Fair enough! (although, as I said, I think that, in practice, one might as well say 22Ω
)Kind Regards, John
Earthing vs. (Supplementary) Bonding
- Thread starter JohnW2
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)Kind Regards, John
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It may be a lot higher (long light circuit) than that of a pipe that may also be touched or possibly vice versa for a high current circuit and long pipe run.
Yes, usually.
Is what you are saying that -
a(nother) way to achieve the desired 50V touch voltage would be to increase the csa of the circuit cpc?
However, that may be difficult compared with a short piece of bonding.
Well, yes but isn't that the point?
SB only being required where necessary so which comes first?
Well, yes but its purpose is to safeguard the person; the facilitating is a secondary effect achieved by the 'bonding'.
Again - well, yes but the ADS is satisfactory unless the location requires bonding - supplementary for additional protection.
Yes, I see that but I do think you are playing with the words to a certain extent.... so I guess you can really 'pay your money and take your choice'. If you take steps to make ADS happen when exposed-cp/MET pd is only 50V (earthing) (although 'normal ADS' might only require this to happen with a pd of, say, 75V) you will automatically have also limited the touch voltage to 50V (bonding). If you take steps to limit touch voltage to 50V (bonding), you will also have caused disconnection to occur when the exposed-cp/MET pd is lower than 'usual' (75V or whatever) for ADS (earthing)
That the SB reduces the Zs is just a consequence of the procedure even though that is the effect.
For example, should a circuit have too high a Zs, the remedy is to increase the csa of the cable (or reduce the mcb rating); it would not be to apply bonding to an extraneous-c-p although that may have the same effect.
It may be semantics but that is what it's called.
I think the trouble is that you do not really agree with the term bonding - joining parts not actually part of a circuit - and consider all G/Y to be earthing in one form or another.
That may be fine but do you not think it would be even more confusing if it were all called earthing.
You could call bonding a supplementary cpc.
The distinction between opd operation and reducing pd is valid.
You seem to have answered a slightly different question (about the R2 of a lighting circuit, rather than the path to earth via extraneous-cp) from the one I asked, but it seems that you probably agree that the two impedances/resistances are likely to be 'of the same order' (low or very low).
Well ... It's not really "another way". If the CSA of the circuit's CPC (hence circuit's Zs) is, as may be the case, such that the pd between exposed-cp and earth (hence 'touch voltage') would be >50V (even though satisfactory 'normal ADS' was satisfied) then, no matter how one achieves it and what one calls it, one has to decrease the impedance of path between that exposed-cp and the MET (i.e. increase the effective CSA of that connection). One could do that by literally increasing the cable's CSA (or by running an additional cable/CPC in parallel with it), all the way back to the MET, or by running a cable from the exposed-cp to 'something else', maybe closer to hand, that had a low impedance path to the MET. In many/most circumstances, the extraneous-c-p would serve that purpose. So it's really more a question of 'the same', rather than 'another', way of restricting the touch voltage to 50V.
Sure, as above, it would usually be much more convenient to connect the required 'additional CSA') from the exposed-cp to some conveniently close low impedance path to the MET than to literally increasing the CSA of the circuit - and, as above, the local extraneous-c-p would serve that purpose.
No, you're thinking of Bernard, not me
Everything we've been discussing, including all the above, assumes the (usual) situation of a very low impedance path between extraneous-cp and MET/earth. However, as Thripster reminded us earlier (and you often mention), the resistance/impedance to MET/earth of an extraneous-cp of can, in theory, be as high as 22kΩ (although goodness knows how that could happen). However, if that ever did happen, then all would be different, and one would definitely be talking about the need for 'true bonding' ...
