One further thought. Is the outside socket (into which the pump is plugged) switched? If so, do you know whether it's a SP or DP switch? Whatever, if it's switched, what happens (i.e. is there an RCD trip) if one switches the pump off with that switch? I'll keep thinking!
Whatever the source of the current after one pole is diconnected, there will still be a need for a continuous path through which the current can flow. Once one pole of the switch has opened, no current from the circuit will flow through one side of the RCD from the corresponding conductor. When the second pole opens, no current will through through either side of the RCD from the circuit. In between those two points in time, current from the circuit can flow through just one side of the RCD (creating an imbalance, hence possibly tripping the RCD) via the non-opened conductor, but only if there is some path to complete that circuit - which, as discussed, in the absence of any conductors to complete the circuit, probably could only be through capacitance to earth. ... at least, that's how I see it!
Indeed, I made the same observation - particularly since, after neutral has been opened (and L not yet opened), it is only N-E capacitance downstream of the switch that would be relevant. However, as bernard pointed out, the capacitance does not necessarily have to be in the cable - it could be from the neutral end of motor windings to earth - where there could be much more scope for a reasonable amount of capacitance than in the cable.
I look forward to hearing what transpires! I know that it could not, in itself, be the whole answer, but I don't think that any of us have asked whether you have checked out the RCD in question, just to make sure that it's not excessively 'trigger happy'?
If you'd stopped there, I think one would be very inclined to blame the pump (although goodness knows what mechanism) - one certainly couldn't blame the sequence of DP switching.
Fascinating. The only thing I can think of is that there was a lot of arcing going on at the L contacts in the original (presumed 'faulty') switch, and that was introducing high frequency transients which, via the capacitance mechanism we've discussed, was resulting in an RCD trip. You're probably 'done with this' now, but I suppose it would have been interesting to try SP switching with the other ('N') contacts of the original switch, to see if those contacts were 'better' and didn't result in the problem.Getting confused, I now thought last thing to try is change the switch. Put on an identical MK 20A DP switch, tried it about 20 times, no tripping!
Fair enough, but in flyingspark's case, that was clearly not the explanation, since the problem persisted when he changed to SP switching and only switched the L.
Fair enough - but I wouldn't get too carried away into believing that, just because it can happen, what the article said is always (or even necessarily often) going to be the explanation for such a phenomenon. After all, it clearly wasn't in flyingsparks' case.
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