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Peter Thomas is Chairman of the PROFIBUS and PROFINET International Training Centre Workgroup (PITC) and a member of the PROFIBUS UK Steering Committee. Keith Armstrong has chaired the IET’s Working Group on EMC and Functional Safety since 1997 and is an EMC expert. The pair discuss EMI/EMC issues that take place at industrial plants across the UK. The first part of the discussion can be read in the previous February edition of PBSI by clicking here.
(Note: The bold text is Peter, whilst the regular text is Keith)
2. Water Processing Area
Peter: This consisted of several devices that connected to each other via a PROFIBUS DP network.
2.1 Several of the motor cables between the drive and the motor appear to have at least one end of the cable screen disconnected, instead of being terminated at both ends. This will allow high frequency EMI to escape from the motor cables that may impact on other systems.
2.2 The screens of the PROFIBUS cables appeared to have been correctly terminated at both ends however several had currents > 80mA (40mA is a typical maximum). The accepted good practice of grounding the screen on entry/exit from a panel had not been followed.
Keith: Both of these points are addressed in paragraph 1.2 of the previous article. The electrical installation business worldwide believes that the most important thing ever is to prevent "ground loops", and connecting a cable screen at both ends causes a "ground loop".
I've even heard of installers refusing to terminate screens at both ends despite the equipment supplier and the customer literally beseeching them to follow the equipment's installation instructions to do so.
I have several case-studies where incorrect cabling (including incorrect type of cable, not terminating screens at both ends, etc.) cost both supplier and customer many millions of GB Pounds, which could not be recovered from the company that performed the installation because they were not worth much and simply went bankrupt when they were sued.
2.3 When terminating the screens of cables at both ends there is a potential for the screens to unintentionally become part of an existing equipotential bonding system and in doing so carry much higher currents than they were designed for. This can be offset by ensuring that the screened cables run in close proximity to cable trays that are RF-bonded (Radio Frequency) at their joints using metal plates or braid straps as opposed to wires. This should be done along their entire length and to the panels at each end. This approach makes the trays act as parallel earth conductors (PEC).
Absolutely correct. I would add that when one has created the minimum-possible mesh-bonded system/installation according to both BS IEC 61000-5-2:1997 and my “lambda/10 at the highest frequency of concern" rules, the more cross-bonding that happens the better everything gets – without exception!
At one extreme is a so-called "single-point grounded" system with no "ground loops" at all. Such systems almost never exist in reality, except for in the most simple of installations. In real-life there is always one, usually more, "ground loops" occurring by accident and are almost impossible to track down. These accidental (and probably undiscoverable) "ground loops" can easily cause excessive cable screen currents, with excessive RF currents often arising at places where you wouldn't expect them to be, anywhere on the site, because of uncontrolled resonances in the "ground loops". A small change to the system can cause these resonances to vanish, only to pop up somewhere completely different, anywhere on the site.
At the other extreme is the BS IEC 61000-5-2:1997 recommended MESH-CBN structure (especially when constructed using my “lambda/10 at the highest frequency of concern” rules), which suppresses almost all resonances by design, and has so many possible paths for 50/60Hz currents that no cable screen ever sees more than a few mA.
If we take an existing "single-point grounded" (supposedly) system and over a period of time modify it with the aim of eventually creating a “proper” MESH-CBN, during those modifications all sorts of weird and wonderful EMI problems can arise, anywhere on the site, until we get to the minimum MESH-CBN, when the resonances are finally all brought under some kind of control.
From then on, as we improve the meshing, any remaining EMI problems caused by "ground loops" get continually less and less until they become completely negligible.
3. Server Room
This room contains several floor-standing server cabinets, two of which are used for the storage of process-related data. There are specific standards associated with providing a low-impedance reference (ground) in IT Server rooms. This usually involves an underfloor mesh bonded network and a bonding ring conductor (BRC) around the perimeter of the room. Neither was found. Additionally, it was noticed that there was no panel to panel RF bonding employed and where braid straps had been provided they had been left disconnected. The two panels associated with process data were manufactured by Schneider who have published a document that is consistent with IEC 61000-5-2:1997.
This comes under the same comments made previously regarding not following the relevant standards and/or not following suppliers’ EMC installation instructions. Before 61000-5-2 was first published in 1997, Schneider published a significant amount of great guidance that was based on its 1995 draft.
From an EMI point of view and based purely on a very brief observational survey, I would consider that the installations may lack robustness and be at risk from intermittent/annoying glitches that could impact production now or in the future. It is clearly impractical to address all of the concerns on an existing installation so a more pragmatic/risk-based approach is recommended. This would require unobtrusive measurements to be taken around the equipment and the interconnecting cables and would need to be done during normal operation (i.e. with the process running). If issues were identified then local solutions could be implemented.