... in such a situation (say a resistance of 20kΩ
, connecting any sort of (true bonding) cable between exposed-c-p and extraneous-c-p would reduce voltage between them (the 'touch voltage') to nearly zero. However, since the maximum current that could flow through that bonding conductor would be only just over 1mA, the presence of the bonding would not make a blind bit of difference to the operation of the circuit's protective device - since, although the touch voltage would be about zero, the potentials of both exposed-cp and extraneous-cp would be at virtually the same voltage, relative to earth, determined by R2/Zs (say 75V as in my previous example), as would have been the case for the exposed-cp without any bondingThat, to my mind, would be an example of 'true bonding' - potential differences ('touch voltages') were more-or-less eliminated by the presence of the bonding conductor, but both parts would remain at a 'high' potential relative to earth. However, it's not the situation that we nearly always have.
We also always tend to assume that the potential that an extraneous-c-p "is liable to introduce" will be earth potential, even though the BS7671 definition reminds us that it only usually earth potential. The hypothetical "20kΩ extraneous-cp" opens up the possibility that the fault resulting in the touch voltage is not in the bathroom's electric circuit but, rather, due to the fact that (due to some fault elsewhere) the extraneous-cp was 'introducing' a high potential relative to earth. That one raises scope for more discussion - but, again, 'true bonding' would certainly be needed to deal with that possibility - since (depending on the 'source impedance' of the actual fault), it is possible that no amount of 'increasing the CSA of the CPC of the lighting circuit' would have much impact on the touch voltage.
Kind Regards, John
I take it that the reason for the first of my later posts has now become self evident?
'The purpose of protective bonding is to equalise potential rather than to carry fault current, although sometimes bonding conductors will also carry fault current where they form part of a parallel earth return path to the source of supply'
Regards
Edited fof spilling mastikes
'The purpose of protective bonding is to equalise potential rather than to carry fault current, although sometimes bonding conductors will also carry fault current where they form part of a parallel earth return path to the source of supply'
Regards
Edited fof spilling mastikes
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If you mean the one in which you stated the generally-agreed guidance on what constitutes and extraneous-c-p, then I have to say that I'm not really sure what point you are/were making. Everything discussed in this thread (including recent discussion about extremely improbable ones with relatively high resistance path to MET) has related to extraneous-c-ps that fulfill that definition/guidance.
I don't think that anyone has ever disagreed with any of that, but it does perhaps help to underline the complication which exists when one is talking about SB. In the case of SB, of course, the implicit requirement is not exactly to 'equalise' potential but to limit the potential difference to a maximum of 50V.
If, in practice, (as will probably usually be the case) SB reduces the potential difference to near zero, that is easy to understand as straightforward bonding (per the 'purpose' you mention above). However, if (as is seemingly 'allowed') it only reduces the pd to just under 50V, and given that the SB conductor will have an extremely low impedance, the SB could only achieve this reduction (from 'whatever' to just under 50V) if an extremely high fault current flowed through the SB conductor. In other words, whilst the 'purpose' of the SB is to reduce the potential difference, in this sort of case the mechanism whereby it achieves this is by carrying a very substantial fault current.
In the (usually overlooked, and presumably extremely rare) situation of an extraneous-c-p introducing a high (rather than earth) potential, it is, of course, quite possible (depending on the situation) that the "SB" conductor would carry all of the fault current, and that would be the mechanism whereby it reduced the potential difference.
In many senses, it is is just semantic, but we often seen people being taken to task for 'confusing earthing and bonding' in manner which doesn't acknowledge how inter-dependent they can be. As above, in some situations, a bonding conductor can only equalise (or reduce/minimise) potential differences IF it carries a high fault current - which must muddle up the two concepts in some people's minds.
Kind Regards, John
You are being over pedantic and argumentative.
As you know, (for others) the figure is derived from the value of 10mA being deemed safe.
Actually the voltage used should be nearer 240V and therefore 24kΩ and accounting for the resistance of the body therefore completely pointless.
Some say the figure should be 5mA and therefore 48kΩ.
That the resistance measured will (virtually) never be anything near this does not detract from the relevance.
What would you state as the determining factor for deciding if extraneous or not?
