When TN-S is TN-C-S

[pc1966]

On another thread there was chat about the N-E voltage to be expected, so I decided to measure my flat out of curiosity as it appears to be TN-S.

A brief history - my flat is a Victorian built 4+4 arrangement and when I moved in many years ago each flat was fed by MICC cables. Something like a decade or so ago the DNO replaced all of that and installed a new panel in the communal area with each flat's cut-out replaced and fed by split concentric cables. So from a casual inspection of each flat you have a red link (no fuse in cut-out) and they appear to be TN-S supplies as earth is separated.

What I found from measurements on L-N on an unloaded spur circuit at the 1.6A of general background stuff on, and at 11.6A if I put on the oven as a test, was a neutral impedance of around 22mR and almost zero residual voltage (i.e. if I remove the current my own flat is taking I get around 3mV residual, around measurement error) so it clearly is TN-S-C at the common distribution board downstairs. (Yes, it could be very good balance out to the street, etc, but seems unlikely as it hardly changes going in to tea time and cookers, etc, going on).

So clearly they are not TN-S from this! Now in my case the extraneous bonding (gas and lead water pipe) is 16mm so well within rating, as as I am a few floors up I don't have any reason to fear PME fault, except over-voltage from phase imbalance trashing stuff.

So the question (if you can call it that) is why make the flats look TN-S by supplying separated earths, if you are actually PME in the local distribution system?

 

[telectrix]

are the N and E of the split cons. connected together at the cut-out?

 

[pc1966]

are the N and E of the split cons. connected together at the cut-out?

No, they are split out before it enters the cut-out.
[automerge]1589131202[/automerge]
The implied neutral resistance I see (22 mOhm) and the 16mm size (as far as I can measure on an off cut I got) point to around 16m length, which sounds about right for 3 floors and a short run to the box, so it really looks like N & E are common there.

 

[Wilko]

Hi - my thought is - historically the original supply will have been TNS but the DNO cabling etc has since been converted to PME.

 

[ipf]

The supplies to the flats were done by a contractor rather than the DNO, is one possible explanation. Hence, why not.

 

[Lucien Nunes]

Is there a BNO? If so, they might have specified it, rather than the DNO, on the basis of the existing arrangement or as Wilko says, the DNO not having committed to TN-C-S at that stage. If they put in split con, they were covered in either case.

 

[pc1966]

No, there is no BNO it is simply under the DNO (SSE here).

The main distribution panel also has a heafty earth (not checked close up but visually looks around 50mm) as well as the SWA cable, another reason I assumed it had to be TN-S all the way.

 

[davesparks]

That hefty earth will almost certainly go to a hefty earth electrode, or to the lead sheath of the original supply cable.
The SWA will probably not be SWA but wavecon cable, DNO's don't normally use SWA.

DNO's can provide TNS from a network that has been partially converted to TNCS subject to the correct earth connections being made, and a long length of old lead cable is one acceptable earth electrode for them.

 

[pc1966]

I guess that leads on to the next question (or subject to debate on this forum for those with time on their hands): how should this be reported on a minor works/EICR/etc for the flat?

An inspection of the flat would probably conclude the same as I initially did, that it is TN-S at the property's feed.

But of course it really is TN-C-S as N&E are joined at places other than solely the feed transformer. An inspection in the local distribution panel would show that I presume (as a N-E link being present), as well as revealing the fuse size (I suspect 60A, but no idea), but that would not normally be possible unless the DNO were out for something at the time.

Discuss!

 

[R-fur]

Classic TNS where the neutral is earthed only at the star point of the transformer is a myth. In reality the DNOs use a system called MEN multiple earthed neutral where the neutral is earthed at every joint and interconnected at every pillar. From this system they can give you a (TNS) or (TNCS) supply.