Earth wire for bath: BS6231 or BS6491 ?

[EFLImpudence]

Frequently when you ask a question, I feel the answer is so obvious that you must have something else in mind.


Anyway, any metal part which enters a bathroom, IF it is earthed by any means anywhere else, will be an extraneous-conductive-part to the bathroom.
Measurement of the impedance from that part to the MET will determine whether it is earthed or not. This measurement may not actually be necessary but it is a quicker way of determining if it is NOT earthed and so saving measurements from all other exposed- and extraneous-conductive-parts.

If that extraneous-conductive-part is simultaneously accessible from an exposed-conductive-part or from other similar simultaneously accessible extraneous-conductive-parts, then measurement of the impedance between them will determine whether supplementary bonding is required by the regulations for the reasons Flameport has given.

 

[JohnW2]

Frequently when you ask a question, I feel the answer is so obvious that you must have something else in mind.

Indeed - I think that you may have misunderstood the point behind my question....

Anyway, any metal part which enters a bathroom, IF it is earthed by any means anywhere else, will be an extraneous-conductive-part to the bathroom. Measurement of the impedance from that part to the MET will determine whether it is earthed or not. This measurement may not actually be necessary but it is a quicker way of determining if it is NOT earthed and so saving measurements from all other exposed- and extraneous-conductive-parts.

When we are considering conductive parts which are possibly 'extraneous' to a building, it is so incredibly unlikely that it could be "liable to introduce" any potential other than earth potential that determining whether or not it is 'earthed' is adequate to determine whether or not it is an extraneous-c-p.

However, in relation to parts which might be extraneous to a bathroom, flameport appeared to be considering the possibility that, due to factors such as fault currents in conductors/conductive parts etc., a metal part entering a bathroom might introduce a potential other than earth potential. If that possibility is a concern, then simply determining whether or not the part is 'earthed' would presumably not be be an adequate means of determining whether or not it was an extraneous-c-p?

Hence my question - IF "earthed/not earthed" is not an adequate criterion for determining whether a metal part entering a bathroom is 'extraneous' to that location, then I wondered if flameport felt that all metal parts entering the location had to be regarded as extraneous-c-ps ('just to be sure'), or whether there was some other basis on which a decision could be made.

Flameport also mentioned a reason for (admittedly 'foolproof') local SBing is that "...pipes and other services may be altered elsewhere in the building ....", but I suspect that you might well question whether one could/should be expected to try to anticipate unknown changes within a property which might possibly happen in the future.

The bottom line, which I think was the basis of what flameport was saying, is that if one installs ('local') SB of sufficiently low impedance between all exposed- and extraneous-c-ps, then it effectively becomes impossible for a dangerous pd to ever exist between two simultaneously-touchable things, regardless of anything outside of the location (including any subsequent changes to pipework/wiring/whatever outside the location).

Of course, in reality, the need for SB is becoming 'a thing of the past', so this discussion is probably fairly moot.

Kind Regards, John

 

[bernardgreen]

it is so incredibly unlikely that it could be "liable to introduce" any potential other than earth

The regulations make for interesting reading when the word "Earth" is replaced by the word "Neutral" ( in many installations the CPC and "main bonding" are not Earth but are Neutral )

 

[JohnW2]

The regulations make for interesting reading when the word "Earth" is replaced by the word "Neutral" ( in many installations the CPC and "main bonding" are not Earth but are Neutral )

That is why we need main bonding.

In context, it is 'vanishingly improbable' that any extraneous-c-p entering a building (unless you count the PEN conductor itself!) would be liable to introduce supply neutral potential. The issue is the other way around - that the extraneous-c-p will virtually always be "liable to introduce (true) earth potential", whereas the incoming neutral in a TN-C-S installation is connected to the MET - hence the need for main bonding between them.

Kind Regards, John

 

[EFLImpudence]

When we are considering conductive parts which are possibly 'extraneous' to a building, it is so incredibly unlikely that it could be "liable to introduce" any potential other than earth potential that determining whether or not it is 'earthed' is adequate to determine whether or not it is an extraneous-c-p.

Ok.

However, in relation to parts which might be extraneous to a bathroom, flameport appeared to be considering the possibility that, due to factors such as fault currents in conductors/conductive parts etc., a metal part entering a bathroom might introduce a potential other than earth potential. If that possibility is a concern, then simply determining whether or not the part is 'earthed' would presumably not be be an adequate means of determining whether or not it was an extraneous-c-p?

I am still not sure what you are getting at. All earthed parts will become live in a fault (until cleared by an OPD) so would be introducing a potential other than earth.
If the part had no connection to any earth, therefore not an extraneous-c-p nor an exposed-c-p somewhere so not liable to introduce a potential of any kind. We do not have to consider it coming into contact with a stray line conductor.

Hence my question - IF "earthed/not earthed" is not an adequate criterion for determining whether a metal part entering a bathroom is 'extraneous' to that location, then I wondered if flameport felt that all metal parts entering the location had to be regarded as extraneous-c-ps ('just to be sure'), or whether there was some other basis on which a decision could be made.

