Oh, I see what you're getting at. Yes, strictly speaking, that's true. Whist we're at it, we might as well use 63A MCBs for all fixed loads unlikely to create an overload (showers, immersions, conventional ovens/cookers etc.) - provided only that Zs was low enough for fault protection
Well, perhaps I have misunderstood you - but since you've invoked 433.3.1(ii), your position seemed to be that any MCB (with an In ≥ Ib) has 'a correct rating for the shower and the cable' (since the cable, and hence the MCB is "unlikely to carry overload current"), provided only that fault protection is adequate (hence my 'extreme' example) - is that not the case?
That is the case.
Is that a bit like Winston's aversion for compliant sockets on 6A circuits?
Agreed - if you are invoking the fact that a shower does not need any overload protection at all, then (as far as overload protection is concerned) there clearly is no such a thing as an "over-rated MCB" - hypothetically, a 1000A one would do if it were able to provide the required fault protection (which obviously would not be the case).
If anything, I'd say that it is probably less rational than Winston's view - in as much as he doesn't have to scrape the barrel as deeply to think up very unlikely scenarios in which the sockets on a 'lighting' circuit could represent a significant problem as how deeply I would have to scrape the barrel to explain how, in exceptional circumstances, a shower could result in an overload current!
Whilst that's true, in the context we're talking it is not necessarily as simple as that - it also depends upon how risk-averse an approach is taken. As I have implied, if I were prepared to scrape the barrel deep enough, I could present an argument as to to how, say, an electric shower could, in exceptional circumstance produce an overload. Even though incredibly improbable, the 'facts' are that such a scenario would not be impossible - and one doesn't have to look further than this forum that some people are so risk-averse that they feel that incredibly small risks should be addressed.
As you will be aware, if the OP can get the short length of cable out from under insulation (so that the entire run is Method C), then 4 mm² cable would, in fact, be adequate for his 7.8 kW (at 230V) shower - and, if one were happy to take EFLI's approach, that would (Zs allowing) be OK on a B40 MCB (even those CCC was only 37A).
As I said, the world is full of people who refuse to make decisions based on evidence and facts.
... and, as I said, the interpretation of the facts is determined by the degree of risk-averseness of the person doing the interpretations.
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