MCB faulty?

[Detling]

Not boat but home related. One of the MCB's in the distribution box keeps tripping very few hours (between 1 and 12 hours) it is on a lighting circuit. Even with all lights off it sometimes trips and sometimes trips with one or two lights on. It resets with no trouble.  If it was a fault to earth I would expect the RCD to trip but it doesn't, and as I understand it, a MCB only trips on over current, which I do not think is happening here. Do they go dodgy as they get old like me, because this one is over 20?  I can't think of any way an overload could be present with all lights off which is not present a few seconds later when I reset the trip. If it helps this has occurred after an old style bulb in an infrequently used light went bang, that took out the MCB and RCD. and is no longer in the light, but wondering if the MCB enjoyed it's trip and has developed a habit.

[George and Dragon]

That does seem rather odd. I think I would get an electrician to do an insulation test before replacing the MCB as the problem may not be in the MCB.

[GUMPY]

MCB do fail, its an easy replacement and a new one should be less than £5.

I would have replaced it by now.

[Detling]

7 minutes ago, George and Dragon said:

That does seem rather odd. I think I would get an electrician to do an insulation test before replacing the MCB as the problem may not be in the MCB.

Surely an insulation leak would trip the RCD and not the MCB,  I could be mistaken.

[WotEver]

8 minutes ago, Loddon said:

MCB do fail, its an easy replacement and a new one should be less than £5.

I would have replaced it by now.

^^^^ Wot he said. 

Just now, Detling said:

Surely an insulation leak would trip the RCD and not the MCB,  I could be mistaken.

Yes, exactly so. 

15 minutes ago, Detling said:

If it helps this has occurred after an old style bulb in an infrequently used light went bang, that took out the MCB and RCD.

Just the kind of occurrence that would stress an MCB. 

[Detling]

Thanks for confirming my thoughts new MCB required preferably before tonight.

[Tonka]

Had this happen a couple of years ago on the boat. Whenever I tried to put the inbuilt generator on line the rcd would drop. Spent hours looking for an earth leak. It was only when I got back to the mooring and plugged in the shore line and the rcd tripped i decided it was the rcd at fault. Quick trip to screwfix and got new rcd. Fitted and all was well on shoreline and generator

[Tracy D'arth]

Check the bulb holder for metal flash inside, but probably the MCB is tired.