As I've indicated, it's not a major one of my worries, either, but it essential is the issue which this discussion is meant to have been about. As you go on to agree:I did not say that, and to me machines spinning down would be the least of my worries if I suddenly found myself in a pitch black room.
Indeed - and, as I've said, the issue of discrimination, which you jumped in to discuss at length, would only be useful (reduce, but not eliminate, the hazard) in relation to just one of those seven mechanisms.I have no problem mitigating all the mechanisms which may lead to what I consider to be the hazard here, which it lights going out in the dark, but some are easier to mitigate than others. The mechanisms I can think of, some of which we've discussed, are: <list of 7 possible mechanisms>
Exactly, which is why I have mentioned it more than once. In fact, AFAICS battery-operated emergency lighting would be the only effective measure (at least, of those reasonably implemented in a domestic setting) that would be useful in relation to 4 or 5 of those possible mechanisms.Clearly we have agreed the a reasonable mitigating measure for most of these (save a loose/poor connection on the switch wiring) would be emergency lighting ...
[remembering that this relates to just one of the 7 or more possible mechanisms of the hazard] ... One can obviously always achieve a reasonable, perhaps high, probablitiy of achieving discrimination by 'proper' design but, as you say, in a domestic installation, the question relates to what is 'reasonable'. As I've indicated, one hardly needs to do any serious calculations, since the situation/options (domestically) are fairly obvious. If there is a downstream 32A MCB, you are not going to get reliable discrimination from any domestic MCB, since even a 50A MCB upstream will not give reasonable discrimination. Even with a 20A downstream MCB, discrimination probably wouldn't be that assured from a 50A upstream one. Hence, one's only chance of reasonable discrimination would be to have submain protected by a 60A, 80A or 100A fuse, with appropriate cable, (not much point in going above the In of the cutout fuse). However, is it reasonable', in a domestic setting, to go to that length to supply '2 lights and two sockets' in a basement'? Surely it would be far more 'reasonable' to either have emergency lighting or else run the lights from some other circuit? The latter, of course, would still leave several (most) of the possible mechansims of the hazard still present.With regards to discrimination therefore, which seems to be what you are suggesting cannot be reasonably achieved, I agree that sometimes it is difficult, but it can be done with reasonable due care, a PFCC reading, and a few calculations, most of which you can do in Excel, or even by hand.
Kind Regards, John