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
