If a cable is carrying low-level analog signals, the shield drain wires should be grounded at only one end. Grounding at one end only prevents any current from flowing between none equipotential ground points, because ground current flowing in the drain wire or shield can couple into the small-signal wires and corrupt the signal. When the drain/shield is connected at only one end, it serves as a Farady cage to isolate the low level signal(s) from noise/interference.

Hi

This is now not the current best practice, due to the increasing prevalence of high-frequency noise in industrial plant.

The earthing of the screen/shield at one end turns the shield into an antenna, and the frequencies at which the characteristic impedance start making this effect noticeable are now very relevant in a modern plant, being somewhere in the 1-10Mhz range.

The best guys to draw down on the knowledge and experience for this is RF and circuit board designers, some quite bizarre things can occur with resonance, terminating impedances etc etc.

Vendors such as Siemens have installation recommendations for the S7-1500 series PLC to earth the shield at both ends, on the basis that the return impedance thru structure etc is 10 times greater than the instrument cable drain wire impedance. If this can't be known then they suggest to run a Parallel Earth Conductor (PEC) out with the instrument cable, sized to be 10 times less impedance than the drain wire and effectively taking an earth out with you. (There are also less preferred alternative configs where the filed earth is via a capacitor etc.)

This has two impacts, one is to make the shield more like a Faraday cage and less like an antenna - a shield as an antenna has the possibility of induced currents being impressed on the active cable cores, but it also it acts as a current splitter for surge, lessening the surge current (and therefore voltage via V=IR at inputs) by a factor of 10.

In Functional Safety Service (IEC 61511) the undetected failure rates quoted on these S7-1500 controllers are phenomenally low, but Siemens tell me that a surge event can cause partial failures of silicon devices such that it is very difficult to predict likely and /or possible failure modes, so the diagnostics that give you the low failure rates cannot be as complete if surge is not managed.

The relevant Siemens publication is A5E03461486-AB - Designing Interference Free Controllers.

Further general detailed technical information is available in ABB publication:

Technical Application Papers No. 25
Electromagnetic compatibility: theory and
application measures in MV environments.


Plus there is some IEC standards etc.

 

Integrator,

Yes, but--in this case the shields are already earthed at BOTH ends and yet the damage has occurred twice.

So, what would YOU recommend, Integrator?

That is the question here: How do we help the original poster solve the problem?

One of my favorite quotes is, "The good thing about standards is there are so many to choose from!" And, you have just proven my point. Yet another new standard, or set of standards.

Unfortunately, we are NOT at the site where the damage continues to occur. And sometimes that makes all the difference in the world as to what is done to resolve the issue. We have made recommendations based on commonly accepted practices (which become defacto standards after time and continued use and favorable experience). We can only base our recommendations on the information which was provided, and we have done so. You, Integrator, have chosen to cite "new" standards--all well and good, but it would be best if you used those standards to make some recommendation based on the information provided by the original poster and cite the relevant sections/paragraphs of the standards and documents you noted.

Lastly, I would challenge your statement that "This is not not the current best practice ..." because in my current experience unless the instrument/device manufacturer makes specific proclamations about how interconnecting wiring is to be shielded the current best practice is and the AE or electrical construction supervisor knows of or reads the supplied documentation it's not noted on any drawing the electrical construction personnel are given: To ask the commissioning personnel how to earth the shield drain wires, BUT, usually to just ground both ends of the shield drain wire (because people working as electricians in MANY parts of the world are not trained nor experienced nor licensed nor certified to perform the work they are doing and so they just do it the way they "think" it should be done and don't ask questions lest they be thought to be inexperienced....). Commissioning personnel usually have a LOT of wiring corrections to make, many to remedy incorrect grounding practices. Sometimes, it's necessary for commissioning personnel to consult manufacturer's documentation or the manufacturer directly when especially troubling noise problems come up--and THAT'S when the special "new" standards you talk about become evident. (How did YOU first learn about these "new" standards and practices--BEFORE, or AFTER, troubling installations.?.?.? Inquiring minds want to know!)

So, thanks for the information, Integrator, but we're still trying to solve the original poster's problem and waiting for ideas because the OP isn't so sure he's got the answers he's looking for. (And probably hasn't consulted the equipment manufacturer either.)

 

Integrator,

Yes, but--in this case the shields are already earthed at BOTH ends and yet the damage has occurred twice.

So, what would YOU recommend, Integrator?

That is the question here: How do we help the original poster solve the problem?

One of my favorite quotes is, "The good thing about standards is there are so many to choose from!" And, you have just proven my point. Yet another new standard, or set of standards.

Unfortunately, we are NOT at the site where the damage continues to occur. And sometimes that makes all the difference in the world as to what is done to resolve the issue. We have made recommendations based on commonly accepted practices (which become defacto standards after time and continued use and favorable experience). We can only base our recommendations on the information which was provided, and we have done so. You, Integrator, have chosen to cite "new" standards--all well and good, but it would be best if you used those standards to make some recommendation based on the information provided by the original poster and cite the relevant sections/paragraphs of the standards and documents you noted.

