Comsumer unit RCD trips on non-RCD protected circuit activity

[ian]

Hi,

I've got an 8/9 yr old bungalow and a split consumer unit where (only) the ring main and garage circuit breakers are protected by a 30ma/100A RCD trip switch.

Today, this trip started going at almost any opportunity: unplugging and resetting didn't help much as any random device may or may not cause the switch to trip, and sometimes it took a minute before it went. Once it tripped when I dropped a 4 way extension block that had no devices connected to it.

I borrowed a PAT tester to check for leakage and plugged it into the cooker socket as that is not protected by the RCD, but there wasn't much (only found

0.1mA from one device and after checking all bar a couple of devices the rest were solidly 0.0mA). When I plugged a hair dryer into the PAT unit and ran it up on full power the RCD tripped again. This doesn't fill me with confidence as the consumer unit has a split live rail and the RCD is 'downstream' from the cooker circuit breaker - I don't think it was something else switching on on the protected ring as the same set of appliances didn't trip it out for five minutes afterwards.

The general consensus I've got offline so far is that my wiring is stuffed. I'm having the loft converted at the moment but the new rings have not yet been connected to the consumer unit, and there has been no-one doing any work for over three weeks (that's another rant, but serves me right for not DIY, I suppose :) I've asked for the electrician doing the work to return, but not heard back from him so far, so I'm looking for advice on tracking things down a bit.

I will test the remaining devices for leakage (UPSes, which kicked in fine when the RCD tripped, so I think they're ok) but after that the best advice I've had is to remove every device from the ring main and turn it off, then remove all the socket fronts and check the tightness of the wiring, followed by plugging things back one at a time. The wiring check seems fine, but as it's a random selection of devices and never just one that causes the problem, I don't know how putting them back individually might help.

Does anyone have experience of tracking down stuff like this ? Is it worth turning off the consumer unit and doing the N1-N2, L1-L2 tests mentioned in the FAQ ?

Thanks in advance,

[Christian McArdle]

You've probably got an earth-neutral short somewhere or such like. These can cause weird trips even of different circuits. You should do a full periodic inspection, which should hopefully find which circuit is at fault, which could be completely different from what you are expecting.

To do the tests you need a proper continuity insulation tester. A 6 quid multimeter from Maplins will NOT suffice.

Christian.

[ian]

Ok; weird is good, 'cos that's what I've got :)

A quick Google shows a bewildering variety (and cost !) range - do you have any makes and models that would be typical of the sort of device I should be looking at ? I'm not after a recommendation, but something that can show me which ones are suitable for domestic testing as I'm sure it's possible to get distracted by non-essential (for this task) features ?

Thanks in advance,

[Christian McArdle]

Megger BM121 is as basic as they come. Mine is a rebadged Newlec one of these. You'd probably be better off begging or renting one.

Most people these days buy multi testers which also include earth loop impedence and RCD testers. Thinking about it, you could use the RCD tester bit...

Christian.

[ian]

Thanks for that: off to ask around offline and see what's lurking in other peoples' tool boxes...

TTFN,

[Andrew Gabriel]

Not for a full inspection, but it might still find a neutral-earth short.