TN-C-S or TN-S

[eveares]

So this is my supply at my new property and when I first saw during a house viewing back in July and before I gave it any real attention, I thought it was TN-C-S.

Then a EICR later on I had done said it was TN-S, making me think it was split concentric and TN-S due to where the main earth exited the fused cut out lower down and not higher up where I have seen it before online with TN-C-S , and then when I had my recent re-wire signed off last week, the electricians said it was TN-C-S not TN-S.

So is it TN-C-S (aka PME) or TN-S. I guess the only way to be sure is to contact the DNO, however the below may help determine.

When I measured Ze/PFC, I got 0.17Ω for Ze and 1.32kA for PFC. When I measured PSCC I got 0.15Ω and 1.58kA.

 

[plugwash]

It's very difficult to be sure a supply is TN-S unless you have a private transformer. You can't tell for 100% sure whether it's seperate in your supply cable without opening the cutout, and even if it's separate in your supply cable, it may be combined further upstream.

The fact the results for Ze and Zn are so similar (probably within measurement error for most meters) suggests some level of combination.

 

[Ragnar_AT]

Would a TN-S earth be the same size as the L and N? If not I‘d say it has to be TN-C-S considering how similar the impedance is.

 

[JohnW2]

It's very difficult to be sure a supply is TN-S unless you have a private transformer. You can't tell for 100% sure whether it's seperate in your supply cable without opening the cutout, and even if it's separate in your supply cable, it may be combined further upstream.

Indeed.

The fact the results for Ze and Zn are so similar (probably within measurement error for most meters) suggests some level of combination.

They are so similar that I would personally put my money on TN-C-S

 

[eveares]

Would a TN-S earth be the same size as the L and N? If not I‘d say it has to be TN-C-S considering how similar the impedance is.

It's very difficult to be sure a supply is TN-S unless you have a private transformer. You can't tell for 100% sure whether it's seperate in your supply cable without opening the cutout, and even if it's separate in your supply cable, it may be combined further upstream.

The fact the results for Ze and Zn are so similar (probably within measurement error for most meters) suggests some level of combination.

I might measure the loop impedance between Neutral and Earth (with all loads disconnected/turned off) when I am next up at the house and see if I get a reading very close to 0.00Ω, indicating a local bridge and thus TN-C-S.

 

[JohnW2]

I might measure the loop impedance between Neutral and Earth (with all loads disconnected/turned off) when I am next up at the house and see if I get a reading very close to 0.00Ω, indicating a local bridge and thus TN-C-S.

How do you intend to do that? Even if it is TN-C-S (which is what I would suspect) I suspect that induced AC in the neutral conductor might well upset resistance measuring devices. If it's TN-S, there's quite likely to be a non-trivial (again AC) pd between the N and E, which could well muck up any measurements.

[You presumably would have to disconnect all bonding, as well as loads?]

 

[ericmark]

My house is TN-C-S when on the DNO supply and TN-S when the supply fails. But does it matter which? Unless a caravan, boat, or petrol station can't see the problem.

 

[flameport]

TN-C-S (aka PME)

TN-C-S is the connection at the service head.
PME is what may exist within the network.

Older services can still be TN-S at the service head, but that reveals nothing about what may be connected elsewhere in the network.

The real question is whether the N&E are combined at some point(s) between the transformer and the service head.
In the vast majority of cases they will be, either with CNE cable throughout or sections of CNE where repairs and alterations have been done.

 

[JohnW2]

My house is TN-C-S when on the DNO supply and TN-S when the supply fails.

What do you mean by that - i.e. what sort of 'supply failure' are you considering/contemplating?

It certainly would not be "TN-S" if the 'failure' resulted from someone or something chopping right through the cable

 

[eveares]

How do you intend to do that? Even if it is TN-C-S (which is what I would suspect) I suspect that induced AC in the neutral conductor might well upset resistance measuring devices. If it's TN-S, there's quite likely to be a non-trivial (again AC) pd between the N and E, which could well muck up any measurements.

[You presumably would have to disconnect all bonding, as well as loads?]

First with all loads turned off on all boards, measure the voltage between the incoming N and E. Providing that is 0V or less than 4V what is the limit of voltage present when doing a continuity/resistance measurement on my MFT1730, measure the continuity/resistance between N and E. Of course omitting parallel paths like via the gas bonding.

Yes, I know in reality I am likely to get a potential >4V between N and E if TN-S

TBH, I may just leave it as is and presume TN-C-S, if not at the head, then somewhere upstream.

 

[eveares]

My house is TN-C-S when on the DNO supply and TN-S when the supply fails. But does it matter which? Unless a caravan, boat, or petrol station can't see the problem.

Just I am sure years ago (arguably when my knowledge was less) I read online somewhere that you can export a TN-S supply to a outbuilding such as a shed, but can't do so for a TN-C-S supply unless you either TT the outbuilding or export a 16mm CPC from the main board to the outbuilding due to the risk of broken neutrals and the danger that presents with the main CPC conductor becoming live in the event of a PEN fault.

Maybe things have changed since then?, but the above risk still makes sense in my head to me.

Oh and in my cause, there would be a AFDD at the main board upstream of the outbuilding/shed, question is would that AFDD still function correctly in the event of a PEN fault.

I should add, the shed would be about 4 or 5 meters away from the back of the house, so not a massive distance; although the shed would feed a small pond.

 

[eveares]

So in my situation, would you TT the shed or simply export the CPC via 3 core 4mm SWA to the Shed and possibly add a extra earth rod at the shed for extra protection?

 

[securespark]

I'm going to play devil's advocate here and say that from my experience, I'm not sure you can be certain what supply type it is from the figures.

I have had extremely low Ze readings from TN-S supplies.

Also, physically, I would expect the earth to exit the cutout higher up if it were attached to the N, IE TN-C-S.

I can't say exactly what cable type that is, but the earth leaving the cutout appears not to be the circular type, leading me to believe it could be the braided type.

But it is very difficult to tell from the photo.

 

[EFLImpudence]

So in my situation, would you TT the shed or simply export the CPC via 3 core 4mm SWA to the Shed and possibly add a extra earth rod at the shed for extra protection?

It usually comes down to whether a large bonding conductor is required from the outbuilding to the main supply.

 

[flameport]

I read online somewhere that you can export a TN-S supply to a outbuilding such as a shed, but can't do so for a TN-C-S supply unless you either TT the outbuilding or export a 16mm CPC from the main board to the outbuilding due to the risk of broken neutrals and the danger that presents with the main CPC conductor becoming live in the event of a PEN fault.

That contains some elements of truth but like many things that can be 'read online somewhere' it's also full of fails.

If a TN-C-S supply is used for some outbuilding, AND that outbuilding also has extraneous conductive parts such as a metal water pipe, then those parts require bonding to the installation earth terminal, and that bonding will need to be at least 10mm².
For a supply using SWA cable that means using a 3 core 10mm² as a minimum and using one of the cores as a combined CPC and bonding conductor.

The more sensible answer is to not have extraneous conductive parts in the outbuilding, such as using a plastic pipe for water supplies. No bonding required and the cable is sized for the load.

The more useless and often impossible option is to make the outbuilding TT, and not have any connection to the TN-C-S installation earth terminal.
That involves earth electrode(s) at the outbuilding, absolutely no connection to any items connected to the TN-C-S supply, which also means all water and other services must be in plastic, and the outbuilding must be located far away (and in reality that means 10 metres or more) from anything connected to the TN-C-S installation or any other TN-C-S installation such as neighbouring properties. Generally far more expensive and time consuming. In many urban areas impossible to achieve.