EV TN system

I always thought that unless you had an intervening transformer TN-S referred to the whole system from source to final connected equipment.

Logically it does - but that hasn't stopped standard writers who perhaps should have known better coming up with phrases such as "The usual form of a TN-C-S system is as shown, where the supply system is TN-C and the arrangement in the installation is TN-S" (thankfully removed from the latest version of BS 7671 but has been there previously, and I've seen very similar wording on other standards too, so presume it was copied from somewhere common).


Perhaps Ireland has a less unreliable PME system than the UK?


   - Andy.

Yes Andy, that misconception is even perpetuated with a multiple choice question by City and Guilds in the 2391exam asking “what earthing system has TN-C as the supply and TN-S as the arrangement in the installation?” 

I can’t comment on the robustness of the PME installations in Ireland other than to say that in the new housing developments I have seen, significant efforts are made in the infrastructure to keep system neutral at earth potential.

I rather think, like other European countries, Ireland tends not to see loss of system neutral in the EV roll out as an issue of concern. That would seem to imply that it is the U.K. that has the unreliable system.

The use of the terminology in this way pervades IEC and CENELEC standards.


It seems the terminology is used in this way, rather than saying "don't re-combine neutral and earth downstream of a point at which they've been separated" type of statement.


Having said that, with prosumers electrical installations, we will need to reconsider how we use terms like "downstream" going forward !

I must say I disagree, I see no difficulty in saying the post-metering part of a TNC-s installation has separated neutral and earth (SNE in UK DNO-speak), and the pre-metering part has combined neutral and earth. (CNE in DNO-speak)  and defining TNCs that way.

If not we have to invent some new words.

It has in the past been suggested on this previous forum that we could extend the notation and use upper and lower case to distinguish the DNO and consumer side respectively, so TNCS has the separation in the street, so presents as TNS at the meter cupboard, while  TNCs has it at the cut out. I do not think that has caught on..

Mike.

I think the argument against this view, Mike, is that the letters describe the system, that is, the whole system, from transformer to the point in question, not just the installation. Based on that, it would be wholly incorrect to describe part of a TN-C-S system downstream of the TN-C element "TN-S"

If  describing the end to end system is always required/intended, then I would agree, but I'm not sure it always is, at least in the eyes of the standards writers.  I suspect they assume you can also use the same notation to describe a meaningful subsystem, such as just the  distribution, or just the consumer wiring, separately.

This interpretation works, and allows the addition of more information, if the whole system has more than  the two CPC arrangements. If not there is the potential to run aground if you wish to describe anything more complex, for example let us consider the earthing of a TT island derived from distribution with a grounded neutral .


That would need to be  written as  TNC-TT or TNC-S-TT or something for the whole end to end system, (first with no section using a neutral derived CPC, e.g. whole house TT,  and the second if we were talking about an island for an outbuilding perhaps on a TNCs main building) but normally I'd refer to it simply as "a TT installation" - but of course this only refers to the subsystem downstream of the CPC break.

I must admit the longer form would be more descriptive, and perhaps quite helpful, but I'm pretty sure that I've never seen it.

M.