Fair enough - but, no, it's not as simple as that. This forum software seems to have a mind of its own and sometimes intercepts 'for moderation' messages which are short, totally innocuous and with no images, special symbols or anything else 'of interest' ... so just one of life's little mysteries!
OK. I dare say that I wasn't as clear as I could have been, so feel very free to ask for clarifications, further explanations etc.!
This is from the onsite guide
That explains everything
Mind you, whilst the resistivities may be due to them quoting them at a different temperature, I can't imagine where a max Zs of 1.1Ω for a B32 came from - are you sure that's what the OSG says?That is, indeed, the convention, and the point I was making was that, the way you drew/labelled it, it was the Line (R1) that was connected to the incoming earth (hence Re), not the CPC. Other than the confusion it caused me (and possibly others), it doesn't matter.
The BS7671 figures say that they relate to a conductor temp of 70°.
Quite so.
What are you suggesting - that in order to be below the max Zs at 70°, one has to measure below 1.1Ω at ambient temp? If so, I bet that not many electricians have thought of that one!
I'm not sure. I'm having some difficulty in working out what his difficulty is. However, I'm not sure that you're right. Touching two exposed-c-ps, one of which is 'live' (because of a fault), and both of which have different CPC paths back to the MET is no different from "touching a live exposed-c-p while earthed", is it?
Yes, I think you are correct. Sorry.
Isn't that what you are saying - if 1.37 in BS7671 is at 70°.
If that is the case then what is the point of supplementary bonding?
I'm glad you agree.
Exactly - or, to put it the other way around, if one measures more than 1.1Ω at ambient temp (which I presume is how measurements are usually done), the Zs at 70° (or, indeed, any temp above ambient) would be non-compliant - and, as I said, I bet that few, if any, electricians think that way!
I would say that it is obvious that such IS the case, but I don't really understand your question, since SB specifically seeks to address that issue. If there are two simultaneously touchable exposed-c-ps which have separate/different CPC paths back to the MET (i.e. 'different circuits') then, if one of those exposed-c-ps becomes 'live' due to a fault, that exposed-c-p will be at a potential (above MET potential) somewhat more that half of the supply voltage, whereas the other exposed-c-p will still be at/about MET potential - hence the potentially dangerous 'touch voltage' (pd) between them. If one applies SB which locally joins those two exposed-c-ps, they will then be at essentially the same potential (albeit much higher than MET potential), hence 'touch voltage' (pod between the two exposed-c-ps) will be close to zero.
Yes the voltage wrt earth at the exposed-c-p.
We are not limiting that voltage, but
Yep - well 143V between the faulty metal work and the MET. With TN installations that will be much the same as between the metalwork and neutral, but with TT, under these fault conditions, the MET will be at a much higher potential than the neutral (so the metalwork-neutral pd would be appreciably less).
I'm not sure what you mean by 'all that metalwork', but I suspect the answer is 'No'! Any exposed- or extraneous-c-ps connected (via different CPCs, or by bonding conductors) to the MET will still be at approximately MET potential, whereas the faulty exposed-c-p (and anything connected to that metalwork via that circuit's CPC) will be about 143V above the MET potential.
See above - what you say is ONLY true of other exposed-c-ps on the same circuit (i.e. same CPC back to MET). The potential between the faulty part and any other MET-connected part (not on the same circuit) will be 143V, not 0V-ish.
Yep, if they are at the same potential (measured relative to anywhere), then no current can flow between them.
That's more-or-less what the regs say, but it gets complicated and I need to think about how best to try to explain the situation. One problem is that if, say, the Zs at the circuit is already at the maximum for ADS, then any additional resistance in the fault path will prevent the device tripping magnetically.
Maybe. 'Voltage', per se, is a meaningless concept, since it has to be voltage 'relative to something' (i.e. a potential difference between the two things).
Having pondered a little, I think it's pretty easy to explain the thinking behind the 'test' ("50/Ia") in the regs ....
Indeed, the max Zs is the max Zs.
For once, I'm inclined to sympathise with the OSG's approach - and, as I've written several times today, I doubt that many electricians have even thought about this. Given that we feel that it is important to have 'effective ADS', and given that we cannot predict when faults might arise, it would seem that (lighting circuits aside) one really ought to look for Zs figures at ambient temp which give reassurance that it will still be below 'the max' (which, as you say, IS 'the max') under in-service operating conditions which are far from impossible.
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