Is this a knackered MCB?

[neds]

Turn the isolator off and it will allow me to turn the MCB back on. As soon as the main isolator is turned back on it trips all the MCBs again.

The offending MCB is a 6A lighting MCB for one of the lighting circuits. All the lights are off and the MCB will still trip instantly with no load. Had a quick look at the switches and nothing obviously amiss.

Any ideas what could be going on?

Cheers

 

[Spark123]

More likely you have a fault on your lighting circuit.
Had any work done recently, replacing lights, hanging pics etc?

 

[neds]

I thought so, well its not my house. As far as I was aware there has nothing recent been done on that circuit to it. A lamp may have blown but this should have just tripped the MCB out at worst?
If you have done some work on one lighting circuit would that affect the other in any way?

 

[nozspark]

I've had this type of thing happen where the pendant drops are old cotton(?) insulated and the insulation has worn through over time!

Just a thought

 

[Spark123]

I doubt it, unless you had disturbed something.
I asked if any work had been done as it is common for people to make mistakes when changing light fittings.
Could be anything on the circuit, faulty lamp holder, nail through a wire that has just been disturbed, faulty lamp gone closed circuit, mass of connection block shoved above a light melted, rodents attacks, water ingress, failed insulation are a few which spring to mind. Without the knowledge and test equipment you will probably struggle to find the problem.

 

[neds]

Hmm I was thinking the next option would be to remove all light fittings and switches and test one by one. I suppose switched live wires would be easy enough to test (continuity at least) but the actual lighting circuit would be difficult. Is it worth going through this procedure?

I've had this type of thing happen where the pendant drops are old cotton(?) insulated and the insulation has worn through over time!

Just a thought

Will double check but I'm pretty sure they are all the plastic variety.

 

[Spark123]

How do you intend on doing the testing? Don't do it by repeatadly closing the MCB and seeing if it trips. I'd be using either an ohm meter if it is a dead short or an insulation resistance tester (AKA Megger).
The fault could be L-N or L-E.

 

[neds]

When it comes to megger tests then I'm at a loss really. Looks like it may be a call to a spark, I know where my limits are! I was trying to save any cash for the home owner as a call out to a spark could run up to £100's.

Cheers

 

[kkynaston]

I've had this type of thing happen where the pendant drops are old cotton(?) insulated and the insulation has worn through over time!

Just a thought

Had this when we bought our current house. Bulb went on landing and I didn't know whether the switches were off or on. (Yes I know should of flicked off mcb ) Took out the old bulb, as I twisted in the new bulb a short occured on the drop, there was a nice big bang and a blue flash, then the pendant dropped onto the floor in flames

Needless to say the next thing I did was to replace all the pendant fittings thoughtout the house