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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> Another nusiance RCD tripping https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1332424067 Message started by DUB8888 on Mar 22^nd, 2012, 1:47pm

Title: Another nusiance RCD tripping
Post by DUB8888 on Mar 22^nd, 2012, 1:47pm

I have had a problem with a very random tripping of the RCD on my CU. I have isolated all the appliances over time and can't narrow it down. There is no pattern to the tripping, it can happen in the day, in the night, with appliances running or turned off, rain, dry, etc etc.. can't tie it down to a specific pattern. So I called an electrician. He said the wiring checked out and that the RCD needs changing at a cost of £140. The RCD is a 80A with a 30mA differential. I asked what it was tripping at when he did his ramp check and he said 23mA which he said is low. My understanding is they should trip before the 30mA so first question, is this 23mA low and should I get the RCD replaced?I wasn't totally satisfied with his price so I decided to take the cover off the CU and have a look for a part number of the RCD just to price it up. I noticed one of the earths for the downstairs lights MCB loose so I tightened it up. I know the RCD will trip when there is a difference in current between the Live and the Neutral but could the loose earth on a MCB cause the RCD to trip without the MCB tripping?Any help would be greatly appreciated..

Title: Re: Another nusiance RCD tripping
Post by Lectrician on Mar 22^nd, 2012, 3:49pm

An RCD is "within spec" if it trips on a ramp test between half it's rated operating current and it's rated operating current.

What make RCD is it - can you take a photo?

Swapping out an RCD hardly ever cures a nuisance tripping problem - it is usually a wiring issue somewhere which needs tracking down with an insulation resistance tester, and someone competent enough to use one!

Title: Re: Another nusiance RCD tripping
Post by DUB8888 on Mar 22^nd, 2012, 3:55pm

Lectrician wrote on Mar 22^nd, 2012, 3:49pm:

Thanks for your reply. The RCD is a Legrand 086 31, 80A 30mA. The chap who tested the RCD and wiring was a Part P registered Electricrian.

Title: Re: Another nusiance RCD tripping
Post by Lectrician on Mar 22^nd, 2012, 5:00pm

I would say most sparks would be able to source a Legrand RCD for no more than £30, charging you around £40-£50 max. With an hours labour, your looking at around £70-£80 + VAT max.

An RCD tripping at 23mA when ramp tested would not be an issue at all, and would be a fairly typical reading.

Title: Re: Another nusiance RCD tripping
Post by londonman on Mar 23^rd, 2012, 7:27am

Just because he's Part P registered means nothing.

Borrowed neutral?

Mice nibbled part way through a cable...we've had that..a right pain to sort out and identify.

Title: Re: Another nusiance RCD tripping
Post by DUB8888 on Mar 23^rd, 2012, 11:08am

Just had another quote for just the RCD change which was £100. Sounds a bit more reasonable, however like most he believes that at 23mA tripping isn't a definite faulty RCD.
I think I will get another Electrician in to troubleshoot now...