TN-C-S supply - worth adding an earth electrode?

[JohnW2]

The provision of an earth electrode is the absolute responsibility of the DNO. It does not form part of the customer's installation as it is directly connected to the neutral within the cut-out. If this is in place the supply can and should be treated as TNCS.

Thanks. That all makes total sense. I can also fully understand that (once earthed) it 'can and should' be treated as TN-C-S. However, my inquisitive mind still wonders whether it actually is (in terms of definitions) TN-C-S?

Perhaps more to the point in relation to some of the disussions here, I presume that (since 'M' would be only 1), although effectively TN-C-S, this is not regarded as PME? That could, as EFLI has been saying, be relevent in relation to the BS7671 requirements for the minimum CSA of main bonding conductors.

Since it ends up as 'effectively TN-C-S' as far as the installation is concerned, do I take it that the only reason PNB is used is to avoid the problem of separating (or combining) HV and LV earth electrodes in the vicinity of the transformer?

Kind Regards, John

 

[westie101]

Since it ends up as 'effectively TN-C-S' as far as the installation is concerned, do I take it that the only reason PNB is used is to avoid the problem of separating (or combining) HV and LV earth electrodes in the vicinity of the transformer?

Generally yes.

 

[JohnW2]

I have only once come across a TN-C-S supply where the neutral was lost and it resulted in the 4mm² earth wire feeding my friends shed to melt due to the electrician who had wired it bonding exposed-conductive-parts and extraneous-conductive-parts which included an earth mat connected to earth bar used for a radio transmitter. Clearly the shed supply should have been TT but it did point out one of the problems when earthing extraneous-conductive-parts.

True, but that's a very extreme case. The earth mat/bar must have been extremely good. I'm pretty sure that it would take at least 100A to melt a 4mm² conductor, hence would require an earth resistance <2.3&#937; even if a full 230V appeared on the neutral. I would suspect that 'ordinary' domestic TT electrodes are probably not that commonly much under 50&#937; - which would never allow enough current to flow to melt even a 0.5mm² conductor!

Never been involved with LPG it would be interesting to hear how that is bonded and piped to the house. That is about the only case where I can see extraneous-conductive-parts would need bonding and I just don't know how this is done.

I'm not sure what you're asking. My LPG tank is about 45 metres from my house, and most of the underground pipe run is in plastic. At each end (tank and house) there are short lengths (probably no more than a metre or two) of partially buried metal pipe (most of the buried bit having been wrapped in something 'black and sticky'!). At the house end, the metal pipe is main bonded where it enters, just with any other service. At the tank end, neither the emerging pipe nor the tank itself are earthed, but when we get gas deliveries a bonding cable (maybe two) are connected between the tanker and the tank during the delivery.

Were you perhaps thinking of a situation (unlike mine) in which there was continuous metal pipe from the house to the tank? If so, I'm not sure that they do that these days, except when the entire pipe run is above ground (in which case it might not count as an extraneous-c-p, anyway).

Kind Regards, John

 

[WellKnownSid]

Quick update...

Took the cover off the cutout this morning - definitely CNE (before Westie has a heart attack... this is Scotland, and the electrician had already agreed that the seals could be removed)

On closer inspection, the pole does have two earth conductors - one which forms the neutral with the outer of the concentric, this has a removable link at low-level on the pole.

The second is the HV earth from the can of the tx running down the opposite side of the pole. So, a 10 inch separation - not 10 metres!

Then the DNO (Hydro) chap turned up... 0.2 ohms ELI, happy as everything will now be to the 17th edition anyway with new boards / RCDs / RCBOs. Discussed my concern about only a single earth - he agreed, whilst it wasn't technically their responsibility (which seems to end with the ELI reading), it wasn't ideal either.

The conclusion we reached was that an extra earth at the house would "be advisable", given the location, so he's asked the electrician to pop into their office tomorrow and they'll provide him with up an earth rod, some more 16mm^2 cable and also another earth block as there is only limited capacity at the cutout.

 

[westie101]

Works for me (and yes I knew that some electricians in Scotland are trained authorised to work on cut-outs)

 

[JohnW2]

Took the cover off the cutout this morning - definitely CNE

Interesting.

On closer inspection, the pole does have two earth conductors - one which forms the neutral with the outer of the concentric, this has a removable link at low-level on the pole. ... The second is the HV earth from the can of the tx running down the opposite side of the pole. So, a 10 inch separation - not 10 metres!

Are you sure that, although the two conductors are only 10 inches apart as they come down the pole, the electrodes to which they are connected are also only 10 inches apart?

Then the DNO (Hydro) chap turned up... 0.2 ohms ELI, ... Discussed my concern about only a single earth - he agreed, whilst it wasn't technically their responsibility (which seems to end with the ELI reading), it wasn't ideal either.

As I understand it, what you have now discovered is that you have TN-C-S without PME, which I didn't think was 'allowed' in the UK. Westie? Is there perhaps an exception when only one property is supplied?

Kind Regards, John

 

[westie101]

which I didn't think was 'allowed' in the UK. Westie?

Or did the team that installed the present arrangement make a mistake?

 

[JohnW2]

which I didn't think was 'allowed' in the UK. Westie?

Or did the team that installed the present arrangement make a mistake?

Maybe - although the DNO man now seems to be regarding it as acceptable, although "not ideal". Are yiu confirming that I'm right in saying that it's 'not allowed in UK' (or is Scotland perhaps different?!)?

Kind Regards, John

 

[westie101]

Are yiu confirming that I'm right in saying that it's 'not allowed in UK' (or is Scotland perhaps different?!)?

It is not an arrangement I have come across but, to be honest, I would view it as little different electrically than PNB and certainly as far as I can see it introduces no additional hazards.
To all intents and purposes PNB would be treated as a PME supply