This is the argument against TT, and I would be testing the RCD far more often with a TT supply. With my TN-C-S supply, I am not really worried about type AC RCDs as they are secondary protection, but with a TT supply I would be changing to type A at least.
All true - and I think most of us would like to see less than 100 Ω - not the least because, as you imply, the figure might increase appreciably over time, and particularly during periods of very dry weather.
with TT I`d be worried about the 7% possible failure rate and have two RCDs in tandem (hopefully different locations if one is a kitchen)
Yes, the idea of a 300 mA RCD before the CU doubling as an isolator does seem to make sense. But where do you stop?
That "7%" figure has been knocking around for very many years and I think was based on a fairly limited survey (and, I believe, not even in UK, and probably related to 'slightly out-of-spec' as well as 'not working at all'). I would therefore suggest that we "don't really know".
Yes John I agree with all of your points there.
That's obviously true of any sort of surveys in general. However, in thi case I think we're talking about one small servery, conducted very many years ago, in a foreign country, so i don't think it should be regarded as anything but a major 'guess'!
Sure, but the "30 mA" spec is merely a semi-arbitrary 'compromise' (between safety and convenience), in the face of something which 'varies'. One could argue that one which did not trip until, say, 32 mA, might not 'save a life' which would have been saved by one which tripped at 30 mA - but that will to some extent be 'balanced'/'cancelled' by the fact that one which (as is common) trips at 25 mA might 'save a life' which might not have been saved if it has not tripped until the (required by spec) 30 mA!
I'm sure it's accurate enough within its own context, but it tells one nothing definite about how the devices behave in service 'in the field'. Given that, as far as I am aware, a substantial proportion of out-of-spec (or not working at all) RCDs are in that state because of 'stiction', I might guess that the same thing might be at least as common with MCBs - possibly more common since, unlike RCDs, they cannot be regularly tested/'exercised' by use of a test button.
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