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.
I would be more worried about the 40 mS trip time. I know my RCDs did trip at less than 40 mS as I did have a tester, but when I found a horizontal wired going around 4 right-angle bends, so not in a permitted zone, with a pair of hacksaw blades that I was using to cut a channel for a water pipe the hard way, it knocked me out.
both tests RCD to earth so shows it will work, and does a loop test so gives a reasonable indication as to how good the earth rod is. It has three lights, better than 1.8Ω better than 92Ω and over 92Ω not as many options as the Martindale tester, but as already said in the main 92Ω is good enough for an earth rod. To test the RCD there are cheaper testers 
but they don't test time or loop impedance, but at £15 what do you expect? The Kewtech Loopcheck 107 13A Advanced Socket Tester 230V AC is better than 1.8Ω and better than 93Ω or worst than 93Ω so nearly the same as the TIS one.
If one is concerned about 'personal protection' (RCD operating as a result of a current passing through a human body) then there obviously has to be a threshold beyond which one would 'worry' - but the actual threshold chosen (be it 31 mS, 35 mS, 40 mS or whatever) is not much more than 'arbitrary'.
Quite. This is where RCDs definitely have value (in clearing faults before anyone has a chance to get a shock) - and, as you say, the trip threshold is then pretty irrelevant.
As in, the fault clears (or has time to clear) before someone is touching something, whereas we always assume for personal protection the fault occours when someone is already touching it, can make a difference and that difference could be life saving or injury limiting to some degree.
Exactly - and, although many/most people seem to assume that the latter (tripping because someone is receiving a shock) is the main 'value' of RCDs, I feel sure that the former (clearing a fault before anyone gets a shock) is far more important (in terms of potentially saving lives and preventing serious injuries). In relation to low impedance faults, in a TN installation an MCB will probably clear the fault more-or-less as quickly as an RCD, but RCDs will trip in response to faults of sufficiently high impedance that an MCB might never trip.
There is obviously a very serious limitation to the sort of research into this that can be done (ethically) in humans, but a fair bit has been done over the years, one way or another, to supplement the much larger about of research that has been done in non-human animals.
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