MCB won't reset on simple lighting citcuit

[Blaina]

Hello,

I have a simple lighting circuit in my bungalow which has worked without hitch since the place was rewired about ten years ago.
But now I get an occasional tripping of the MCB, sometimes it will reset and stay OK for weeks then it suddenly trips again and won't reset. I normally only use the overhead light in the kitchen for any extended time (The lounge is lit by side lights from the power circuit).
The MCB seems to trip only when the kitchen light is on - sometimes hours after it has been switched on it will suddenly trip for no apparent reason, without anything else on the circuit having been touched or switched on.
What could be the reason for this please?

All help appreciated.....Blaina

 

[bongos]

If your MCB keeps tripping it suggests you could have an intermittent fault between L/N or L/E (I'm assuming that there is no RCD involved anywhere). At certain times enough current is flowing to trip the MCB. It could be a faulty MCB but this is much less likely.

The title of your post suggests that you are unable to switch the MCB back on at all? Is this the case? Try turning the main switch off for all circuits and then flick the lighting MCB back on. If the lighting MCB trips again when you turn the main switch back on then it confirms that you have a fault on the circuit and not a dodgy MCB.

Is it only the kitchen light that causes it to trip? Another idea might be to turn the power off and then open the light fitting in the kitchen and check all the connections are sound. If everything seems ok here then it could be a loose connection at any other light fitting or any junction boxes under the floors.

This won't be easy to find and might be best to call an electrician to sort.

 

[Blaina]

By the method you suggest I have confirmed that MCB is OK, so I will just have to go through process of elimination for loose connections.
Fortunately it's not a huge or complicated circuit, and is quite accessible via the loft.

Many thanks for your help and advice...(forgive my ignorance but what is an RCD!?)

 

[bongos]

An RCD is a current operated device that detects any inbalance between the Line & Neutral. In perfect conditions the current in L & N are both the same but during a L/E or N/E fault there will be some leakage to earth. The RCD detects this inbalance and if the current leakage is outside it's tolerances it trips. For domestic installtions the common tripping limit is 30mA.

If your house was rewired 10 years ago it is likely that your sockets and some other circuits will be protected by an RCD (as well as their own MCB). RCD's are good but they are very sensitive and can be prone to nuisance tripping which can be a nightmare to identify.

 

[Blaina]

Thanks again for your advice - very helpful..