Earthing a bath
- Thread starter Sickboy
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That's the reason why, in general, protection in TT installations has to come primarily from RCDs, since TT earth impedances are such that disconnection time requirements (for MCBs/fuses) probably can't be achieved.
However, in the present context, it's not as simple as that. The 'dispensation' in the regs for not having supplementary bonding in bathrooms requires (amongst other things) both that there be RCD protection AND that the (earth fault current) disconnection time requirements of the regs are satisfied - and, as I said, I don't think the latter is going to be achieved with a TT installation. I presume that means that Supplementary bonding usually still is required in TT installations.
Kind Regards, John.
And there we have it.
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.
I understand that the Zs on a TT is not typically low enough to trip an MCB.
But if the RCD operated within the required disconnection time on, say a L-E fault in a TT, then surely that requirement is then satisfied?
But if the RCD operated within the required disconnection time on, say a L-E fault in a TT, then surely that requirement is then satisfied?
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Indeed, I think it effectively impossible that this would ever be achieved. The arithmetic is simple enough. A 50Ω earth fault loop impedance would be considered quite good for a TT installation. At 240V, that means that a L-E dead short would only result in a current of 4.8A. That, in itself, would obviously never trip even a 6A MCB (and certainly not a higher-rated one), let alone within the required disconnection time.
Yes, you are right, and I was wrong. Although the text of 411.2.3 seems to imply that they are talking only about protection by an overcurrent device (i.e. MCB/fuse), Note 2 to Table 41.1 seems to indicate that achievement of the disconnection times by RCD is acceptable - which, as you imply, makes sense. Apologies for any confusion.
Kind Regards, John.
It's actually dawned on me that we are probably again having a semantic argument which is probably of no great importance (so long as we all understand what we're talking about), but,to respond ....
Consider a situation in which there were no incoming metal supply pipes (possible, if service(s) entered the building as plastic). By your definition/logic above, the pipework wholly contained within that building would still be deemed to be 'liable to introduce a potential' because it almost certainly would be connected to the MET (by CPCs, if not also any explicit bonding). Is that your belief?
In the situation I have described, the reality is that only conductor entering the building that was 'liable to introduce a potential, generally earth potential' would be the DNOs 'earth' itself - and it would obviously be silly to suggest that this needs Main Bonding to itself.
However, I have one admission/U-turn to make. Looking at the regs I see that, like you, they do talk about 'e-c-ps' throughout the building, even though they are technically not able to introduce a potential other than that of the MET, because of the presence of MPB. However, within the spirit of what I've been saying, 701.415.2, whilst still talking about 'e-c-ps' in bathrooms, effectively says that they do not need to be considered as e-c-ps (in relation to Supplementary bonding) if MPB is present and these parts can be shown to be 'effectively connected' to that MPB (and the protection requirements are also satisfied). Sorry if I confused things a bit there.
Kind Regards, John.
I have to say that I would consider that to be too wide a definition of 'e-c-p'.
Consider a situation in which there were no incoming metal supply pipes (possible, if service(s) entered the building as plastic). By your definition/logic above, the pipework wholly contained within that building would still be deemed to be 'liable to introduce a potential' because it almost certainly would be connected to the MET (by CPCs, if not also any explicit bonding). Is that your belief?
In the situation I have described, the reality is that only conductor entering the building that was 'liable to introduce a potential, generally earth potential' would be the DNOs 'earth' itself - and it would obviously be silly to suggest that this needs Main Bonding to itself.
However, I have one admission/U-turn to make. Looking at the regs I see that, like you, they do talk about 'e-c-ps' throughout the building, even though they are technically not able to introduce a potential other than that of the MET, because of the presence of MPB. However, within the spirit of what I've been saying, 701.415.2, whilst still talking about 'e-c-ps' in bathrooms, effectively says that they do not need to be considered as e-c-ps (in relation to Supplementary bonding) if MPB is present and these parts can be shown to be 'effectively connected' to that MPB (and the protection requirements are also satisfied). Sorry if I confused things a bit there.
Kind Regards, John.
So it's the how that you don't like?
A pipe bolted to the manifold of a boiler introduces an earth potential to the bathroom but you don't want to consider it an e-c-p because that makes the definition too wide?
What on earth (
) is wrong with the simple test:"Does it introduce a potential, generally an earth potential, into the location, yes or no?"?
I see nothing in BS 7671 to support the notion that the reason for a pipe being able to introduce a potential is of any relevance to dealing with the fact that it does.
