Tripping MCB

RobFJ · 22 May 2020

I've got an intermittent fault on a lighting circuit that I've been monitoring for over a month.

There are three unusual things about it :

it only happens about every third day and then there are usually multiple trips over the space of an hour or so before things settle down again.
about half the trips are between 1600 and 1800 - the other half are between 0100 and 0400 when there is no human activity and all lights are off.
if I reset the MCB and RCD immediately they quite often trip again and usually reset on the third attempt.

I've switched MCB's (both type B) to see if it was an MCB fault but the fault persisted on the other MCB; I also tried a Type C to see if that made a difference but again the fault is still there.

Does anybody have any further ideas before I have to start ripping up floorboards and cutting holes in ceilings !

Many Thanks

JohnW2 · 22 May 2020

RobFJ said:
if I reset the MCB and RCD immediately they quite often trip again and usually reset on the third attempt.

Are you saying that both MCB and RCD are tripping - and, if so, is that always the case?

Kind Regards, John

Colin Brenton · 22 May 2020

The pros will be able to give better advice, but if it were me, I'd split the circuit about half way, and see if the problem continues (if it does, the fault is on the 50% of the wiring that is still powered). Divide and conquer

Obviously, check the lights and switches for damaged insulation, water ingress, trapped/pinched wires etc.

Better/easier this than holes all over the house. Good luck!

chivers67 · 22 May 2020

Find the last light and disconnect it and work backwards until the Fault disappears or get an Electrician in to fault find. Unless you have a multi meter and can measure resistance?

Or what Colin above suggests

Harry Bloomfield · 22 May 2020

Ideally, you need a megger to check the insulation resistance, to confirm there is an insulation breakdown somewhere. When you confirm there is a fault, to work your way around the circuit. Depending on how it is wired, the live and neutral run/loop will be accessible at every ceiling rose, where the loop can be broken, for further tests.

Usual way of finding such a fault, is to isolate in the mid point and see if it is before or after that point, then quarter the circuit.

Chances are, it will be a floorboard nail or screw, which has nicked a cable under a floor, or a fixing in a wall for a picture or similar.

ericmark · 22 May 2020

If I am reading correct the RCD is tripping if either MCB is one, this would suggest an earth to neutral fault. But not sure if the MCB is tripping, but unless the MCB was to trip swapping to type C would do nothing so not so sure.

So if the MCB is not tripping then the test is to remove the neutral, but with such an intermittent fault the first thing I would do is get out the installation tester and with the neutral isolated test neutral to earth. Think my installation tester cost £35 OK not calibrated but you don't need it calibrated, to my mind trial and error is not the way forward, way forward starts with installation tester.

AndyPRK · 23 May 2020

A picture of the tripping devices may help.
You say two Mcb?

Deleted member 18243 · 23 May 2020

Are there any outside lights on the circuit? Seals could have failed and let rain in, which can trip MCBs.

Not that there's been much rain lately.

RobFJ · 23 May 2020

andy11 said:
Are there any outside lights on the circuit? Seals could have failed and let rain in, which can trip MCBs.

Not that there's been much rain lately.

Thanks to everyone for comments and suggestions - much appreciated. There is an outside light, on a 2-way switch which, on closer inspection, is either fully illuminated or slightly illuminated depending on whether it should be off or on. (Easy to spot in darkness - almost impossible in sunshine

) There must be a junction box hidden en-route as the wire colours at each termination point won't match. Time to call in the professionals

At least 90% of the circuit has now been ruled out with everyone's advice. Thanks again to everyone.

RobFJ · 17 Jun 2020

Just to conclude, The fault turned out to be a partial short on an external cable running under the mortar in the outside wall. Its insulation had been slightly damaged 20+ years ago when a pergola had been put up and had got worse until it failed. Moisture would get into the cable completing the short and causing sparking that would trip both the MCB and burn off the moisture. It would next trip only when the moisture level was sufficient to again complete the short - hence why the fault was so unpredictable. Regs were different in the 80's (or it was a bodge) as there was no armour cabling or conduit. Now fixed and fully compliant

ericmark · 17 Jun 2020

Over the years there have been land mark changes, 1966 earth on lights and 2001 we started on RCD increased 2008 but most things were active by 1992 when the wiring regulations became BS7671. However 1992 seems to be the point when electricians started taking exams to show they knew the regulations, there have been regulations since 1882, but in the 70's the boss might have a copy in his office, but ask to look at it and he wanted to know why, so Chinese whispers was the main way the rank and file learnt what was permitted, in some ways it still is, the early regulations were like the regulations and the on site guide combined, so it would suggest distance sink to nearest socket, so you had to unplug kettle to fill it.

I would be interested to read an old copy of regulations now, but earliest I have is 2001. But reading old guides which were then printed by the Union not IET it seems not that much has changed since the 12th Edition in 1950, that was when the ring final was born I think, the 13 amp plug and socket.

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Tripping MCB

Deleted member 18243

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