New Consumer Unit

[davesparks]

Trainee question! Would I be right in saying that although it's TN-C-S it might not necessarily be PME?

You are right in saying that yes, all PME supplies are TNCS but not all TNCS supplies are PME. for a start there is a system called PNB (protective neutral bonding) which has just one N-E connection via an earth electrode wheras PME has multiple earth electrodes.

True, but I think if you ring WPD they will say all TNC-S is now PME, I just call it the same, because it generally is as far as I am concerned.

That is a brilliant answer to give to a trainee asking a reasonable question!

 

[TJ Anderson]

You are right in saying that yes, all PME supplies are TNCS but not all TNCS supplies are PME. for a start there is a system called PNB (protective neutral bonding) which has just one N-E connection via an earth electrode wheras PME has multiple earth electrodes.

That is a brilliant answer to give to a trainee asking a reasonable question!

Is the earthing requirement different for PNB & PME? I cant see that it would be.

 

[HandySparks]

I have a consumer unit to change, the earth on the gas is easy enough but the water is in 6mm and literally the other end of the house and would be a nightmare running in a new cable

Just to add, it is green and yellow, so not like its just green.

Your average 1930's built house is going to have suspended floors downstairs and T&G floorboards both up and downstairs, so spring a few floorboards and run a 10mm bond to where you need it.

This is of course assuming you have already confirmed that bonding is actually required in the first place?

As davesparks says, see what the ground floor construction is. If suspended floor, then it may be easy. In fact, since the existing cable isn't the original you might find that you can use it as a pull through.

 

[davesparks]

Is the earthing requirement different for PNB & PME? I cant see that it would be.

No the earthing and bonding requirements are the same.

 

[uksparks]

Yes it could be split in the head, without measuring it I dont know.

The floor in the front room is suspended but the middle room and kitchen appears to be solid unfortunately.

yes I accept its not a very accurate answer to give a trainee.

 

[Leesparkykent]

Yes it could be split in the head, without measuring it I dont know.

The floor in the front room is suspended but the middle room and kitchen appears to be solid unfortunately.

yes I accept its not a very accurate answer to give a trainee.

Thats exactly how most of the older terraced houses round my way are as well. How will you test it to confirm it's TNS or TNC-S?

 

[davesparks]

Thats exactly how most of the older terraced houses round my way are as well. How will you test it to confirm it's TNS or TNC-S?

As a general rule :
TNCS - PEFC = PSCC
TNS - PSCC > PEFC

 

[spark 68]

How will you test it to confirm it's TNS or TNC-S?

By comparing Ze and Zp-N, if the readings are identical then it will be TNC-S, this is because of the CNE link in the head and the PEN conductor are common to the N and E.

 

[HandySparks]

By comparing Ze and Zp-N, if the readings are identical then it will be TNC-S, this is because of the CNE link in the head and the PEN conductor are common to the N and E.

Anyone tried checking for TN-C-S / TN-S by measuring the resistance between the incoming neutral at the first accessible point (assuming reasonably close) and the earthing terminal on the head?

Main switches all off, so no current flow and check for any voltage, just in case.

I would have thought that this might be a more reliable and direct way of telling? Clearly if there's any significant voltage or resistance between these points, then there's no local connection between the two.

 

[spark 68]

Anyone tried checking for TN-C-S / TN-S by measuring the resistance between the incoming neutral at the first accessible point (assuming reasonably close) and the earthing terminal on the head?

Main switches all off, so no current flow and check for any voltage, just in case.

I would have thought that this might be a more reliable and direct way of telling? Clearly if there's any significant voltage or resistance between these points, then there's no local connection between the two.

Well I check them at the same time, obviously because I am checking Ze the main switch is off anyway, and in domestic the tails are usually fairly short and I am testing from the live side of the mains switch, and with only a short run for the 16mm EC it makes no practical difference.

