EMC / EMI; Electromagnetic Compatibility & Immunity

[PJW]

The London Study Group had a talk from an EMC expert recently on this topic; note that two of the 2010 module 7 questions were closely related to this subject.

I am hoping that he may permit me to share his Powerpoint presentation on this forum. In the meantime I am attaching a document that I was inspired to produce; I found that I didn't need all the time I had set aside last weekend for reviewing answers for the forthcoming mod2/3 Signet event, which gave me the opportunity to do this.

It is not directly aimed at answering any specific exam question, but hopefully will provide a resource useful for tackling some of these; I have gathered together information from a range of documents and my own experience to attempt to shed some light onto an area of mystery for many. For the IRSE exam you only need an appreciation of the overview, so I have spared you 3 dimensional Maxwell equations- divs, grads, curls and all the rest of university maths that in truth I have all but forgotten

[Jerry1237]

Peter,

A good article. I would like to clarify one point though. The definition of
Electromagnetic Compatibility (EMC) is subtly different to that within the document. EMC is defined as the equipment will operate in its intended environment (railway is heavy industry with levels at 20V/m) and be suitable imune to emissions of other equipment operating in that environment as well as its own emissions being of a suitable level for that environment.

I would also suggest adding the design of CAT5 cable to show how the effects of cross-talk can be mitigated. Simply, the four twisted pairs are twisted at different rates with specific standards on how many of the twists can be unwound during installation plus cable length etc.

Earthing maybe a good addendum to this as well. Identification of how a poor earthing strategy can kill EMC and cause undesired behaviours.

Jerry

[AlastairHayden]

Very good article!

[PJW]

Thanks for your comments.
I take your point re EMC being specific to the environment regarding what is acceptable- I thought I had alluded to that when quoting the different values to which equipment is tested; a mixture of likelihood of being subjected to that level of E-M disturbance and the consequences of any malfunction that might result.

Have you found any formal definitions?; I have had a bit of a look and have only discovered the following:

BE EN 50121 states:
"This set of standards provides both a framework for managing the EMC for railways and also specifies the limits for the electromagnetic (EM) emission of the railway as a whole to the outside world and for the EM emission and immunity for equipment operating within the railway. The latter must be compatible with the emission limits set for the railway as a whole and also provides for establishing confidence in equipment being Fit For Purpose in the Railway environment. There are different stationary emission limits set for trams/trolleybuses and for metro/mainline railways. The frequency covered by the standards is in the range from d.c. to 400 GHz."
Rather surprisingly I haven't been able to find an actual definition of EMI / EMC within that suite of standards, (but life is too short to trawl the lot!)

London Underground standard 1-222 defines EMC:
The ability of a device, unit of equipment or system to provide immunity and function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances into that
environment It quotes that this is obtained from "SI 1992 No.2372" but I haven't followed that up.

Similarly I did actually mention a little about earthing and the need for a FTE in the context of EMI (although rather hidden away in other text I accept), but certainly the whole subject of earthing and bonding is another large "mystery black art" area which certainly overlaps EMI/EMC.

It was good you mentioned the CAT5 cable; to be honest I wrote this more thinkuing of trackside, traction interference and track circuits etc and only very sketchy of the subject inside control rooms etc.

So one day when I get more time perhaps I'll update and extend, but for now it must suffice........

Not yet done this directly, but have posted a related topic re lighning transient suppression in Module 5 area since it relates to a question asked in that module

Peter,

A good article. I would like to clarify one point though. The definition of
Electromagnetic Compatibility (EMC) is subtly different to that within the document. EMC is defined as the equipment will operate in its intended environment (railway is heavy industry with levels at 20V/m) and be suitable imune to emissions of other equipment operating in that environment as well as its own emissions being of a suitable level for that environment.

I would also suggest adding the design of CAT5 cable to show how the effects of cross-talk can be mitigated. Simply, the four twisted pairs are twisted at different rates with specific standards on how many of the twists can be unwound during installation plus cable length etc.

Earthing maybe a good addendum to this as well. Identification of how a poor earthing strategy can kill EMC and cause undesired behaviours.

Jerry

[PJW]

I have pleasure in attaching the aforementioned. Obviously the slides were made to illustrate a talk, but even if you weren't there they should be useful although you may need to do a bit more research around the subject if your basic electromagnetic theory is a bit rusty.

Also this appears to be a useful link; I can't vouch for accuracy of all information but certainly seems highly credible:
http://www.electrical-installation.org/w...r-measures

The London Study Group had a talk from an EMC expert recently on this topic; note that two of the 2010 module 7 questions were closely related to this subject.

I am hoping that he may permit me to share his Powerpoint presentation on this forum.