Discuss Nuisance RCD tripping - faulty RCD? in the Electrical Forum area at ElectriciansForums.net

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Shooby

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Hi everyone.

This has been an off and on issue for quite a few years, and seems to return for short periods of a few days or weeks and then goes away for months even a year or more. During the dodgy spells, the RCD trips at pretty random times - it was just after 10pm last night, after we'd all gone to bed. On other occasions it would happen in the mornings, at around 7am, even tho' nothing was being switched on at the actual times.
Weather seems irrelevant - it's been warm and dry here for the past couple of weeks; on other occasions it's been wet and windy.

The RCD can usually be reset successfully, tho' there have been times when it would trip again quickly, and I would then try isolating circuits until it would stay 'on'. The randomness of it all, however, means that it decides itself when to stay 'on', and the circuit I'd isolated at the time was a red herring. (I was darned sure it was the outside lights on one occasion, as isolating this MCB seemingly 'allowed' the RCD to reset. With the ext lights MCB still 'off' for a couple of days while I checked the wiring, the bludy thing tripped again...)

We had an extension built last year, and the sparky did his normal check on the house wiring before extending the socket and lighting circuits for this, and clearly didn't find anything of concern, tho' I didn't point out the issue we'd been having as they just hadn't happened for a year or so before then.

As there is no single circuit I can isolate via their MCB that stops this happening - tho' I haven't been able to check them all as the issue is so random and infrequent - I suspect it's a 'cumulative' leakage in many circuits which is building up towards the '30', and then something - an external 'spike' or summat? - trips it over the edge. For instance, during one spell it would happen at around 7-ish (I think it was) each morning, but there was nothing in the house being switched on at these times. Sometimes it would refuse to reset for a couple of goes, then it would be fine, and stay fine. Other times it would reset the first time, and either stay fine, or trip again 20 minutes later (as it did this morning).

To sum up - no single circuit being isolated has stopped it from happening. There is nothing in the house that can be pinpointed as coming 'active' at these times.

I'm considering having our stupidly large double CU (16 MCBs... 7 do just 'lights', a few rooms each) rationalised down to, say, a 12-way CU with RCBOs, so that they share the leakage. Would this be a reasonable move? I'm guessing the cost would be - what? - £800-ish?

Or is it possible for the MEM A100HE RCD to be faulty - too sensitive?

Thanks.
 
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Mikegh

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Your last point is logical yes, an all-rcbo board will distribute the leakage

Best to get the installation checked first for faults or a faulty rcd
 
telectrix

telectrix

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as above. could be cumulative leakage when no actual fault is present. however, turning off MCBs will not stop tripping if the fault is N-E, not L-E.
 
loz2754

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Switching off MCBs in the hope of narrowing down a possible fault is only part of the diagnostic process. An experienced electrician with the right test equipment should be able to track this down within an hour or 2.

Switching off the MCB only isolates the line, not the neutral, and an RCD can trip if there is a neutral to earth fault. The neutral conductors would have to be removed from the common neutral bar to properly isolate each individual circuit completely.

An earth leakage clamp test, and insulation resistance tests, would be the way to go.
 
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Shooby

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Thanks everyone.

Ah, of course - the MCB only isolates the L.

I'll get the guy who did the extension - I was pleased with his work.

Thanks all.

(I'll report back with findings!)
 
Lucien Nunes

Lucien Nunes

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If the fault is between neutral and earth, tripping might be provoked by increasing load on any circuit, not just the faulty one. If the supply is TN-S that includes load outside of your property, so it is possible for your trip to occur only when two neightbours take a shower at once, etc. So a comprehensive test of insulation is definitely the next move. We have heard of smart meters causing random trips when installed with their antennas too close to the RCD, too.
 
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Shooby

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If the fault is between neutral and earth, tripping might be provoked by increasing load on any circuit, not just the faulty one. If the supply is TN-S that includes load outside of your property, so it is possible for your trip to occur only when two neightbours take a shower at once, etc. So a comprehensive test of insulation is definitely the next move. We have heard of smart meters causing random trips when installed with their antennas too close to the RCD, too.
Interesting, Lucien.

I have an earth rod, I understand - how does this fit in with the above?

Cheers.
 
Lucien Nunes

Lucien Nunes

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Same as TN-S but always subject to the many different possible multiple earthing scenarios in the distrbution main. With an N-E fault, any EMF between your own neutral and earth drives current through it and creates an imbalance at the RCD causing a trip. The potential mainly occurs due to voltage drop along the neutral conductor due to load. The higher the load on the system, the greater the difference between the neutral at the point of the fault and that at the nearest point at which the neutral main is earthed (and hence at a similar potential to the rod.) When that difference divided by the resistance of the fault path exceeds the RCD threshold, it trips.

But an N-E fault is only one possible scenario. Until the installation is fully tested you can't be sure this is what is happening.
 

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