But how much more than a standard RCD of reputable make is that?
Mmm - not sure
Yes, but they call that resolution because it is resolution! It could have a 'resolution' of 0.001 mA (i.e. could display a result such as 20.123A - albeit that's a bit daft with an accuracy of only 1%!) but a low sensistivity such as it could not measure (at all, or reliably) below, say 500 mA.
It wasn't me who suggested this approach, and it certainly isn't foolproof (see below). I would prefer to use a custom-designed 'monitor/indicator' something like I described in the OP.
I don't see why that could be any more guaranteed than, say, a passive 10mA RCD - whether is trips before the RCD would surely depond upon the respective characteristics, sensitivities and speeds of operation of the two devices, plus some 'random' factors, wouldn't it? What makes you think it would trip before the RCD?
Indeed, or just 'random leaks' (rather than 'delayed functions'). As I've said, it would take only very minor modifications to the concept of a cheap electronically-actuated RCD to produce the sort of device I postulated. Simply disconnect the SCR (and the latch which triggers it) and tripping solenoid and, instead, connect the output of the sense amplifier to a little bit of voltage-sensing electronics with some sort of non-volatile display or memory. The only real difference is that one will have replaced the electromechanical consequence of a detected leak (which takes a finite time to happen) with electronic sensing which will be almost instantaneous, and therefore will detect (and 'remember') a leak before an RCD operates.
Indeed, as I said, those would be the problems with what you suggested
Indeed. The presence of a VO-ELCB coil (IIRC, usually about 500Ω
in the path to earth would put the EFLI of the circuit up into the clouds, but it would have little effect on the operation of the RCD in most situations. If it did prevent an RCD operating, that would be because it’s presence had reduced the fault current to <30mA, in which case one wouldn’t expect/want the RCD to operate.... unless, as we’ve both said, the earth leakage path was via something (e.g. pipework, or a human being!) other than the CPC
Quite so. I can think of few things one could do which would be more dangerous when one already suspected an L-E fault!!
If the appliance is resulting in sufficient leakage to trip the RCD, then there’s really no sensible/safe way of stopping it tripping the RCD. The sort of ideas you are coming up with (VO-ELCB coils or diodes in the CPC path) are essentially attempts to reduce the fault current to a level which won’t result in tripping. I’m not sure that really helps and, in any event, any such methods (at least, any I can think of) would rely on the (not necessarily true) assumption that the leakage current was travelling along the CPC.
Indeed, but an electronic detector such as I have proposed could be designed so as to be able to detect the current imbalance within those timeframes.
If one had a means of detecting electronically the occurrence of a transient current imbalance >30mA, if that showed that no such imbalance had occurred when it was in-circuit (to the suspect appliance) when the RCD tripped, that would more-or-less ‘prove’ (at least, render it very probable) that the appliance under test did not cause that trip – although that would obviously not mean that it never caused RCD trips. However, in practice, if a trip occurred which was not due to the appliance you were testing/monitoring, it would be extremely likely that the ‘culprit’ was something else!
Sure, a regular RCD tester cannot reproduce all the situations which might result in a trip.
Ultimately, that (or simply replacing the RCD temporarily with a new one) might prove necessary, but swapping the RCDs in a CU is not a trivial exercise, and it would be much nicer if one could first largely eliminate (or ‘convict’!) some suspect appliances by use of a simple plug-in device such as I have postulated.
There are, and one has to wonder what is the difference between the cheapest ones, like the above, and those costing many times more, many of which appear to have a similar spec.
I presume that by 'latching' you mean 'passive', since that is what one would want - i.e. an 'ordinary' (as in CUs etc.) passive RCD - i.e. one that did not 'trip' as a result of lost power, but only in response to detecting an L-N imbalance. As I'm sure you know, RCD sockets and RCD plugs often are 'active' ones (which would be no good for the purpose being discussed) - so I suppose it's possible that the in-line one might also be 'active'.
Indeed, the terminology is pretty confusing. I used to find it very difficult to remember which was 'passive' and which was 'active (since it's not intuitively obvious - at least, not to me!). Then someone sugegsted that I thought about it in terms of a power cut - in that situation, a 'passive' one remains passive (i.e. 'does nothing', just remains in its 'on' state) - and I've remembered that every since!
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