Do they introduce a potential, generally an earth potential, into the location, yes or no?
So would a metal pipe, with a sound electrical connection to the supply earth via the cpc of a Class I item be introducing a potential, generally an earth potential, into the location, yes or no?
Is the MET at earth potential?
If a part is at that potential, and you take that part into a location, is it introducing that potential into the location, yes or no?
No it doesn't.
It just says that supplementary equipotential bonding may be omitted if...., not that the parts in question are not e-c-ps. Read the second (iii) in 701.415.2
Whether or not the conditions are met, things can still be e-c-ps.
What on earth (
"Does it introduce a potential, generally an earth potential, into the location, yes or no?"?
As I said, I think this has probably degenerated into an unhelpful semantic discussion.
I cannot deny that your position is consistent with the literal words in BS7671, without any thought or interpretation. However, I personally think that such a literal blind application of the words probably results in an 'unintended' interpretation of the regs.
I find it hard to believe that the intention was that something (which has no other connections to anything outside of the building) should be regarded as an 'extraneous conductive part' solely because it is connected to the MET and hence to the DNO's 'earth'or a local TT earth electrode. We all know that the concern about e-c-ps is that they may introduce a potential which is different (closer to 'true earth') than the potential that can exist at the DNO's 'earth' terminal' (and hence the installation's MET) under supply-side fault conditions.
What I don't fully understand is your view on the Main Bonding of the e-p-c's you are postulating. I would think we are probably agreed that the regs require that e-c-ps should have Main Protective (Equipotential) Bonding, which should be attached close to the entry point. You appear to be saying that, in the scenario I outlined, you would regard the internal pipework as a e-c-p because it was connected to the MET, and hence DNO's (or TT) earth via CPCs. Does that really mean that you think it requires Main Bonding - and, if so, where would you attach such main bonding, given that the part (pipework) in question does not 'enter' the property anywhere?
Kind Regards, John.
(Again) I don't understand the difficulty.
The point of bonding is to ensure that all conductive parts are at the same potential.
The MPB ensures that all e-c-ps entering the property are at the same potential as the MET.
If conductive parts enter the bathroom (e-c-p's), and those parts are effectively connected to the MET, then there is no 17th edition requirement to bond further (subject to the other conditions).
Supplementary bonding is needed when this condition is unsatisfied, so for example a metal radiator which is supplied by metal pipes NOT connected effectively to the MET & MPB (e.g. remote plastic pipes) may introduce a potential, earth or otherwise. As far as the bathroom is concerned, it is an e-c-p. So is a metal waste pipe. The supplementary bonding ensures that there is no potential difference between that radiator (or waste pipe) and any other metalwork.
And a pipe which is bonded to the MET should be connected to another incoming pipes NOT connected effectively to the MET, so the same reason applies - both can introduce a potential, and it might not be the SAME potential.
If I've got any of this wrong, please do correct me.
The point of bonding is to ensure that all conductive parts are at the same potential.
The MPB ensures that all e-c-ps entering the property are at the same potential as the MET.
If conductive parts enter the bathroom (e-c-p's), and those parts are effectively connected to the MET, then there is no 17th edition requirement to bond further (subject to the other conditions).
Supplementary bonding is needed when this condition is unsatisfied, so for example a metal radiator which is supplied by metal pipes NOT connected effectively to the MET & MPB (e.g. remote plastic pipes) may introduce a potential, earth or otherwise. As far as the bathroom is concerned, it is an e-c-p. So is a metal waste pipe. The supplementary bonding ensures that there is no potential difference between that radiator (or waste pipe) and any other metalwork.
And a pipe which is bonded to the MET should be connected to another incoming pipes NOT connected effectively to the MET, so the same reason applies - both can introduce a potential, and it might not be the SAME potential.
If I've got any of this wrong, please do correct me.
No, the concern is that the e-c-ps may end up at a different potential to each other and/or to the cpcs of circuits serving the location.
That's why they are bonded together.
Nothing to do with getting them to the same potential as the MET, which is why they are not connected to it.
I really don't see why a pipe which is connected to the incoming cold water supply pipe via a cpc an earth conductor and a PEB is electrically different to one connected to the same pipe by a Yorkshire tee...
The ones I'm postulating don't require main bonding as they are not services entering the building.
If this is a response to one of my posts, then I don't understand the difficulty, either, since I agree with everything you say ...
Totally agreed.
Again, totally agreed.
Again, totally agreed. I don't think this is really any different from the preceding paragraph. Both are really saying that (assuming both exist) anything that isn't connected to the MET has to be connected to something which is connected to the MET.
