New Consumer Unit

[uksparks]

Hi,

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.

Would you be happy changing it and leaving a 6mm in? Im going on Tuesday to do some pre work tests etc so can have another look.

Ill be quite honest, in all the consumer unit changes I have done, I have always updated the earth to 10mm as its never been an issue, but this is the first one where its going to be a complete pain in the --- doing it.

Whats your thoughts?

 

[uksparks]

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

 

[Leesparkykent]

What's the earthing arrangement?

 

[VoltzElectrical]

If it was good enough for previous editions of the regs....

 

[uksparks]

I 'think' if i remember correcly it was TNS.

 

[uksparks]

Like I say, I always update it, but never had this predicament before, quite honestly its never going to be a problem, more chance of the commet in 2022 hitting earth.

 

[Octopus]

I looked at one today where running a cable from the CU to the incoming water would mean going round most of the house, yet behind the wall from the CU is the cloakroom with a nice copper water pipe - makes me think that sometimes thinking outside the box is sensible!

 

[uksparks]

Tell a lie, Ive just found a picture and it looks like its PME not TNS.

 

[uksparks]

Unfortunately this is a mid terrace, quite an old property, probably 1930 I think, no access either side due to neighboring houses.

Its kind of a non starter, plus the bathroom and kitchen are both downstairs at the back of the house which is quite deep front to back and no water at all in the rest of the house, so would literally have to go from front to back and probably trash half the house in the process.

 

[Leesparkykent]

I 'think' if i remember correcly it was TNS.

IMO it's happy days as long as no sign of mechanical/thermal damage on the 6mm. But if there is a cold water supply nearer the DB like a downstairs WC or the like and wouldn't be to much disruption I would still bang in a 10mm and mark it down on the cert that the new MEB to water is not within 600mm of the water stopcock but existing 6mm is.

 

[Des 56]

Perhaps its best to keep in mind the reason for the bond
Dumping it where it happens to suit may not achieve what it is installed in the first place to provide

If you do leave the 6mm in situ,there is guidance on the Esc site regards this matter

 

[TJ Anderson]

Like I say, I always update it, but never had this predicament before, quite honestly its never going to be a problem, more chance of the commet in 2022 hitting earth.

You are right it won't be a problem. I would just note on EIC, 6mm Main Protective Bonding Conductor - No signs of thermal damage. Also your Gas is in 10mm, Cross bonded to water through boiler etc. It has been fine for years like it is, and you are leaving installation safer than you left it by adding RCD protection. Crack on with it mate.

 

[Octopus]

Bonding BS 7671 rule 131.8 (BRB) is now 132.16 (BGB) - don't know what it is in the (BYB)

 

[Resu]

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

 

[Deleted member 38070]

id put a 10mm in, even if i had to go into roof space and back (maybe 16 if that log a run) itll be worth it in the end (you know you want to ;-)

 

[uksparks]

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

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.

 

[Resu]

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.

Hey, I don't doubt it mate, was just an observation!

 

[Leesparkykent]

Unfortunately this is a mid terrace, quite an old property, probably 1930 I think, no access either side due to neighboring houses.

Its kind of a non starter, plus the bathroom and kitchen are both downstairs at the back of the house which is quite deep front to back and no water at all in the rest of the house, so would literally have to go from front to back and probably trash half the house in the process.

View attachment 27753

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

 

[davesparks]

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?

There are a lot of 1970/80's terraces and wimpey homes around here which follow the basic layout of gas and electricity by the front door and water main under the sink at the rear of the house with 6mm main bonds. The water main is however a black plastic pipe and rarely actually requires a main bond in the first place so I am normally perfectly happy leaving it as it is.

 

[7029 dave]

Looked at the pic, IMO if the 6mm bond is mech okay and no thermal damage leave it. Measure it as well for piece of mind. (you know the value you are looking for.)

 

[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.