is this TN-S or PME?

[THRIPSTER]

TN-S?

• THRIPSTER
• 28 May 2013
• 1

What configuration is this?

TN-S?

• THRIPSTER
• 28 May 2013
• 1

Configuration photo two

The green, sweated cable connects to the MET. The braid, connected immediately to the RHS of the cut out goes to the Henley - there is no other connection within the Henley. The Henley has a label which states PME - I doubt that tbus is correct - what do you think?

Regards

 

[westie101]

I'd say definitely TN-S

 

[THRIPSTER]

Cannot understand why somebody would go to the trouble of connecting the (later) braid to an unused Henley and label it as PME?I mean , the braid isn't even connected to the neutral - who would do such a thing and to what purpose?

Regards

 

[wundaboy]

Edit: should have read OP first.

Does the Ze test okay?

 

[ericmark]

Although it looks like TN-S the definitions mean if some where between the transformer and the consumer the neutral and earth share the same conductor then it's classed as TN-C-S the point where it separates does not have to be in the consumers premises.

Hence why it's down to the DNO to tell you what it is.

 

[ban-all-sheds]

What if that point is a mile from the consumer and a foot from the transformer?

 

[JohnW2]

What if that point is a mile from the consumer and a foot from the transformer?

As we've discussed before, AFAIAA it would still technically be TN-C-S - although I'm not at all sure that it would be appropriate, helpful or reasonable to regard it as such! Having said that, I suppose that a break in that one foot of CNE would result in the same sort of 'lost neutral' issues as people fear with TN-C-S supplies (potentially worse, due to the absence of formal PME).

Kind Regards, John

 

[westie101]

Sorry guys but some are a bit off the mark here.

1/ It is possible to have a TN-S (SNE) service fed from a TN-C-S (PME/CNE) main

2/ It is possible in certain conditions to insert a length of CNE cable in an SNE main, all services past that point and still fed from the SNE cable will still be TN-S.
This would be the situation with that one foot of cable, there are other network conditions that need to be met

Whilst you may wish to discuss theory, the above is practice

 

[JohnW2]

Sorry guys but some are a bit off the mark here.
1/ It is possible to have a TN-S (SNE) service fed from a TN-C-S (PME/CNE) main
2/ It is possible in certain conditions to insert a length of CNE cable in an SNE main, all services past that point and still fed from the SNE cable will still be TN-S. This would be the situation with that one foot of cable, there are other network conditions that need to be met

I guess it's a question of which 'mark' one may be off. What you state obviously must be based on the definitions you work with. Other definitions (I think including BS7671- I don't have my copy to hand) take TN-C-S literally - i.e. if the N and E are combined at some point and then split (anywhere), it's regarded as TN-C-S.

However, the terminology is largely an irrelevance. What you say is the thing of practical importance - that what a DNO describes as TN-S may have a CNE segment somewhere along its path. This could be of some interest to those who worry about 'lost neutrals', since they might have to extend their worries to 'TN-S' (without PME) as well as TN-C-S/PME supplies!

Kind Regards, John

 

[THRIPSTER]

Looks to me as if the DNO has been asked to PME but only done part of the job - cannot think why anbody would bother to fit braid with a clamp to the incomer and then terminate in an otherwise, unused Henley. Will have to ask supplier what they say it is - will update in six weeks or so.

Regards

 

[ban-all-sheds]

Other definitions (I think including BS7671- I don't have my copy to hand) take TN-C-S literally - i.e. if the N and E are combined at some point and then split (anywhere), it's regarded as TN-C-S.

In that case there's no such thing as TN-S.

 

[JohnW2]

In that case there's no such thing as TN-S.

In terms of 'that' definition of TN-C-S, I think that's precisiely the conclusion we came to last time we discussed this! However, that's largely 'academic'.

"Seriously, though", whilst there will inevitably always be places close to the transformer where there is technically a CNE, most of us would probably not get too excited about that, or regard it as precluding an ('essentially') TN-S supply. However, if I understood him correctly, westie seemed to be saying that it is acceptable to have segments of CNE anywhere along the length of a TN-S supply - which I do find a little more surprising.

Kind Regards, John

 

[westie101]

However, if I understood him correctly, westie seemed to be saying that it is acceptable to have segments of CNE anywhere along the length of a TN-S supply - which I do find a little more surprising.

I said "It is possible under certain conditions" which is not "anywhere"

 

[JohnW2]

However, if I understood him correctly, westie seemed to be saying that it is acceptable to have segments of CNE anywhere along the length of a TN-S supply - which I do find a little more surprising.

I said "It is possible under certain conditions" which is not "anywhere"

Fair enough. Is it common to have a CNE segment, other than very close to the transformer, in a TN-S service?

Kind Regards, John

 

[westie101]

Tut, tut John

Read what I wrote

CNE cable in an SNE main

we cannot do this in a service unless we convert it.