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Hello,
My daughter has just moved into a house and asked me to fit a new, fancy light fitting in the main bedroom. However, I discovered that none of the MCBs in the consumer unit turn the power off to that bedroom (and one other at the front of the house), despite there being two MCBs that are marked as being for 'Upstairs lighting'!

Is it likely that one of the MCBs has failed (and so won't trip off)? Or is there another likely cause for the problem? I know I can (and should) turn off all the power at the CU but having found this issue I am concerned that she may have a bigger problem to solve.

Many thanks for any suggestions.

Well before an electrician answers, you could do some deduction: are there any MCB's which when in the off position do nothing to any of the house outlets (specific if you want, those marked for lighting) Lighting MCB's can range from 6 to 16 ampere mostly depending on the size of cabling. If a particular MCB does nothing it could be not connected, I have several redundant in my consumer unit, or failed.

Thanks for the quick reply. I'm not at my daughter's house at the moment, so I'm going from memory, but I believe that all of the MCBs in the CU do turn something off when in the off position, except for one of the MCBs that are labelled as 'Upstairs lighting' - which maybe points to the MCB being faulty? I have no idea how common it is for one to fail in the 'on' state. The only thing that kills the power to the affected bedroom is to turn the main power switch off.

My friend had the same problem few years ago, he had only one fuse box (cu) under staircase, he called an electrician who found another one behind the fridge he had in the same area. You may have something similar?!

I don't think that's the case - but I'll double-check next time I'm there.

If it is as you say, and there is not another consumers unit somewhere, and you have turned all mcb's off one at a time (Might be labled wrong, unusual for two lighting circuits on the same floor) And the lights are not fed via a fused connection unit, then either it it is wired wrong or there is an mcb problem, so you should get an electrician to look for you, may be worth while getting an EICR done too. (like an MOT for a vehicle)

Hum, before you spend money on a spark ( and yes I’m a spark) turn said light on. Then turn off the circuit breakers one by one and see what happens, the light stays on turn each one off in turn and don’t switch it back on.

Then report back

Thanks - I will try that, but it will have to wait until I'm at my daughter's place again (so don't expect a quick update!).

davec56 wrote: ↑Mon Jun 14, 2021 3:40 pm Thanks - I will try that, but it will have to wait until I'm at my daughter's place again (so don't expect a quick update!).

No rush, better safe than sorry

Mmm. This could turn into one of those that we had a few years ago. Daughter had a problem but Father lived in London. When prompted, it was disclosed that the Daughter lived Barnsley?

A member Lecky covered that area and solved the problem.

Link here.

viewtopic.php?t=58806

Blimey. Was it really that long ago. - 2012

Tewkesbury and Worcester - so not that far apart! Hopefully I've got the electrician that I use coming to have a look on Thursday, so I'm sure he will sort it out.

Neelix wrote: ↑Mon Jun 14, 2021 2:19 pm Hum, before you spend money on a spark ( and yes I’m a spark) turn said light on. Then turn off the circuit breakers one by one and see what happens, the light stays on turn each one off in turn and don’t switch it back on.

Then report back

This is also what I would do, I have had it where two MCB's control the same circuit, at some point in the past the circuits had been split, so two lighting circuits, then they were linked again. In fact had it in my own house, a 4 switch plate controlled hall, landing, and two sets of outside lights, two independent supplies, one for landing light, and one for other three, and some how they had go mixed up. Found went went to all RCBO consumer unit.

My house the split is side to side for sockets, and up/down for lights one middle and upper floor, and lower floor independent RCBO's for lights and sockets, grouped upper two floors on left of consumer unit and lower floor to right. But easy to mix up and it took me 2 months to find out where one supply went, turned out to be a FCU likely originally outside socket. Rear outside lights are switched from hall at middle level, in spite of middle level not extending to rear that far, lower floor originally just a garage.