RCD still tripping after damp socket disconnected

[Dazbert]

Hello! The RCD for our downstairs circuits tripped overnight. I narrowed it down to the MCB for the 'kitchen sockets', which seems to cover everything in the kitchen (except the boiler), plus two outdoor sockets. I switched off all sockets and isolators, and unplugged all appliances, but no luck. So following forum tips, I took apart the outdoor sockets and BINGO! it was full of rain. I removed it, dried the wires and taped them up, and blow me down, it still tripped. I connected the two lives, neutrals and earths (to each other as they were in the socket, not altogether!), in case breaking the ring caused a fault, but no difference. I'm about to call someone out, as I'm out of ideas, but thought I'd ask in case I'm missing something obvious.

 

[Dazbert]

I just found my multimeter, opened a socket up and got 65kΩ between earth and live. I'm not an electrician, but with nothing plugged in I'm guessing that means something has chewed through my wiring Oh, no, hang on a minute, scratch that - it's a socket with a USB port on it, so I was measuring the resistance across the transformer. Doh! I disconnected live, and the resistance has gone high again. Circuit still trips though...

 

[EFLImpudence]

65,000Ω is not low enough to cause the RCD to trip.

I don't think that necessarily indicates chewed cables.

Any more wet items?

 

[Dazbert]

Thanks! You're right, it indicates that I am very stupid - there's a USB outlet on that socket, so I guess I was measuring the transformer. I disconnected the live, and now there's high resistance.

There's a second outdoor socket, but the seal was good on that one. I'm not aware of any spills anywhere, certainly nowhere accessible. Water could have gotten into the cavity through the leaky socket, but I don't suppose that matters if everything is insulated.

 

[EFLImpudence]

Does the RCD trip as soon as the 'kitchen' MCB is switched on every time?

 

[Dazbert]

Yes, instantly.

 

[EFLImpudence]

That does indicate that somewhere there is a constant fault from a Line(live) wire to earth - MCB only switches off Line - although your measurements say no.

Need test equipment ?

Are you sure the boiler is not on the kitchen circuit?

 

[Dazbert]

Yep, the boiler is running with the kitchen MCB switched off. Notably, the RCD trips when I turn the kitchen circuit on, but the MCB remains on, although I guess that might just mean that the RCD gets there first. I think I've reached the limits of my diagnostic abilities, and it's time to call in someone with the kit and the knowledge. Thanks for trying to help though. If the thread stays up, I'll post back with the answer when I find it.

 

[EFLImpudence]

Yep, the boiler is running with the kitchen MCB switched off. Notably, the RCD trips when I turn the kitchen circuit on, but the MCB remains on, although I guess that might just mean that the RCD gets there first.

Possibly - or just a relatively "small fault".

A slug somewhere.

I think I've reached the limits of my diagnostic abilities, and it's time to call in someone with the kit and the knowledge.

Yes, your meter might not be accurate enough.

Thanks for trying to help though. If the thread stays up, I'll post back with the answer when I find it.

Yes, please do that.

It shouldn't be too hard to find; it's the once a month trips that are difficult.

 

[JohnW2]

Yep, the boiler is running with the kitchen MCB switched off. Notably, the RCD trips when I turn the kitchen circuit on, but the MCB remains on, although I guess that might just mean that the RCD gets there first.

As EFLI has said, that would be very common with a 'small fault', such as often occurs when water gets into the electrics.

It's not so much a matter of the RCD 'getting their first' - it's probably that the MCB would never trip, even if there were no RCD. A 'live' (technically 'line') to earth fault current of 0.03 A (30 mA) results in instant tripping of an RCD, but it could take 160 A to instantly trip a 32A Type B MCB.

Kind Regards, John

 

[DetlefSchmitz]

I don't suppose there's an external flood light on that circuit that you've forgotten about?

 

[Dazbert]

I don't suppose there's an external flood light on that circuit that you've forgotten about?

No. Good idea, but while there is one, it's on a different circuit. It'd be really nice to discover a final dodgy appliance that I've forgotten about, but I think the in-socket 5V transformer was the last one.

 

[Dazbert]

Update: electrician finally pinned it down to a fault in the cable, seemingly two cables, running from an indoor socket by the back door to an outdoor socket (NOT the one that was leaking), then from there back indoors again. He isolated the outdoor socket by disconnecting the ring at both indoor ones. So now I have power to all indoor sockets, but effectively 2 spurs rather than a ring. He reckons this is fine if he splits the two at the fuse box and puts each on a lower-rated MCB. I'm not so sure, but because of where it is, rewiring is going to be a significant job (bifold door and beam to cross, with a solid floor and ceiling joists going the wrong way).

Not sure why the cables are shorted, they're only 2 years old. Guess I'll find out when we get the outdoor socket wired back in. Thanks for all the replies, and sorry I don't have a more definitive answer to what caused the problem.