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Hi I hope this is the right forum to ask some advice for what I hope is a straight forward question.

Short version: Is an RCD required in domestic installation where there are 2 consumer units, each with its own RCD?

The layout of the system is
Meter >
RCD (BG 80A/30mA) > split >
CU1 with RCD (MEMAD100ME) 100A 100mA + 10 trip: B32A for cooker, B16A for outside sodium lights, 8xB6A Lighting circuits

CU2 with RCD (MEMAD100ME) 100A 30mA + 5 trips, all B32A for sockets & gates

Long version: I'm having a problem with a nuisance tripping RCD. It will trip (taking all circuits in the house with it) and if you immediately reset it then its fine. This is happening seemingly at random perhaps once or more times a month. Its not when anything is obviously coming on, heating, kettle, iron, etc. The frustrating thing is that it is ALWAYS the RCD closest to the meter that goes taking everything with it. It has tripped once when we were on holidays, defrosting the fridge freezer. We have electric gates and it did seem to happen more often when they were moving (not every time) but I have turned them off its still happening occasionally, slightly more often in bad weather. I don't know if it matters but we are rural and have the power coming from overhead lines and a pole in the garden.

I've spoken to 2 electricians and both say its very very difficult and expensive to troubleshoot a fault like this and to be honest while I'm not on the bread line I can't afford a bill of hundreds and hundreds with no definitive solution. I'm nervous that they both went down the lines of 'turn everything off then on one at a time to see what does it' and when I explained it happened occasionally and not on demand neither made any reference to systematically testing each circuit, although this was maybe implied by the 'difficult and expensive' bit.

I was pondering how to try narrow it down and what the purpose was of the MEM AD100ME switches because they have never triggered. When I looked them up and realised they were RCDS as well. Does that sound right, essentially 2 in series? And with a lower maximum Amp?

It would just be much more straight forward if it was a single circuit that tripped not everything every time. At least if the 'last' RCD wasn't needed then it would trip one CU or the other and you'd have a start. There is space on each CU so I don't know if it'd be sensible to move circuits one at a time to narrow down the culprit?

I should make clear that I've every intention of getting an electrician to carry out any work but I don't want to be taken for a ride if there is something obviously wrong in the setup or a straight forward test that can be done (by a _good_ electrician) to track down which circuit is the problem, or if its a combination thats hovering close to the limit and being pushed over the top. If wiring needs replaced I'll do it of course if that's the safest thing to do.

Thanks for any help in advance, I just don't know who else to ask.

-Steve
 

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