Lastly, I would challenge your statement that "This is not not the current best practice ..." because in my current experience unless the instrument/device manufacturer makes specific proclamations about how interconnecting wiring is to be shielded the current best practice is and the AE or electrical construction supervisor knows of or reads the supplied documentation it's not noted on any drawing the electrical construction personnel are given: To ask the commissioning personnel how to earth the shield drain wires, BUT, usually to just ground both ends of the shield drain wire (because people working as electricians in MANY parts of the world are not trained nor experienced nor licensed nor certified to perform the work they are doing and so they just do it the way they "think" it should be done and don't ask questions lest they be thought to be inexperienced....). Commissioning personnel usually have a LOT of wiring corrections to make, many to remedy incorrect grounding practices. Sometimes, it's necessary for commissioning personnel to consult manufacturer's documentation or the manufacturer directly when especially troubling noise problems come up--and THAT'S when the special "new" standards you talk about become evident. (How did YOU first learn about these "new" standards and practices--BEFORE, or AFTER, troubling installations.?.?.? Inquiring minds want to know!)

So, thanks for the information, Integrator, but we're still trying to solve the original poster's problem and waiting for ideas because the OP isn't so sure he's got the answers he's looking for. (And probably hasn't consulted the equipment manufacturer either.)

If you had read and understood what I wrote, then you might consider that just earthing the cable at both ends is not the answer, depending on the return path for earth.

Next step could be to run the parallel earth conductors out with the cables to provide a good earth at the far end, but also provide a current divider so surge would decreased by an order of magnitude.

Alternate or next step would be to install a surge event counter on the earthing/LPS system so it could be seen whether events correlated or not to lightning.

I do design/commission, I first become aware of this design thinking as part of design, but I have seen many strange things in the 30+ years I have been working with EIC in various forms including RF, DC, LV/HV etc. Usually associated with por installation practices (eg putting the "pig tail" on earth wires thus rendering them useless for lightning) or unexpected resonance (where I high Q factor can generate huge spikes in voltage that very counter-intuitive), followed by unexpected coupling and poor earthing.

The problem with standards is how people use them - standards like these are not "code", they are guidance, and the user needs to understand why they say what they do and apply to their own situation accordingly. This is why I also supplied the info to get a detailed understanding of how/why, but you would need to read and understand it.

In an increasingly performance-based and risk assessed standards world (as opposed to prescriptive "code like" standards), unfortunately many who have survived so far just following the "rules" are being left behind.

Now, you are expected to be able to a certain amount of thinking for yourself, not just rely on words on a piece of paper written by someone that may not have ever dreamed to anticipate your particular circumstance.

There are certain nationalities that cope with this better than others, culturally. Americans seem to be one of these that cope least well with such circumstances, but I am sure they will catch up with new generations coming thru being accustomed to a new way of thinking.

Unfortunately, regardless of where you come from, you seem to be wanting to rely on a fixed set of rules drawn up by someone who may or may not have considered the circumstances being examined - I would say maybe not, because problems exist, so now you should take care and time to examine all relevant information tat may come your way and evaluate it to see if it is relevant and helpful.

So what have YOU done to try and understand the problem in more detail and consider first principles, rather than just continue to apply the standard "rules" for no changed result?

 

Integrator,

We are pretty certain the original poster has not tried the "standard rules” so there's still that to try.

I did not read all of the documents you cited. My experience if there's a problem with a device or installation is to check the wiring and installation (including earthing practices) to see how it was done and if it seems to be correct (as per my experience--which included routing, earthing and shield drain wire termination) to then consult manufacturer's documentation (manuals; installation instructions; data sheets; recommended wiring practices (if they exist)) and make any necessary changes.

If there are still problems, I meggar the wiring and replace or repair it as necessary. I have even run temporary wiring from the device/instrument to the control system or monitor to see if that solves the problem--which means there are unseen issues with the existing wiring and/or routing.

If that doesn't work, I then contact the manufacturer either by email or phone for assistance, detailing what steps were undertaken AND what the results were. Usually, with that information the manufacturer will be able to provide valuable assistance to resolve the problem.

But, I have never been pointed to the documents you cited. It was never necessary to go to those lengths to solve similar issues.

But, thank you for making me--and others--aware of the existence of these documents.

I don't believe the original poster knows precisely how the equipment was installed nor the details of ambient conditions on the site, and that we never will, either.

 

Thanks

 

Integrator,

We are pretty certain the original poster has not tried the "standard rules” so there's still that to try.

I did not read all of the documents you cited. My experience if there's a problem with a device or installation is to check the wiring and installation (including earthing practices) to see how it was done and if it seems to be correct (as per my experience--which included routing, earthing and shield drain wire termination) to then consult manufacturer's documentation (manuals; installation instructions; data sheets; recommended wiring practices (if they exist)) and make any necessary changes.

If there are still problems, I meggar the wiring and replace or repair it as necessary. I have even run temporary wiring from the device/instrument to the control system or monitor to see if that solves the problem--which means there are unseen issues with the existing wiring and/or routing.

If that doesn't work, I then contact the manufacturer either by email or phone for assistance, detailing what steps were undertaken AND what the results were. Usually, with that information the manufacturer will be able to provide valuable assistance to resolve the problem.

But, I have never been pointed to the documents you cited. It was never necessary to go to those lengths to solve similar issues.

But, thank you for making me--and others--aware of the existence of these documents.

I don't believe the original poster knows precisely how the equipment was installed nor the details of ambient conditions on the site, and that we never will, either.

CSA AND Integrator,
Thank you guys for your points. As to whether i have contacted the manufacturer, that was done when the first case occurred. It was blamed to poor earthing. At the initial stage, the system the Panel was earthed to an existing earth system. This time, the manufacturer recommended that a dedicated Earth system be prepared for it. That was done prior to repairing the systems then. Despite the dedicated and functional earthing system, it has happened again.

As to whether i know how the equipment was installed, i would want to say that that i took the lead in installing those equipment. I have already provided details of the installation in one of my previous posts.