I would think not if that would include isolated parts - which begs the question of how we determine which parts are isolated.

Flameport also mentioned a reason for (admittedly 'foolproof') local SBing is that "...pipes and other services may be altered elsewhere in the building ....", but I suspect that you might well question whether one could/should be expected to try to anticipate unknown changes within a property which might possibly happen in the future.

I would not consider that nor could we consider the converse.

The bottom line, which I think was the basis of what flameport was saying, is that if one installs ('local') SB of sufficiently low impedance between all exposed- and extraneous-c-ps, then it effectively becomes impossible for a dangerous pd to ever exist between two simultaneously-touchable things, regardless of anything outside of the location (including any subsequent changes to pipework/wiring/whatever outside the location).

Yes, but not all metal parts as that would introduce paths to earth which were not there before; a hazard with other faults.

Of course, in reality, the need for SB is becoming 'a thing of the past', so this discussion is probably fairly moot.

In effect it is, but the regulations are still the same - it is just because of RCDs having such a low Ia.


Of course, a bathroom itself is not much different electrically than a kitchen where SB does not have to be considered at all - although one could if one wished - it is just the wet naked people that make the difference.

 

[JohnW2]

Yes, but not all metal parts as that would introduce paths to earth which were not there before; a hazard with other faults.

... but 'a hazard with other faults' which some people may feel has to be balanced against the hazards of alternatives. It seems that that was probably flameport's thinking when he wrote (of SB) ...

.... it's to ensure items within the bathroom remain at the same potential regardless of what happens elsewhere. .... AND .... pipes and other services may be altered elsewhere in the building, often by those with no knowledge or care about bonding or any other electrical issues. What may be connected today can easily change next week due to unrelated repairs or modifications.

I'm sure he'll correct me if I'm wrong, but I took that to mean that he felt that the risk of a being created in the future by something (currently not earthed) becoming earthed in the future outweighed the hazard which would be created by 'unnecessarily earthing' (via SB) now.

In effect it is, but the regulations are still the same - it is just because of RCDs having such a low Ia.

I'm not so sure about 'the same'. Omission of SB is a bathroom merely requires RCD protection (which the regs require for all circuits serving a bathroom, anyway) and a couple of other conditions which will nearly always be satisfied.

I also have to say that, in my personal experience, exposed-c-ps in bathrooms are not at all common (electric towel rails etc. being about the most common)l, so the question of 'simultaneously touchable' exposed- and extraneous-c-ps often cannot arise (and even when there are exposed-c-ps, they are probably most commonly on the ceiling - again not simultaneously touchable) - so another sense in which the discussion about bathroom SB may be heading in the 'moot' direction.

Kind Regards, John

 

[EFLImpudence]

... but 'a hazard with other faults' which some people may feel has to be balanced against the hazards of alternatives. It seems that that was probably flameport's thinking when he wrote (of SB) ...

I doubt that.

I'm sure he'll correct me if I'm wrong, but I took that to mean that he felt that the risk of a being created in the future by something (currently not earthed) becoming earthed in the future outweighed the hazard which would be created by 'unnecessarily earthing' (via SB) now.

I doubt that as well.
It would be negligent to do the converse - i.e. not fit required bonding in case the part became isolated in the future.

I'm not so sure about 'the same'. Omission of SB is a bathroom merely requires RCD protection (which the regs require for all circuits serving a bathroom, anyway) and a couple of other conditions which will nearly always be satisfied.

It is the same.
It's just that with an RCD the relavent impedance (R≤50/Ia) is so high that it would never occur, but if it did then SB would still be required - in accordance with 415.2.

I also have to say that, in my personal experience, exposed-c-ps in bathrooms are not at all common (electric towel rails etc. being about the most common)l, so the question of 'simultaneously touchable' exposed- and extraneous-c-ps often cannot arise (and even when there are exposed-c-ps, they are probably most commonly on the ceiling - again not simultaneously touchable) - so another sense in which the discussion about bathroom SB may be heading in the 'moot' direction.

That is true but people usually disregard the simultaneously accessible bit and state that every part and CPC should be bonded.

 

[JohnW2]

I doubt that. .... I doubt that as well.

Well, it would indicate a view different from yours and mine but, as I said, the way I read him that seemed to be what flamepoint was implying.
It would be negligent to do the converse - i.e. not fit required bonding in case the part became isolated in the future.

It is the same. It's just that with an RCD the relavent impedance (R≤50/Ia) is so high that it would never occur ...

Fair enough, but since RCD protection is, itself, one of the requirements for SB to be omitted, as you say, it would never occur that 701.415.2(vi) was not satisfied if the other requirements for omission of SB were satisfied - so it would seem to be essentially a redundant requirement.

That is true but people usually disregard the simultaneously accessible bit and state that every part and CPC should be bonded.

It seems that they usually do, but that does not alter what the regs 'actually say'!

Kind Regards, John