubs, each of which have 5 independent segments going to switch room and plant-based PROFIBUS slaves.

The network was running at 1.5MBits/second and was correct ly terminated. The engineering staff of the client had estimated that the length of the main backbone was around 160m. The permanent monitor had been connected to the monitoring port of HUB 04.

Case Study PROFIBUS Waveforms


In July 2015, this network was particularly troublesome and resulted in the engineering staff having to spend more time than usual on the network.

Control Specialists were contacted for assistance, initially being asked to comment upon the data derived from the permanent monitor. The image above shows several oscilloscope traces for nodes downstream of the Hubs at the time of a fault.



This was particularly interesting because the waveforms were from the permanentmonitor on the backbone which, by definition, is showing a cleaned-up version ofthe waveforms downstream of the hubs.

This suggested that the fault lay on the backbone as opposed to anythingdownstream of the hubs.



When a portable analyser was eventually connected, the reported fault had clearedbut came back intermittently with no warning.

Following on from a presentation given by Indu-Sol GmbH about the effect ofpotential equalisation currents in PROFIBUS networks, it was decided to measurethe currents flowing in the screens of the PROFIBUS cables.

Case Study Network screen currents


If the currents flowing through the screens of cables are >> 40mA, they mayindicate installation-related problems.

The current in the screen will be due to one of two main reasons:-

1. Compensating currents due to inductive coupling.2. Potential Equalisation currents.

Compensating currents are good currents and can be redu ced below 40mA by existing cable segregation techniques. Potential Equalisation currents are bad currents and should be investigated further.

Investigation in this particular instance indicated that the likelihood was the fact that the cable between Hub 3 and Hub 4 came within 0.3m of a large power cable for part of its length. However, further investigation on the site showed evidence of several instances of bad practice from an EMC point of view so a second visit was arranged to look at currents flowing in the PE cables.

Shielding (Network Cables)


Despite the recommendation of connecting cable shields (screens) to earth at both ends, it is still common practice to follow the old rule of earthing at one end only.

Earthing the screen at one end only on a balanced system like PROFIBUS or PROFINET only protects against electrostatic (capacitive) interference.

In order to protect against electromagnetic (inductive) interference, a counter-balancing current must flow in the screen.

But the earth loop must be a low impedance. Specific limits for screen currents and loop impedances wil l be required. We

currently use >0mA and

Case Study EMC measurement tools


Control Specialists Ltd wondered if galvanic EMC-related issues might be the cause of the problems.

To assist in determining this, an LSMZ I clamp meter and an MWMZ II impedance clamp meter were used. The latter injects a 2.05 KHz signal into the screen and calculates the impedance at that frequency.

LSMZ I Current Clamp MWMZ II Impedance Clamp

Case Study Measurements


The diagram below shows the measurements taken for each cable on the backbone using the LSMZ I and MWMZ II clamps. The following limits were applied:-

Screen Currents: > 0mA and 40mA) flowing between hub 3 and 4 were also where a significant number of faults had occurred.

Note: PLC EARTH = PROFIBUS cable between PLC and f loor-mounted earthing bar.

Case Study Network screen currents


Investigation in this particular instance indicated that the likelihood was the fact that the cable between Hub 3 and Hub 4 came within 0.3m of a large power cable for part of its length.

However, further investigation on the site showed evidence of several instances of bad practice from an EMC point of view so a second visit was arranged to look at currents flowing in the PE cables.

EMC & Equipotential Bonding The current situation


The increasing use of high-frequency switching dev ices (e.g. invertors) increases the likelihood of industrial networks like PROFIBUS and PROFINET being affected by EMC-related issues.

These EMC issues will not always be airborne, many will be galvanic as a result of inadequate equipotential bonding.

Conventional bus analysers are of little use in identifying galvanic-driven EMC issues.

The lack of awareness of EMC on the factory floor is staggering, with many competent engineers quoting and following outdated practices.

There is no obvious single source of guidance rega rding EMC. There is a real need for the promotion of an increased awareness

of EMC within industry.

International / European Guidelines


IEC 61000-5-2 1997 covers guidelines for the earthing and cabling of electrical and electronic systems and installations aimed at ensuring electromagnetic compatibility among electrical and electronic apparatus or systems.

EN 50310 (VDE 0800-2-310) specifies requirements and recommendations for connections (bonds) to earthing networks in buildings in which information technology (IT) equipment is intended to be installed.

