Downstairs lighting circuit MCB randomly tripping since electrician's visit last week.

Maybe too late if you've already booked him in, but you could potentially use one of these live-AC detectors to follow the cables... power-off all the other MCBs then hope you get lucky with the proximity of the offending cable to the plasterboard. I've no idea how good they are in reality. https://www.ebay.co.uk/itm/3-in-1-D...AC-live-Wire-Intelligent-Finder-/291816705001

Alec_t said:

If the problem has only arisen since the downlighters were installed, my money would be on the feed wiring to the downlighters having been damaged, or badly routed such that insulation has melted at some point, causing an intermittent short of an unswitched wire.

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That was my initial suspicion but it's not easy to say. The MCB only started tripping last weekend and the (3 x 5W in this circuit) downlighters have been installed since April. This week when the electrician removed the melted transformer it seems to have stopped the MCB from tripping even though there is clearly still a fault, so who knows how long the mysterious 2kW fault has been on the go for if - without the melted transformer - it is no longer tripping the MCB?

garyo said:

Maybe too late if you've already booked him in, but you could potentially use one of these live-AC detectors to follow the cables... power-off all the other MCBs then hope you get lucky with the proximity of the offending cable to the plasterboard. I've no idea how good they are in reality. https://www.ebay.co.uk/itm/3-in-1-D...AC-live-Wire-Intelligent-Finder-/291816705001

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I've used one of those in the past and it seemed like a random noise generator, didn't seem to find anything that I knew was there.

garyo said:

Maybe too late if you've already booked him in, but you could potentially use one of these live-AC detectors to follow the cables... power-off all the other MCBs then hope you get lucky with the proximity of the offending cable to the plasterboard. I've no idea how good they are in reality. https://www.ebay.co.uk/itm/3-in-1-D...AC-live-Wire-Intelligent-Finder-/291816705001

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Better to leave the lighting one powered off, as was suggested by John D. Those detectors are not 100% reliable, it is better to depend on IR testing and visual inspection.

As the MCB seems to have stopped tripping, is there any other reason that the meter may flash at that rate?

Good point EFLI.

garyo said:

I've no idea how good they are in reality. https://www.ebay.co.uk/itm/3-in-1-D...AC-live-Wire-Intelligent-Finder-/291816705001

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ban-all-sheds said:

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Chocolate teapot proves useful
http://www.bbc.co.uk/news/uk-england-york-north-yorkshire-29126161

"Chocolatiers have managed to produce a chocolate teapot that held boiling water for two minutes to make a drinkable brew."

Hmmmm.

EFLImpudence said:

As the MCB seems to have stopped tripping, is there any other reason that the meter may flash at that rate?

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Well, the fact that the rapid flashing stops when the MCB is 'switched off' would seem to indicate that, almost certainly, a high current is flowing (to somewhere) in the circuit concerned when the MCB is 'switched on' and all known loads on the circuit are 'off'. Any other attempt to explain those facts would require one to postulate co-incidence of two or more obscure faults, wouldn't it?

Kind Regards, John

stillp said:

Better to leave the lighting one powered off, as was suggested by John D. Those detectors are not 100% reliable, it is better to depend on IR testing and visual inspection.

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Yeah, as mentioned I am leaving it off.

These two rooms that now have downlights, may be worth checking above where the old original lights were, as its likely 3 plate wiring, a problem there could exist and trip the mcb even with the lights switched off

JohnW2 said:

Well, the fact that the rapid flashing stops when the MCB is 'switched off' would seem to indicate that, almost certainly, a high current is flowing

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It would but that's why I asked "As the MCB seems to have stopped tripping, is there any other reason that the meter may flash at that rate?".

EFLImpudence said:

It would but that's why I asked "As the MCB seems to have stopped tripping, is there any other reason that the meter may flash at that rate?".

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Indeed, and I answered that question by offering my opinion as to what was 'almost certainly' the cause of that, the corollary of which is obviously that my opinion is that it is 'almost certainly not' due to 'any other reason'.

In fact, having just tried, I cannot really even think of any credible scenarios in which 'multiple co-incidental obscure faults' could do it. For the meter to be flashing inappropriately rapidly could, of course, be due to a faulty meter, but for that behaviour to cease when one particular final circuit (which had no known active loads, anyway) is de-energised makes it very hard to see how anything but genuinely high current in that circuit (which should have no current) could be responsible. Had you thought of any other possibilities?

Kind Regards, John

Some years ago whilst doing inspections and remedial work for a local authority prior to home improvements I came across a property with a socket circuit fault – I can’t exactly remember but it was something like a dead short to earth from live. RCD hadn’t tripped but was found to be faulty anyway and so replaced with healthy one. However, replacement RCD still not tripping even though I knew that there was an unresolved earth fault on the system.

After disconnecting various socket legs I narrowed the fault to a section of cable that disappeared into the structure of the building and could not be accessible without dismantling stuff.

The 3 bed property was occupied by one elderly lady who made very little use of the electrics but for whom I had to put back the circuit as I found it so she would have power to sockets she was using. I reported the problem back to HQ and explained that I thought the current was exiting via earthed metal (old disused hot air ventilation system) and not enough current was being created to trip the RCD. I explained it could be dangerous for a plumber disconnecting a pipe and unexpectedly making love to a very high fault current. The response was “Does the RCD work?” "Well yes" , response “then there is no problem.” Needless to say that report had an appended page which listed the problem in some detail.

After such a long winded reply my summation is that there may be a similar fault on your system with a damaged cable going straight to earth. In the first instance, get the faulty RCD replaced and get the circuit properly IR tested as suggested by others.