Can anyone clarify what sort of earthing we have.

[ericmark]

The question I ask "Why does it matter if TN-C-S or TN-S?" it's a TN system and the rules with exceptions of cars, caravans, boats and petrol stations are the same for both types of TN supply. OK the ELI limits change between 0.8Ω and 0.35Ω but that's down to the DNO.

If anywhere in the supply after step down transformer neutral and earth share the same conductor it is a TN-C-S supply and the move from TN-C to TN-C-S does not have to be at the head so looking at the head we may be able to say with certainty it's TN-C-S but we can never say it's TN-S which is why when we fill out the installation certificate it states we can use the inquiry method as only way to be 100% that it's TN-S is to ask the supplier.

But I still ask why does it matter?

 

[JohnW2]

The question I ask "Why does it matter if TN-C-S or TN-S?" ... But I still ask why does it matter?

I think that the perceived hazards associated with, say, exporting a TN earth to an outhouse (the issue about which the OP says he is interested/concerned) are primarily in relation to TN-C-S, rather than TN-S, aren't they?

Kind Regards,
John

 

[aesmith]

I think that the perceived hazards associated with, say, exporting a TN earth to an outhouse (the issue about which the OP says he is interested/concerned) are primarily in relation to TN-C-S, rather than TN-S, aren't they?

Kind Regards,
John

That's my concern. The intention is to provide a couple of power points in each of two buildings, and these buildings have a lot of metalwork.

 

[aesmith]

Your situation, of a single customer being supplied by a transformer may be a sort-of exception (I'm sure that others will be able to talk more authoritatively about that) - but I would then be more comfortable with a second earth connected to (or very close to) my installation - just in case someone 'snipped' the neutral supply cable!

It may be a difference in regional practice, but this is the almost universal way of serving rural houses in NE Scotland - 11kV overhead lines, pole-mounted transformer supplying a single customer. Our last three houses had this arrangement. I will see if I can get an authoritative answer from DNO.

By the way I've found the references I remembered, it's under 5.6.4 "Special requiremements for PME-fed installations" in the Electrician's Guide.

Danger can arise when the non-current carrying metalwork of an installation is connected to the neutral, as is the case with a PME-fed system. The earth system is effectively in parallel with the neutral, and will thus share the normal neutral current. This current will not only be that drawn by the installation itself, but may also be part of the neutral current of neighbouring installations .

It follows that the earth system for an installation may carry significant current (of the order of tens of amperes) even when the main supply to that installation is switched off.

As said I am not clear if that is a normal concern, or only during a remote fault. Nor am I quite clear whether the concerns are significantly lessened where there's only a single supply, although it does seem that other supplies on the same LV circuit are part of the issue.

Tony S

 

[JohnW2]

That's my concern. The intention is to provide a couple of power points in each of two buildings, and these buildings have a lot of metalwork.

Quite. We're obviously talking about hazards which only really arise in the case of very rare supply-side faults(loss of N & E connections) - but that is much more likley (although incredibly unlikely!) to happen with TN-C-S. To lose N & E, but not also L (loss of which would obviously remove the hazard) would require an incredibly improbable happening to a TN-S cable!

Having said all that, in the situation you mention, I would personally favour not exporting the TN earth and, instead, TTing the outhouses - which, apart from anything else, avoids the need to main bond the metalwork in the outhouses back to the installation's MET.

Kind Regards, John

 

[JohnW2]

By the way I've found the references I remembered, it's under 5.6.4 "Special requiremements for PME-fed installations" in the Electrician's Guide.

Danger can arise when the non-current carrying metalwork of an installation is connected to the neutral, as is the case with a PME-fed system. The earth system is effectively in parallel with the neutral, and will thus share the normal neutral current. This current will not only be that drawn by the installation itself, but may also be part of the neutral current of neighbouring installations . .... It follows that the earth system for an installation may carry significant current (of the order of tens of amperes) even when the main supply to that installation is switched off.

Yes, but as I keep saying, I think they are using "PME-fed installation" rather sloppily - I think they actually mean TN-C-S (which, 'nearly always', is PMEd). The hazard to which they refer results from TN-C-S, not from the multiple earthing (the purpose of which is,as I've said, to reduce those hazards).

Kind Regards, John.

 

[dhutch]

Sound like a concentric supply cable.

Sounds like SPLIT concentric to me, and TN-S.

Split Concentric

And the white and green strands are?


Daniel

 

[EFLImpudence]

I would personally favour not exporting the TN earth and, instead, TTing the outhouses - which, apart from anything else, avoids the need to main bond the metalwork in the outhouses back to the installation's MET.

Isn't the main reason for not exporting the TN earth nearly always cost and convenience?

The requirement for TNC-S meaning large bonding conductors are required.

When do you think it would be electrically favourable to TT an outbuilding?

 

[JohnW2]

I would personally favour not exporting the TN earth and, instead, TTing the outhouses - which, apart from anything else, avoids the need to main bond the metalwork in the outhouses back to the installation's MET.

Isn't the main reason for not exporting the TN earth nearly always cost and convenience? ... The requirement for TNC-S meaning large bonding conductors are required.

Indeed - that was the point I was making (which is why I referred to TN earths in general, not just TN-C-S).

When do you think it would be electrically favourable to TT an outbuilding?

