Earthing a bath

[JohnW2]

unless I'm missing something, nothing in 411.3.1.2 says that Main Protective Bonding is not required for e-c-ps which are not services entering the building; it appears to simply say that e-p-cs must have main protective bonding, without any (common sense) exclusions such as you are now suggesting.
....or am I missing the bit which says that?

411.3.1.2 calls for main protective bonding conductors complying with Chapter 54.
Read 544.1.2

You are again applying common sense (which I applaud), but it still amounts to making an interpretation. 544.1.2, which is obviously but a small part of Chaper 54, merely talks about MPB/MEB for an e-c-p which is a gas, water or other service which enters the premises; it says nothing about e-c-ps which aren't. Taking everything literally, that does not alter the fact that (without the application of common sense) 411.3.1.2 requires MPB/MEB for any e-c-p - it can still be compliant with Chapter 54 (particularly 544.1.1, relating to conductor sizing), even if it is not an e-c-p for which 544.1.2 is relevant.

You will have noted that we are not disagreeing about what, on the basis of a common sense interpretation of the regs, is actually required. I would hope that you will sooner or later admit that such common sense interpretation often is required, in order to avoid silliness - although I don't think I would dare to 'hold my breath'!

Kind Regards, John.

 

[ban-all-sheds]

544.1.2, which is obviously but a small part of Chaper 54,

544 is the only part of which deals with protective bonding conductors.

merely talks about MPB/MEB for an e-c-p which is a gas, water or other service which enters the premises; it says nothing about e-c-ps which aren't.

Then they don't require main protective bonding.

Unless you are going to claim that there can never, ever be any such thing as an e-c-p as talked about in 701.415, then at some time, when you do have something which you believe is an e-c-p, and you think that the omission of it from 544.1.2 is irrelevant then you are going to have to run a main bond back to the MET.

Whatever your definition is which can result in an e-c-p in a bathroom, have you ever seen anybody agreeing that it should have a main bond ran back to the MET?

Taking everything literally, that does not alter the fact that (without the application of common sense) 411.3.1.2 requires MPB/MEB for any e-c-p - it can still be compliant with Chapter 54 (particularly 544.1.1, relating to conductor sizing), even if it is not an e-c-p for which 544.1.2 is relevant.

You keep going on about common sense.

Surely common sense says that whatever hazards of an e-c-p are to be mitigated by bonding, then those hazards exist, and should be mitigated, whatever causes the part to be an e-c-p.

Read the definition of an e-c-p again. It does not qualify it according to how the part comes to be an e-c-p, it either is capable of introducing a potential into the location or it is not.

If it is, then whatever the mechanism responsible for that state of affairs, it is an e-c-p, and must therefore be dealt with as such.

To say that it's not an e-c-p because it acquires its potential by being connected via a length of cable to something which comes out of the ground rather than being directly soldered to something which comes out of the ground seems to be the exact opposite of common sense.

 

[JohnW2]

Whatever your definition is which can result in an e-c-p in a bathroom ....

It was actually you who introduced the concept of an e-c-p in a bathroom - a concept which I initially challenged, but then I retracted the challenge when I saw that the regs use the same terminology.

Whatever , have you ever seen anybody agreeing that it should have a main bond ran back to the MET?

Of course not, and I assume that almost rhetorical question implies that you feel the same. As I said before, I applaud you for applying common sense in interpreting the regulations.

Kind Regards, John.

 

[holmslaw]

..

 

[JohnW2]

Been using and reading this forum for many years and I have never seen any post from anyone that understands the reason for bonding of extraneous services. ..... Once you link these items you might realise why we could all be living in potentially dangerous houses with pme supplies and the 'correct' main bonding

As far as I can see, it's certainly the case that the requirement for main bonding arises primarily from the fact that DNOs have been allowed to supply (with TN-C-S) a 'protective' 'earth' terminal which can, under certain supply-side fault conditions, rise to a potential high enough to be a serious hazard in the presence of anything connected to a true earth (like metal service supply pipes) - main bonding seeks to reduce that hazard As I've said repeatedly, I really don't understand the logic of requiring main bonding in a TT installation.

In the pre-RCD days, I can see that the provision by DNOs of a low impedence return path was a valuable safety feature, allowing overload devices to provide short disconnection times in the presence of L-E faults in an installation. However, now that we have RCDs, I can't help feeling that we'd probably all be safer (and, to my mind, not needing main bonding) if we all had TT installations!

In fact (and I guess the answer is probably No), I wonder if it's permissible to ignore a DNOs PME 'earth terminal' and, instead, use their supply for a TT installation?

Kind Regards, John.

 

[securespark]

I believe that if a piece of copper pipe is, by some route or other, connected to something buried in the ground OR to the MET (which, of course, is itself connected to something buried in the ground) then that piece of copper pipe is introducing a potential, generally Earth potential, into the location.

Like a copper pipe connected to a damp external wall, the wall being buried in the ground?

 

[JohnW2]

Like a copper pipe connected to a damp external wall, the wall being buried in the ground?

Quite. In fact, just the damp wall alone is capable of 'introducing a potential' (and if it's damp, I guess it counts as a 'conductive part')

Kind Regards, John.

 

[bernardgreen]

In fact (and I guess the answer is probably No), I wonder if it's permissible to ignore a DNOs PME 'earth terminal' and, instead, use their supply for a TT installation?