That's all very true but, despite what you suggest, I don't think I have been pedantic and/or argumentative about any of it, nor even 'challenged' any of it! In fact, as I've said, the figure really makes little difference in practice - one can use 22kΩ (or 48kΩ
if one wishes but, as I've said, in practice one could probably use, say 20Ω, since the it is incredibly unlikely that figures between that and 22kΩ (or 48kΩ will ever be seen in practice.That's a very interesting question - and I suspect that the answer is probably that there is no test that would work satisfactorily ('be safe') in all situations.
Given that, I have said, the BS7671 definition allows that the potential introduced by an extraneous-c-p may (albeit very unusually) be high, rather than 'earth potential', a criterion based only on the resistance to the MET (or to an exposed-cp) would seem totally inadequate....
...if the extraneous-c-p were introducing a high (probably 230/240V) potential then the higher the resistance between that extraneous-cp and the MET (or an exposed-c-p), the higher would be the 'touch voltage' (pd between extraneous-cp and exposed-cp or MET/CPC). If one wants to cater for that situation (which BS7671 implicitly recognises as a possibility), it would surely be totally inappropriate (one might say crazy) to employ a definition which said that the part didn't qualify as an extraneous-cp, and therefore didn't need bonding, if the resistance between it and MET (or exposed-cp) was above some figure (whether that figure be 20Ω, 22kΩ, 48kΩ or whatever), wouldn't it?
Nor is this totally hypothetical. If the part in question were, say, a pipe which was normally electrically 'floating' (hence extremely high resistance to exposed-cp or MET - so you would say that it was not an extraneous-cp, and therefore didn't need bonding), if that were to somehow acquire a high potential relative to earth, then the touch voltage (between it and exposed-cp or CPCs/MET) would be the full 'introduced voltage' (quite probably 230/240V). Hence, despite the well-meaning 22kΩ (or 48kΩ
criterion (intended to limit current through a person to 10mA), a part could satisfy 'the criteria' for not being an extraneous-cp, yet leave (in the absence of bonding) a situation in which at least a couple of hundred mA could pass through a person if they simultaneously touched it and something (e.g. an exposed-cp) connected to MET/earth.So, I don't really know what criterion for something being an exposed-cp would be 'safe' in all situations - but I do know (for reasons explained above) that the criterion being discussed would not be! It could well be that the only course that would be totally safe in all situations would be to forget about 'tests' and simply regard any conductor which entered a building (or 'location') as being an extraneous-cp.
Kind Regards, John
As I said previously, if that scenario is thought to be a possibility then the part should be earthed and whether it was extraneous (before) or not is irrelevant; it now will be....if the extraneous-c-p were introducing a high (probably 230/240V) potential then the higher the resistance between that extraneous-cp and the MET (or an exposed-c-p), the higher would be the 'touch voltage' (pd between extraneous-cp and exposed-cp or MET/CPC). If one wants to cater for that situation (which BS7671 implicitly recognises as a possibility), it would surely be totally inappropriate (one might say crazy) to employ a definition which said that the part didn't qualify as an extraneous-cp, and therefore didn't need bonding, if the resistance between it and MET (or exposed-cp) was above some figure (whether that figure be 20Ω, 22kΩ, 48kΩ or whatever), wouldn't it?
Nor is this totally hypothetical. If the part in question were, say, a pipe which was normally electrically 'floating' (hence extremely high resistance to exposed-cp or MET - so you would say that it was not an extraneous-cp, and therefore didn't need bonding), if that were to somehow acquire a high potential relative to earth, then the touch voltage (between it and exposed-cp or CPCs/MET) would be the full 'introduced voltage' (quite probably 230/240V). Hence, despite the well-meaning 22kΩ (or 48kΩ
But, that's not fair, is it?
An electrical installation has to be a compromise as not all possible occurrences can be safeguarded against at the same time.
