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Discuss MCB triops every 20, 30 mins. in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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HI all,

Got a problem here

about 12 lights on a circuit, IR tests out ok, but the MCB trips about every 15 or 30 mins.

cant for the life of me find the bloody fault, taken everything apart but nothing,, i switch on all lights nothing for 15 mins then trips, i turn off all light nothing, then suddenly trips, how the hell can i test something that's not there, its really doing my head in, Board changed, down stairs neutral borrowed on the landing light so them two circuits on the same trip, no shaver, no fans

one thing i haven't try'd is Changing the LE lamps, but it still trips when all lights are off,, OMG its doing my head in

any ideas going forward

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  • Thread Starter Thread Starter
  • #3
no its an extension and i was temporarily putting ends in the old board when it started, so ive changed the board now, and changed the MCB for a 10 amp from 6, there has been a chippy working alot there, but cant see where he could have nailed a cable, thats what im thinking of now, but its just that it happens every 30 mins or so, not every 5 mins, or every 2 mins. so bloody strange.


Or a plate screw just pinching the common in the switch enough to drive you mad.

AP Electrical

Can you take some of the circuit off and see if that helps locate ie remove leg which feeds6 of the lamps
  • Thread Starter Thread Starter
  • #8
pendants and chrome bathroom lights, had all the switches off, most of the switches is where the feeds are, i assume its a dead short and not an earth fault as it doesnt trip the RCD, whats bugging me most is the time on it, the 30mins+, i could live with something getting HOT then tripping but then it wouldnt take that long to trip again if i turned it back on has its hot already if you know what i mean,

i have about 8 new lights in the extension and about 7 old light from the old part of the house. going back on thursday as they are away. so on one will be in the house.

one thing that has happened is a loft extension, so joists have been layed on top of the loft rafters so thats my next cosentration point, but still the timing of the bloody think bugs me, the bloody fault is either there or not.

thanks for the reply' s guys



Can you disconnect the "new" from the "old" to try and narrow down where the issue may be?


one thing guys, this circuit has been live for weeks with no problems
In which case where have other trades been doing work, not looking to point fingers but there could be a correlation between the work and your "fault".

Have fun and do let us know what you find. Faults like this are a pain to locate!


Have you put all the lights on then clip an amp probe at the CB ? also if stuck swap the supplies over with another circuit and see what happens

ie check what load the circuit is drawing plus if ok and you swap it with another CB then see if that trips as well.

If you dont have an amp probe check Screwfluxout as they have a budget one for about £10 bucks
Last edited by a moderator:

tony mc

What were your IR results between Line & Neutral.

And if clear have you tried keeping the test on for a least 1 minute to see if it drops?


Probs wont help you too much but i had the same fault on a med centre the other week tripping randomly anything from 5 mins to 5 hours. Bunch Tested circuit, clear. Replaced mcb, Same. Dissconnected all lights tested again, clear. Eventually what i fouind it to be was when it had been installed at least 15 years ago in galv pipe and singles they used ceramic connector in a through box then into the fitting in flex. and it was a line single trapped between ceramic block and through box and over the years with heat etc it had blown apart and then obv tracking across at random times and tripping! Took me 9 hours to find this bugger! Had a well deserved pint that night!


Split the circuit mate and try it it might give you a bit of a clue
  • Thread Starter Thread Starter
  • #18
ok guys, thanks for your help and reply's,, it was a steel beam put down for the loft bedroom, it was trapping the cable, had to take the landing ceiling down to find it, two days labour x2 men down, but glad ive found it as its a weight off my mind.

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