Absolutely correct! I would add that most large systems/installations suffer from problems that reduce their output rate or degrade their output quality. Such problems are often intermittent, with no obvious pattern to them, but (like a manufacturer with some percentage of products being returned under warranty) they can be lived with.
In my experience, these are often caused by the kinds of poor EMC installation practices you describe above, and because their causes are unknown and they are therefore not controlled, a small change to the system/installation can cause them to 'flare up' and become show-stoppers. This can happen whether the changes are intentional or not, for example they could just be the ageing of some of the equipment (the EMC performance of equipment always degrades over time).
In line with recommendations for the qualification of PROFINET networks, the same should apply to EthernetIP networks. Such an approach checks the low-level device-to-device communication prior to handover. Relying on the absence of PLC alarms/diagnostics is not considered good enough and could leave you with a network that is at risk from intermittent failure that will be difficult/time-consuming to resolve. This is a relatively simple task to undertake.
Yes, PROFIBUS/PROFINET are simply examples of data communications, and all data communications suffer from the same EMC issues. Some suffer more because their error correcting protocols are less robust, but they all suffer.
A problem with modern digital data communications is that their error correction hides EMI problems, until it is too late. For example, on an old analogue television you could always tell if it was suffering EMI from the noise in its picture, so you knew if there was a problem that needed to be solved before it got so bad that you couldn’t watch the programmes you wanted to. But with modern digital TVs the picture is always perfect, or else it isn’t there at all, so you have no idea whether EMI is present or not. I’m told that many people return their digital TVs to the shop they bought them from, saying that they are broken, only to find that the next one they are given is also broken. They all work just fine in the shop, but not in their home.
I know one person who, having had several replacement TVs from the shop, realised it must be something else so had a completely new aerial installation put in, even replacing the coaxial cable buried in the wall – a disruptive and costly exercise. But the new aerial had no effect at all, his new TV still wouldn’t work in his house (although it did in the shop). Eventually he found that all he had to do was move his DECT phone in its charging station a little further away from the TV!
Many industrial data communications could be on the point of not working at all, all they need is a little more noise (perhaps when the next legacy motor drive is replaced by a modern VSD) to become unacceptably unreliable. Some people call this the ‘digital cliff’ – the digital system works just fine despite EMI right up until it ‘falls off the cliff’ and stops working. I’ve seen exactly this scenario bring a 1GW electrical power generating station to a halt for several weeks.
It’s all about the acceptable level of financial risk, and almost all installations where they have cut corners on good EMC design/construction (e.g. by not fully applying BS IEC 61000-5-2:1997) may be feeling pleased with themselves for ‘getting away with it’. They can’t know if they are – in effect – standing too close to the edge of a crumbling cliff whilst blindfolded. Of course, simple/quick checks with clamp-on RF current probes and portable spectrum analysers can identify potential EMI issues, but that doesn’t tell us how close the data communications are to their cliff edge. You were telling me the other day that PLC’s do not always reveal how much error correction is going on, but that PROFIBUS and PROFINET network analysers were available that could do so, revealing how close a system/installation is to the ‘cliff edge’ of unreliability due to excessive EMI. It seems to me that using a special data communication’s analyser like yours, that reveals how overworked the error correction is, is something that all industrial installations should do for risk management reasons.
If a problem is found, some simple/quick EMI measurements will identify if the problem is likely to be caused by EMI, and whether any steps that are taken to try to reduce the level of that EMI are actually doing so.
About the authors:
Peter is a Chartered Engineer (C.Eng)/(Eur Ing), Chairman of the PROFIBUS and PROFINET International Training Centre Workgroup (PITC), a member of the PROFIBUS UK Steering Committee and an Instructor at the Application, Training & Engineering Centre of Endress + Hauser. He is a Certified PROFIBUS Engineer, Certified PROFIBUS Designer, a Certified PROFINET Engineer and a Siemens Certified Network Professional - Security. He has 30+ years of practical experience in several areas of Process & Manufacturing Automation, the last 24 of which have been with Control Specialists Ltd.
Keith has been a member of the IEE/IET since 1977 and a member of the IEEE since 1997 and was appointed IET Fellow and IEEE Senior Member in 2010. Keith started Cherry Clough Consultants in 1990 to help companies reduce financial risks and project timescales through the use of proven good EMC engineering practices. Keith has chaired the IET’s Working Group on EMC and Functional Safety since 1997, and is the UK’s appointed expert to the IEC committees on 61000-1-2 (the basic standard on EMC for Functional Safety), 60601-1-2 (risk management of EMC for medical devices), and 61000-6-7 (generic standard on EMC for Functional Safety).
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