The answer is YES you can ignore the PME "earth" and install as a TT.

That came from the DNO technical department which has quoted me for the new supply to a cottage that at present shares a meter and electrical installation with the next door shop ( they were recently one property but now divided into two ). I am buying the cottage so dealing directly with the DNO.

From that discussion :-

A CPC earth derived from an earth rod is often better than a PME earth

Walls that have a permant moisture content, such as stone walls in old buildings, have to be considered as extraneous conductive items that will import the ground potential into the building. This imported earth [true ground] may not be at the same potential as the earth imported from the PME supply.

Therefore in strict compliance with regulations the inside surface of a stone wall with permanent moisture content should be bonded to the MET of a PME installation.

 

[JohnW2]

The answer is YES you can ignore the PME "earth" and install as a TT.

Interesting, even if it surprises me a little. If I had a PME installation (which I don't), I might be tempted!

From that discussion (with DNO):-
A CPC earth derived from an earth rod is often better than a PME earth

I'm not totally sure what they mean by that. If they are talking about the impedance of the connection to true earth, then they may, of course, be right. However, in terms of disconnection times etc. what obviously matters is the loop impedance, which (with PME) is usually extremely low but is essentially a matter of the (neutral) return path to the supply. Although it is generally called the EFLI, true earth has really not got anything much to do with that with a PME installation.

Walls that have a permant moisture content, such as stone walls in old buildings, have to be considered as extraneous conductive items that will import the ground potential into the building. This imported earth [true ground] may not be at the same potential as the earth imported from the PME supply.
Therefore in strict compliance with regulations the inside surface of a stone wall with permanent moisture content should be bonded to the MET of a PME installation.

Indeed. However, the problem there is that, as far as I am aware, BS7671 does not make any distinction between TN and TT systems as regards main bonding requirements (apart from the matter of sizing of main bonding) - so, although it obviously makes no logical sense at all, if the walls are damp enough to be regarded as 'conductive', the regs would theoretically requiring them to be bonded to the MET (goodness knows how!) even in a TT system!

Kind Regards, John.

 

[Sickboy]

OK, I've been trying to follow this but as someone with very little understanding of electrics I am completely lost.

I may have confused the issue to begin with by using saying the pipes were earthed rather than bonded.

I haven't tested the pipes in any way. The bath is a replacement and the existing hot and cold pipes have an earth cable between them. I presume this means they are bonded?

The pipes are all now hidden from view but the tap is exposed and comes out from the wall. You can therefore touch the tap and the bath at the same time. The bath is steel but not in direct contact with any other metal.

All the pipes in the house are copper and I don't think there are any plastic connections in the pipework.

Should I be extending the earth cable from the pipes to the bath or not? Or do I need to get someone in to get the whole lot tested?

On a similar note, why is a bath treated any differently from the metal frame on a shower cubical which never has any bonding/earthing?

Thanks
John

 

[JohnW2]

OK, I've been trying to follow this but as someone with very little understanding of electrics I am completely lost.

I can understand that. The vast majority of this thread which has evolved is of no importance to your question. The answer, which I think you were given very early on, is that there is no requirement in the regulations for the bath to be bonded/'earthed'. The one possible sort-of exception might be if there were a metal waste pipe going from the bath and into the ground (in which case that pipe, not the bath, might need bonding), but that is most unlikely. I don't think you really need to worry about anything more than that, unless you're interested in the debate!

I may have confused the issue to begin with by using saying the pipes were earthed rather than bonded.
I haven't tested the pipes in any way. The bath is a replacement and the existing hot and cold pipes have an earth cable between them. I presume this means they are bonded?

This doesn't alter what I've said above but, out of interest, is there simply a cable between the two pipes, or does the same (or a different) cable go from one of the pipes to some part of the electrical installation (e.g. a socket, light or switch)?

The pipes are all now hidden from view but the tap is exposed and comes out from the wall. You can therefore touch the tap and the bath at the same time. The bath is steel but not in direct contact with any other metal.

The reason that bonding of the bath is not required is that, at least as far as the regulations are concerned, there is no way that it could 'aquire any potential' and therefore represent a hazard. You will have seen that some people (but NOT the regulations) feel that there is a very small risk of a hazard, in very unusual circumstances. I don't think you should concern yourself about that.

Should I be extending the earth cable from the pipes to the bath or not? Or do I need to get someone in to get the whole lot tested?

As above, provided there is not a metal waste pipe (which may then need bonding), there is no requirement for this in the regulations.

On a similar note, why is a bath treated any differently from the metal frame on a shower cubical which never has any bonding/earthing?

They aren't treated differently under the current regulations. None of them are required by the regulations to have any bonding.

Kind Regards, John.

 

[ban-all-sheds]

Interesting, even if it surprises me a little. If I had a PME installation (which I don't), I might be tempted!

Can't see why you couldn't equally not use a provided TN-S earth....

 

[JohnW2]

Interesting, even if it surprises me a little. If I had a PME installation (which I don't), I might be tempted!

Can't see why you couldn't equally not use a provided TN-S earth....

Agreed, I was really thinking of TN in general, but was continuing to talk in terms of PME since that is what holmslaw introduced. Mind you, I think that the potential hazards are appreciably greater with TN-C-S than with TN-S.

Kind Regards, John.