So, as above, I don't see any differences between us. The only thing I would add is that all of the above relates to what is actually required by regulations. The bit of the discussion which started this tangent was relating to some people liking the idea of 'bonding together' all metallic parts in a bathroom which could be touched simultaneously, even if not required by regulations. That is theoretically unnecessary for anything which enters the bathroom, since the regs only allow the absence of supplementary bonding if all such conductors are effectively connected to the MET (and hence to each other), but it is something one can talk about in relation to things (like the metal bath) which don't 'enter' the bathroom. They will usually, of course, acquire a connection to the MET (hence equipotential with everything else) via the pipework, but there will be cases in which that it is not the case because of plastic pipes. That then opens up the scenario of the silly person using a hairdryer or radio in the bathroom (on an extension cable) and dropping it into the bath of water they're in - then, in a panic to get out, touching some nearby (connected to MET) pipework, or maybe a basin tap! If I were that silly person (which I obviously wouldn't be!), I'd be happy to know that there was a bit of G/Y cable connecting the bath to some pipework which was connected to the MET (even though that is not required by the regs), wouldn't you?
Kind Regards, John.
You seem to miss my point. It is you who seems to want to take the words of BS7671 literally, without allowing any common sense 'interpretation' - and, 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? Needless to say, I totally agree with the common sense of what you're saying (and it's therefore what I'd follow in practice), but you have said that we have to go with the letter of BS7671, without applying any such common sense in interpreting what it says.
Kind Regards, John.
Everyone agrees then! I thought there was some difference of opinion between yourself and Mr Sheds. The 'again' was just a response to another 'bonding'- themed thread.
Sorry if I came across antagonistically - wasn't intentional.
So the point about the comforting piece of G/Y in the case of a stupid person is what? to provide a path to earth to trip a circuit breaker, or to ensure the bath does not attain the potential of a phase conductor, or both?
I get the point about wanting to trip a breaker quickly, but not that it is safer to have an earthed bath than a non-earthed bath per se. I am pretty neutral ( ) on this and would not feel any safer knowing that my bath was connected to earth than knowing that it was fully isolated from the electrical system. Same goes for other isolated, exposed conductive parts.
Sorry if I came across antagonistically - wasn't intentional.
So the point about the comforting piece of G/Y in the case of a stupid person is what? to provide a path to earth to trip a circuit breaker, or to ensure the bath does not attain the potential of a phase conductor, or both?
I get the point about wanting to trip a breaker quickly, but not that it is safer to have an earthed bath than a non-earthed bath per se. I am pretty neutral (
) on this and would not feel any safer knowing that my bath was connected to earth than knowing that it was fully isolated from the electrical system. Same goes for other isolated, exposed conductive parts.Oh, there are always differences of opinion with Mr Sheds - not currently helped by the fact that, almost in the same breath, he seems to want us to take the words of the regs absolutely literally, with no common sense interpretation allowed, and also wants to apply common sense interpretation himself. Such is life
Goodness - no need to apologise; if you think your post was antagonsistic, you can't read many of the posts here
As for your question, hopefully the effect of the G/Y would be to cause a protective device to operate before the inhabitant of the bath had time to discover that the bath was at a high voltage relative to that of pipes/taps/whatever within reaching distance!As above, the primary point of the 'earthing' would be to cause a breaker to trip if the bath found itself in contact with an unpleasant potential.
In the specific context of a bath, I would agree entirely if one could be confident that it (and its contents) would always 'be isolated from the electrical system'. However, when it comes to Joe/Jane Public being the user, the scenario of someone dropping that live hair drier, radio or whatever into the bath of water they were inhabiting is far from beyond credibility. In fact, the situation is actually better with a metal bath, since one can at least 'earth' it, as I have suggested, and therefore hopefully get a protective device to operate under those circumstances. The risk still exists with a plastic bath and it's far more difficult to establish a path to earth from the water within the bath in that case.I am pretty neutral (
As for 'other isolated, exposed, conductive parts', if you are thinking of things like the infamous door handles, then I totally agree with you. However, there are some other situations which I would regard as similar to the bath, the most obvious being a kitchen sink (in which case substitute 'hand mixer' or 'electric kettle' for 'hair dryer and 'radio'!) - where I would again probably derive some comfort from the presence of a G/Y cable.
Kind Regards, John.
411.3.1.2 calls for main protective bonding conductors complying with Chapter 54.
Read 544.1.2
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