It was you that asked me what criteria could be used to define what was, and was not, an extraneous-cp - otherwise I probably would not have raised the matter!
Your view seems to be that, since "not all possibilities can be safeguarded against at the same time", we should restrict our thinking about extraneous-cps to their ability to 'introduce earth potential'. I accept that, given the rarity of the alternative possibilities, that might not be an unreasonable 'compromise' - but if that's what were wanted, then BS7671 really ought to remove the "usually" (earth potential) from it's definition of of an extraneous-cp. So long as the word remains there, the test we have been discussing is inadequate in relation to the definition.
Kind Regards, John
Yes, but the 23kΩ (or 48kΩ
above which bonding is not necessary is adequate, albeit the actual figures a nominal limit.We do not have to cater for the part becoming live by contact with a conductor.
Where parts are extraneous becoming live by being attached to an exposed part is a possibility.
But doesn't that definition relate to parts which are extraneous?
If isolated, surely, as above, they won't.
Indeed - in fact, far 'more than adequate' in relation to an extraneous-cp which 'liable to introduce earth potential'.Yes, but the 23kΩ (or 48kΩ
So what do you think that BS7671 had in mind when it defined an extraneous-cp as a conductive part which is "liable to introduce a potential, generally Earth potential"? 'Generally' surely means/implies 'not always' - so what potentials (and situations) other than Earth potential do you believe they were thinking of, if not the possibility of the part (somehow) acquiring a potential above earth potential? ... or are you suggesting that they were wrong to include the word "generally"?
Eh? I'm talking about BS7671's definition of what IS (and not) extraneous. If the definition only applied to things which we had already decided were extraneous, there would surely be no point in having a definition which told us what was (and what was not) extraneous, would there ?!!
Kind Regards, John
Just that. It is 'usually' earth potential but 'not only' or 'not exclusively'.
Extraneous-c-ps are connected to exposed-c-ps; they are not themselves exposed-c-ps therefore...
No. I do not see the difficulty.
Extraneous-c-ps are connected to exposed-c-ps; they are not themselves exposed-c-ps therefore...
Of course the definition only applies to parts which we had already decided were extraneous; how could the definition of an extraneous-c-p apply to a part which isn't an extraneous-c-p?
If it is determined that a part is extraneous then it may introduce earth and other potential.
If it is determined that a part is not extraneous it will introduce neither.
I think what you are, in my view, mistakenly, getting at is that there can be an isolated part which cannot introduce earth potential but may introduce (up to) 240V.
That then makes it an exposed-c-p doesn't it?
It therefore, as I have said, must be earthed and if reaching into a bathroom etc. may cause it to require bonding as well.
Needless to say, I don't believe that I'm mistaken but, yes, I am saying that there can be such a part - and that it would fulfill the BS7671 definition of an extraneous-c-p (since it may 'introduce a potential').
Only if, per BS7671 definition, it is a conductive part of 'equipment' (which, in turn, BS7671 defines as "Any item for such purposes as generation, conversion, transmission distribution or utilisation of electrical energy") - and I really can't see that a bit of pipe appearing through a bathroom wall can qualify as such, can it?
You appear to be working with your own concepts/definitions of "extraneous-c-p" and "exposed-c-p" to refer to any conductive parts that may introduce 'earth potential' and 'potentials above earth potential' respectively. Whilst those definitions are essentially self-fulfilling in relation to the arguments you are presenting, in neither case do they seem to correspond precisely with the BS7671 definitions. Whether your definitions or the BS7671 ones are the more useful/appropriate is, I suppose, a matter for debate!
Kind Regards, John
Please give me an example as I'm obviously missing something?
Well, then in that case it won't become live.
You really should be asking the regs - it is they which have a definition of an extraneous-cp which, by implication, includes parts which can introduce a potential other than earth potential. I am really just the 'messenger'.
It won't become live unless something happens which you would probably say we 'do not need to safeguard against'.
Kind Regards, John
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