Touch voltage

Only IF, as your example, Ze is 0.8Ω. Most these days have to be considered as TN-C-S despite appearing TN-S, so who knows? I obviously have to agree that if only 0.3Ω is left for the circuits then they might not be compliant with the numbers; 45A shower circuit max.Zs- 0.78Ω; that does not mean they won't cause ADS satisfactorily in the actual circumstances. 240V, conductors not at 70°, MCB at 4 In. etc.
All agreed. However, to have a safety feature which 'might well work satisfactorily' if/when it is needed 'in anger' is hardly an ideal situation, is it?
... You seem to think the electricians do not know what they are doing. You might be right so where does that leave us?
That's not what I'm really suggesting, and I'm not criticising or 'blaming' anyone.

However, in all the years I have been talking/listening to electricians, and seeing what they advise, I don't think I've even seen/heard this issue being discussed (or even mentioned) before - and it's only in the past couple of days that I (and, very possibly, others) have even been aware of the 'lower max Zs figures' quoted by the OSG. All I've ever seen/heard, or seen being advised, is the need for a circuit to have a Zs (by implication, when measured at ambient temp) no greater than the (BS7671) 'maximum Zs' - so I can but assume that a lot of electricians (and myself) have always believed that that was adequate.

As I said when this started, it might be an exceptional case in which the OSG is actually 'less potentially misleading' than is BS7671 itself!

Kind Regards, John
 
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Well, all I can say is refer to Note 2 under all the maximum Zs tables and to Appendix 3 in the BBB (App 14 in BYB).
 
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Indeed - and, as will be apparent from what I recently wrote, I had totally forgotten all of that discussion.
Fair enough - but that obviously pre-dates my 'joining' the forum, so I wouldn't expect to 'remember' that!

Kind Regards, John
 
a Mod said:
MOD: To answer @JohnW2 question regarding exercising control over the forums software, the answer is no, we cannot. Please address concerns to [email protected]
Fair enough. I'm not sure about 'concerns' - it's really just an 'annoyance' - and, in any event, I'll continue to undermine the intent of the software in the manner in which I've been doing!

Kind Regards, John
 
Well, all I can say is refer to Note 2 under all the maximum Zs tables and to Appendix 3 in the BBB (App 14 in BYB).
Apologies for having gone suddenly quite - I got distracted by a pressing 'real work' issue which suddenly arose, but I'm now largely addressed that issue!

Anyway, as I said in my initial (uncharacteristically brief!) response, those "Note 2"s to the 'maximum Zs' tables (e.g. 41.4 for MCBs) is certainly clear enough.

As I read those notes, they don't say that one has to remain below those 'maximum Zs' figures at the 'known operating temperature' of the conductors, or at some conductor temperature which the designer 'believes will never be exceeded' BUT, rather, when one follows the references it cites, that those max Zs figures must not be exceeded if (hypothetically) the temperature of both the Line conductor at the CPC did become 70° (for PVC).

In other words, there appears to be no scope for 'designer discretion' - which seems a bit daft, given how few real-world circuits (particularly in domestic installations) will ever have cable conductor temps anywhere near 70° (indeed, quite impossible with a 6A lighting circuit!). Hence, contrary to what you initially suggested, I don't think that the OSG is really 'considering the worst possible case' but, rather, is considering THE ('one and only') case which BS7671 allows to be considered (for measurements at ambient temp) - which, I agree, is a 'worst case' (which will virtually never be seen in practice), but a 'worst case' which is imposed by BS7671, not thought up by the OSG.

That surely means that, in reality, measurements close to those tabulated 'max Zs' figures are effectively never going to be low enough (in realtion to measured Zs), since it must be essentially unknown for measurements to be taken when conductor temp is 70° (which, as above, in most cases is a temperature that the conductors will never reach, or even approach, in service).

Hence, since in the real world, where I imagine that such measurements are nearly always undertaken close to ambient temp, it would have seen much more sensible for the BS7671 tables to be written in terms of the 'OSG' figures (with a stated assumption of measurement at 20, 30 or whatever), with exactly the same Note as at present, indicating that if measurement were undertaken at a different one from that stated, the max Zs measurement at that temp could/should be determined. It just seems daft that the regs should present 'default' figures which are effectively never going to be coirrect!

The point I've been making still remains that the max Zs figures in those BS7671 tables will essentially never be 'low enough' (as 'maximum' figures) in relation to measured (rather than calculated) Zs, since no-one is ever going to undertake the measurements when the conductors are anywhere near 70!

Kind Regards, John
 
John W2 and EFLImpudence Thank you for all your help with this.
I think I've got a good grasp of it...
 
John W2 and EFLImpudence Thank you for all your help with this.
I think I've got a good grasp of it...
You're very welcome, and I'm glad that you have gained a reasonable understanding.

You also deserve some thanks. By quoting the OSG's 'maximum Zs' figures, you reminded me of something that I had completely forgotten - that I feel sure that many electricians are probably comparing their Zs measurements with 'max Zs' figures which are actually too high, since they are using the figures tabulated in BS7671 (which essentially assume that measurements have been taken with a conductor temp of 70°), rather than the appreciably lower 'OSG figures' which are what one should use in relation to measurements undertaken at ambient temp!

Kind Regards, John
 

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