I think we've been over this ground many times before. In the case of TN-S, in which case the chances of the potential of the exported earth ever being dangerously above true earth potential are (IMO) 'vanishingly small', I doubt that there is ever an electrical reason for not exporting the earth. With TN-C-S, if one wants to cater for those very rare supply-side faults which could result in the exported earth being significantly/'dangerously' above true earth potential, then it's rather different. If, for example, the infamous hypothetical person standing on the surrounding soil with bare wet feet is able to touch bonded metalwork (frame of a metal greenhouse, metalclad light switch just inside door etc.), then there is clearly a potential (tiny) hazard, which wouldn't exist if the outhouse was 'isolated' from the TN-C-S earth and locally TTd.

Kind Regards, John

 

[westie101]

And the white and green strands are?

Fillers with no metal in them that help to keep the N & E seperated

 

[dhutch]

Fillers with no metal in them that help to keep the N & E seperated

Ahh, cunning. Thanks for putting my curiosity at bay!

 

[aesmith]

Yes, but as I keep saying, I think they are using "PME-fed installation" rather sloppily - I think they actually mean TN-C-S (which, 'nearly always', is PMEd). The hazard to which they refer results from TN-C-S, not from the multiple earthing (the purpose of which is,as I've said, to reduce those hazards).

I still feel there are differences, firstly because the hazard will only arise in the course of a fault within my own supply, and secondly if it does occur it will mean everything stops working so it will be apparent that something's seriously wrong. The fault condition wouldn't persist unnoticed. I guess we'll have to disagree over whether these differences are significant or not. I suppose your point may be that the hazard needs to be designed for, even if the chance and potential duration are much less, and that view has merit.

I'm now considering whether I could side-step the whole issue for the larger building, because the workshop area is going to be lined anyway, and it may be possible to make contact with the metal frame impossible.

Tony S

 

[JohnW2]

Yes, but as I keep saying, I think they are using "PME-fed installation" rather sloppily - I think they actually mean TN-C-S (which, 'nearly always', is PMEd). The hazard to which they refer results from TN-C-S, not from the multiple earthing (the purpose of which is,as I've said, to reduce those hazards).

I still feel there are differences, firstly because the hazard will only arise in the course of a fault within my own supply, and secondly if it does occur it will mean everything stops working so it will be apparent that something's seriously wrong. The fault condition wouldn't persist unnoticed. I guess we'll have to disagree over whether these differences are significant or not.

I don't really understand what 'differences' you're now talking about - perhaps you could clarify?

All I've been saying is that what (very rare) hazards exists arise because of the TN-C-S, regardless of whether there is one earth or multiple earths (i.e. 'PME'). Multiple earths reduce the chances of a supply-side fault on a TNS neutral 'causing problems' (which is why, generally, it is compulsory to PME a TN-C-S supply). What are you talking about?

I'm now considering whether I could side-step the whole issue for the larger building, because the workshop area is going to be lined anyway, and it may be possible to make contact with the metal frame impossible.

As I said, the 'surest' solution is probably not to export the TN earth at all - but, instead, to TT the outbuildings.

Kind Regards, John

 

[aesmith]

I don't really understand what 'differences' you're now talking about - perhaps you could clarify?

Maybe I'm on the wrong track altogether. The differences I perceived were that in a multiple customer PME supply my earth potential could be affected by a fault on another customers supply, and this could persist even if our own supply was switched off. I can't see that either of those would apply on a single customer TNC-S supply, but I'm happy to have it explained if in fact they could.

It also seems to be suggested that on a multi customer PME supply there could be significant currents carried by bonded items even in normal operation. I have to say that the IET documents that I read do not 100% say that this occurs in the absence of a fault. Again I would be happy to be corrected if this applies also in a single supply with a single E/N connection.

Tony S

 

[JohnW2]

Maybe I'm on the wrong track altogether. The differences I perceived were that in a multiple customer PME supply my earth potential could be affected by a fault on another customers supply, and this could persist even if our own supply was switched off.

The potential issue is that of a fault in the neutral supply to both you and other customer(s) - leaving your installation's neutral and earth connected to the other customer's neutral and earth, but with neither connected to the supply neutral. As I've been saying, that potential problem can arise with any multi-customer TN-C-S supply, and that would still be the case even if there were no 'multiple earths' (i.e. 'PME]) (although not to have PME would not be allowed, at least in England).

I can't see that either of those would apply on a single customer TNC-S supply, but I'm happy to have it explained if in fact they could.

If there are no other customer's connected to the sample supply, then they are out of the picture. However, if you're the sole customer on the supply, if that supply neutral breaks, the potential of your installation's neutral (and 'earth' {CPCs} - hence metal electrical accessories and any bonded pipework etc.) will try to rise to close to the full supply voltage (i.e. nominally 230V) above 'true earth' - with only the path (if any) to true earth via any bonded items opposing that.

It also seems to be suggested that on a multi customer PME supply there could be significant currents carried by bonded items even in normal operation.

That is true, but it is also true with a single-customer TN-C-S supply. The path to earth back to the transformer via bonded items is in parallel with the 'proper' route via the supply N/E conductor - the return current to the transformer will therefore be shared between the two paths. If, as can happen, bonded water supply pipes provide a very low impedance path, a substantial proportion of the total 'neutral' current can flow via that route - hence the requirement for larger bonding conductors with TN-C-S installations.

Kind Regards, John