MCB Tripping On Lighting Circuit - Out Of The Blue?

[TMess]

Firstly, no alterations to any lighting circuits have been undertaken in years. Nevertheless, yesterday, the MCB on our Consumer Unit for our upstairs lighting circuit tripped. I switched it back on and a couple of seconds later, it tripped again, we once again I flicked it on and it stayed on all night. This morning, it tripped once again, and then again after I turned it back on.

Before calling out a real sparky, is there anything obvious I can check that could be causing this (light bulbs, 12v transformers, etc.)?

 

[Rockingit]

It could be that you've had a transformer go if you've 12v lighting fixtures. My suggestion would be to try to isolate the circuits one at a time (switch on the wall is probably fine for the job) and see if you can narrow the field.

 

[Rockingit]

The reason that it will trip straight away again after it's gone is because it's still hot - they work by a thermal mechanism - so leave it a few minutes to cool down internally.

 

[Deleted member 26818]

When you re-set the MCB, were any of the lights connected to the MCB still switched on?
Did you try re-setting the MCB with the lights switched off, if so did it make ant difference?
Is there an extractor fan connected to the circuit?
Are there any outside lights connected to the circuit?
Did you conduct any form of fault finding at all?

 

[AdieB]

As above plus has it been raining do you have a leak in loft which has come into contact with electrics?

 

[tony mc]

Or a faulted MCB ?

 

[TMess]

It could be that you've had a transformer go if you've 12v lighting fixtures. My suggestion would be to try to isolate the circuits one at a time (switch on the wall is probably fine for the job) and see if you can narrow the field.

Even with all lights turned off at the light switch, the MCB still trips after some 30 seconds from having been re-set.

 

[TMess]

When you re-set the MCB, were any of the lights connected to the MCB still switched on?
Did you try re-setting the MCB with the lights switched off, if so did it make ant difference?
Is there an extractor fan connected to the circuit?
Are there any outside lights connected to the circuit?
Did you conduct any form of fault finding at all?

1. Yesterday, yes.
2. Today I made sure they were all switched off before re-setting the MCB. No difference, see above reply.
3. Yes, this only turns on with that room's lights.
4. No.
5. If you are referring to the use of any testing equipment, then no. This is beyond my knowledge.

 

[Deleted member 26818]

That would suggest that there is a fault on the circuit, rather than a particular light fitting causing a high load.

 

[TMess]

As above plus has it been raining do you have a leak in loft which has come into contact with electrics?

It's been foggy these last few days, however there are no obvious leaks from the rain we have had.

- - - Updated - - -

Or a faulted MCB ?

I swapped MCB's from an adjacent circuit. The 'old' MCB works on that circuit, the 'new' one still trips.

 

[TMess]

That would suggest that there is a fault on the circuit, rather than a particular light fitting causing a high load.

How could this have come about, seeing as no alterations or additions have been made to the circuit in years? Anything obvious I can still check for prior to calling our sparky in?

 

[Richard Burns]

Looks like you have a not obviously locatable fault, something like a damaged cable where it enters a light switch or passes thorough an opening or under boards.
Have you had any other work done recently that may have damaged a cable, changes to floor coverings, items installed on a wall, any drilling or nailing?
The last possibility is rodents nibbling a cable and getting through to the live wires.

 

[Deleted member 26818]

There are a number of possible reasons.
Could be just a loose connection somewhere, could be a damaged conductor, ar as AdieB suggested water, either through rain getting in, or even a leaky radiator or pipe.
There's even the possibility of rodent damage.
The most likely cause, is a loose or a number of loose connections.

 

[TMess]

We haven't undertaken any work which would have affected the upstairs lighting circuit. Unsure about whether rodents could nibble cables upstairs?

In all honesty, is an electrician likely to find this fault much quicker than I could (i.e. using their testing equipment)? Or is it purely a case of ripping floor boards up?

 

[Rockingit]

Even with all lights turned off at the light switch, the MCB still trips after some 30 seconds from having been re-set.

See my additional post #3! It will do..... you have to let it cool down for a couple of minutes first else you'll go round in circles.