EN 60204-1 2006 provides requirements and recommendations relating to the electrical equipment of machines so as to promote the safety of persons and property, consistency of control response and ease of maintenance.

Supply Distribution Systems


Supply Distribution Systems (TN-C-S)


Distribution Systems (EN 50310)


Equipotential Bonding


EN 50310 - Multiple bonds , including those using building structures, should be used rather than a single bond since this reduces the impedance (inductance) of the resulting bond.

Type A Earth Bonding Network Star


A recommended, but not ideal, improvement can be made used bonding conductors as shown.

Note - It is important to ensure that the screens of network cables are not inadvertently used as bonding conductors. This can be easier said than done.

A conventional Type A Star network whilst considered good for electrical safety is poor for EMC at all frequencies.

Type D Earth Bonding Network MESH BN


IEC 61000-5-2 and EN 50310 specifically recommend 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.

The lower diagram shows multiple MESH-BNs connected together to create a Common Bonded Network (CBN). Here, all Protective and Functional earthing is combined (CPFE).

Note specific recommendations will be required for improving existing installations.

Power Supply Earthing


Sections 6 and 9.4.3.1 of IEC 60204-1 address the earthing of Protective Extra-Low Voltage (PELV) supplies (ac and dc). Here the need to earth at the point of supply is stipulated.

Consumers

Multiple earthing of dc supply lines can make the automation devices connected to them susceptible to unintended consequences.

Case Study Other EMC measurements

The LSMZI and MWMZII were also used to measure the currents and calculate the impedances of the following:-

Potential equalisation installation. Variable Speed Drive cable screens

Overall a structured, and documented, approach had to be taken so that a clearer picture of the issues could be obtained.


Case Study Variable Speed Drives


The four channels of the EMC INspektor were connected as shown.

Variable Speed Drives


The UK trade associations GAMBICA and REMA have produced a technical guide to meet a demand for an authoritative guide on best practice for the installation of Power Drive Systems.

It is the result of a study that takes note of well-established fundamental theory, technical papers, and the results of specific investigations.

Motor Cables (GAMBICA/REMA)


It is strongly recommended that the motor cable be screened . This is essential in order to meet many of the relevant EMC standards, and is recommended in any case to reduce the risk of interference to other equipment.

The screen will have to carry relatively high amplitude current pulses, therefore its RF impedance and connection method at each end are critical .

Effective forms of screen are: Copper braid, Copper tape, Close wound galvanised steel wire armour, Aluminium tape armour, Steel braid and solid copper cladding

To be effective, the screen must have good electrical continuity along the cable length.

The braided or helical construction must be such that current can easily pass from turn to turn along the length.

The screen coverage must be as close as possible to 100%, and the resistance must be low, especially at high frequenc ies.

Variable Speed Drives (Motor Cable)


The PE of the cable between the drive and motor can carry surprisingly high currents.

This can be significantly reduced using symmetric cables.

Case Study Measurements


Variable Speed Drives


It would be timely to wake up industry to this problem, and start to deal with it, because the next generations of VSDs will be smaller and less costly, and run cooler, because of the use of newly-developed types of power switching devices which can operate at 10 to 100 times faster than current devices.

These will cause truly horrendous problems if the present incorrect installation techniques are allowed to continue.

UK EMC Expert and Author of books on EMC

PI Recommendations


Coming Soon !

New Guidelines

Conclusions

EMC and its effects on control systems are nothing new. However misunderstandings and the lack of awareness on all sides make it essential that PI take a leading role in promoting awareness and good practice in this respect.

Industrial Network Engineers clearly need to know more than just configuration, protocols and network-based fault-finding techniques. They also need a knowledge of EMC, equipotential bonding techniques, low voltage distribution systems and the standards that apply to them.

The installation and operational qualification phase of a new installation should incorporate tests to establish conformance to these standards. A structured approach will be required.

It will not be easy to include the necessary knowledge within the Certified Installer training but what is there needs to be updated.

The PI recommendations for grounding are a good start but there is still work to do.


Thankyou


Clear Recommendations

Clear Guidelines Better Training

Peter ThomasControl Specialists Ltd

[email protected]/in/petermthomas

Presentation available at http://www.slideshare.net/ProfibusUK

Engineering

Profibus and Profinet shield currents - Peter Thomas