Probably a stupid question...

Yes I would agree with you and record this as a TN-S supply. 

tonydon said:

the earth and neutral must be bonded at the star point of the transformer secondary.

Have you actually checked that this is the case? It is quite possible (even likely) that it is expected that the neutral earth links are intended to be in your switchboard, especially if it's a single split bus board.

If you're on site I would talk to the designer of the LV system, who should be able to tell you, as they will have needed to know / decide.

Note that the armour may be only bonded to the gland plate (and thereon to the transformer tank) and not the neutral in the spill box... or indeed it might be deliberately isolated at one end to prevent circulating current.

Sometimes the N-PE link is in the first switchboard rather than on the transformer itself - but I agree it's normally TN-S (not least because if you receive the supply at HV and have private transformers, all LV side including the transformer secondaries will be part of the 'consumer's installation' as so prohibited from having a combined earth and N conductor by the ESQCR).

   - Andy.

Is the earth your downstream  loads connect to just returned to the armour, or is there an NE link in or near your switch gear ?

If the former, yes TNS, if not then probably PNB.

Mike.

"if you receive the supply at HV and have private transformers, all LV side including the transformer secondaries will be part of the 'consumer's installation' as so prohibited from having a combined earth and N conductor by the ESQCR)."

Interesting answer.  we have 2 subs the first was put in in 1954 with the original "college building" as it was then.

The second was installed about 15 years ago.  If I remember correctly we had to pay for the full installation.... so private??

Unfortunately we cant open the main isolator (1954) without disconnecting the entire site..... So, cant see if the PE and N are connected within the isolator....  Probably have to wait until the next time we have to turn the whole site off......

Thanks for the reply

if not then probably PNB.

There are several forms of PNB (depending on which way around the earthing of the N and the N-PE split are arranged) - so still quite likely to be TN-S.

TN-C-S can be with or without multiple earths - PME or PNB,

PNB can be with a section of combined neutral & earth conductor (TN-C-S) or separate all the way (TN-S).

(It gets more confusing when DNOs get involved as they often deem that "PME conditions" apply to what are actually PNB supplies, just so they can keep their options open for later conversion to TN-C-S /PME at a later date (e.g. when new consumers are added to an existing transformer)).

   - Andy.

AJJewsbury said:

There are several forms of PNB (depending on which way around the earthing of the N and the N-PE split are arranged) - so still quite likely to be TN-S.

TN-C-S can be with or without multiple earths - PME or PNB,

PNB can be with a section of combined neutral & earth conductor (TN-C-S) or separate all the way (TN-S).

(It gets more confusing when DNOs get involved as they often deem that "PME conditions" apply to what are actually PNB supplies, just so they can keep their options open for later conversion to TN-C-S /PME at a later date (e.g. when new consumers are added to an existing transformer)).

Just to clarify 100 % ... PME exists only in distribution systems for supply to the public, and nowhere else. The arrangements for which PME conditions apply are described in ENA Engineering Recommendation G12/4 and the ESQCR. They are TN-C-S and PNB arrangements in the public distribution network, and have to meet some specific criteria.

If you are a private LV network with TN-C-S earthing arrangement, it is not PME. In practice, though, most (but not all) private LV networks will have separate N and PE, except upstream of the primary switchboard, if the neutral is earthed at the primary switchboard, and upstream of the main switchboard being TN-C only effectively at the transformers or generators. GN8 refers to this as a PNB arrangement.

tonydon said:

Given that the transformer is just the other side of the wall in our switch room, and that the earth and neutral must be bonded at the star point of the transformer secondary.

Are we right in thinking the supply is TNS??? 

It may well be an "effective TN-C-S" system, as technically N and PE will be coupled at both transformers, and earth nest at one transformer, or the main switchboard.

However, in this case, PME conditions don't apply, and effectively from a BS 7671 perspective it's treated as a TN system that isn't PME ("TN-S" if you like, but perhaps not fully "TN-S").