PME vs TN-C-S

PNB is also (most of the time) PME in a public distribution network,

If it helps understanding at all (or just reduces the opacity of the soil water emulsion slightly), my head distinguishes between PNB being PME and PNB should be treated as if it might be PME.


In my head, PNB clearly can never be PME - any system has a certain number of points where the N is deliberately earthed - if it's one it cannot be PME, if it's greater than one it cannot be PNB. The definitions do not overlap.


On the other had we have the problem of DNOs changing things - what was PNB one day might be converted into PME the next - or indeed what was originally a pure TN-S system being converted into PME - usually entirely unbeknown to the consumer, The lead sheath earth at home is as likely to be PME as TN-S (probably more than likely these days) - most of the time we simply can't tell. So the advise is to treat any DNO earth facility as the worst case - i.e. as if it were (or could soon become) PME. You might think of this as being where "PME conditions apply" rather than the earth terminal actually being PME,


   - Andy.

Also PNB might be either TN-S or TN-C-S - depending on where the N-PE link is compared to the source electrode connection.


If the N-PE link (or links) is downstream of the source electrode's connection to the supplier's N conductor then it's TN-C-S and you have the risk of some diverted N currents (whether you have one customer or many).


If the N-PE link is upstream of the source electrode's connection to the supplier's N conductor then it's TN-S and should be immune to divertedn N currents even if there are multiple customers (like a TN-S supply to a single customer with 4 submains).


   - Andy.

gkenyon:

Chris Pearson:

Can it ever be PME if there is only one earth rod?

Yes - at the very least PNB. See G12/4, Clause 4.11. The reason PME conditions apply if there is only one electrode is that there can be fortuitous electrodes such as extraneous-conductive-parts. Since up to 4 customers can be supplied with the arrangement, diverted neutral currents can flow through these ...

This seems to be the operative sentence:

Earth terminals provided using PNB shall be treated in all respects as PME earth terminals.

 

AJJewsbury:
In my head, PNB clearly can never be PME - any system has a certain number of points where the N is deliberately earthed - if it's one it cannot be PME, if it's greater than one it cannot be PNB. The definitions do not overlap.

Same in my head! The get-out is "treated as".


My confusion stems in part from the fact that my step-daughter's house is PNB (only earth is close to the transformer) but well over 40 m from the transformer with presumably a TT installation at the end of the line some distance away. Officially the DNO says that it is PME.

For declared PME systems, it's also worth bearing in mind, that connection of the PEN conductor on one feeder circuit, to the PEN conductor of another circuit (even if that's from a different transformer) is quite normal as it reinforces the earthing of that conductor.


This interconnection may well be used instead of bashing in more electrodes (particularly at the end of circuit positions)


Borrowed Neutral, anyone 


Regards


OMS

It might be less worrying to think of it as a borrowed earth. It is both after all.

Sure - I think the phrase we would use is "reliably at or around earth potential"


It was just a bit of an insight into how PME services may be delivered in practice by a DNO or similar, for compliance with ESQCR


Regards


OMS