Bearing in mind this test (for TNC-S v TNS) is only really necessary on the odd occasion where it could be an either/or situation

 

[uksparks]

Thats exactly how most of the older terraced houses round my way are as well. How will you test it to confirm it's TNS or TNC-S?

as Dave says in #27. I did get caught out once by the nic bloke, to be fair it said pme on the head, but the readings were different and he insisted it was split concentric.

 

[spark 68]

I meant to add as we need to test Ze and PSSC then it makes sense to do these tests together, before reconnecting the EC and the doing your Zdb and the other PFC measurement.

Plus I would have done a Ze test at the pre-test stage before even quoting a price

 

[HandySparks]

Well I check them at the same time, obviously because I am checking Ze the main switch is off anyway, and in domestic the tails are usually fairly short and I am testing from the live side of the mains switch, and with only a short run for the 16mm EC it makes no practical difference.

Bearing in mind this test (for TNC-S v TNS) is only really necessary on the odd occasion where it could be an either/or situation

Sure, only required if it's not clear by inspection. But a while ago I saw a head that looked like TN-C-S, but when opened up (not by me), was clearly TN-S, supplied using split concentric underground cable.

Just thought that although different values of PEFC and PSCC say 'TN-S', similar values don't necessarily mean 'TN-C-S'. In which case, a direct resistance check may decide the matter.


Edit: And yes, agree, if testing PEFC (and Ze), you'd test PSCC (and Z line) at the same time, anyway.

 

[spark 68]

Yes agree HS, sometimes it is not clear, but then again I always carry out a few essential pre-tests before I give a price, which the OP clearly hasn't , one day he will regret not doing a few preliminary tests and checks before quoting and this will end up biting him on the arse one day, can you guess why I always pre-test now ?

 

[Tidy Max]

I always pre-test, it doesn't take long at all and is worth it.

 

[Simonslimline]

could well still be TNS even though it looks like TNC-S.

Same in the flat i live in. A service head in each flat with the MEC connected in to the side of the cutout. The cutout is fed from a split concentric from a TP service head in the basement which is PME.

 

[R G]

Hmm . not sure about TNCS/PME as already said test the Zs and Zp-n , I wouldn't be quoting until that and the preliminary testing of the circuits is done along with the usual visual checks , or you could get yourself in deep s--t .give them the quotation of all that you find..

 

[spark 68]

Same in the flat i live in. A service head in each flat with the MEC connected in to the side of the cutout. The cutout is fed from a split concentric from a TP service head in the basement which is PME.

It can get awkward on flats such as yours because the incoming supply DB can be TNC-S at the incomer, but each flat is technically TNS as a separate dwelling/installation in it's own right, although for bonding requirements and certification purposes are treat as TNCS/PME

I had some documentation from UKPN which goes into far more detail into these "hybrid systems", for us though they are treat as TNC-S according to UKPN.

 

[Simonslimline]

It can get awkward on flats such as yours because the incoming supply DB can be TNC-S at the incomer, but each flat is technically TNS as a separate dwelling/installation in it's own right, although for bonding requirements and certification purposes are treat as TNCS/PME

I had some documentation from UKPN which goes into far more detail into these "hybrid systems", for us though they are treat as TNC-S according to UKPN.

Yeah i have 10mm bonding in the bathroom to water pipes which are fed from a plastic incomer from the street. Never tested it to see if its actually extraneous though.

 

[spark 68]

Yeah i have 10mm bonding in the bathroom to water pipes which are fed from a plastic incomer from the street. Never tested it to see if its actually extraneous though.

These really blur the lines, because you only have one origin (the incoming supply) usually with one CNE link at the origin and separate earths at each flat, but then according to BS7671 each dwelling is classed as a separate installation, however the gas and water services are distributed and shared as well.
So we end up with a hybrid system, and the DNO make the rules, their supply, their rules.

If you search for the UKPN documents on line they are free to download, but they can be a bit boring to read lol.