MCB tripping when light switch turned on

Lex Luthor

Just moved into a house and when I turn the light switch for one of the bedrooms to the ON position, it trips the mcb for that circuit (ie. all bedroom lights)

What could be causing this? There is no bulb in the light fitting so it can be either a short in the switch or fitting?

Stoner

Lex if the switches are metal and earthed a common cause for this is , the switch wire could be loose and touching the metal on the switch and when you hit the switch a live to earth fault occurs.

You could check the connections.

There are many other possible causes, snagged cables, bad connections etc. it can come down to a process of elimination

I know you are involved in the industry and know to be careful when working on these things

Lex Luthor

Stoner wrote: »

Lex if the switches are metal and earthed a common cause for this is , the switch wire could be loose and touching the metal on the switch and when you hit the switch a live to earth fault occurs.

You could check the connections.

There are many other possible causes, snagged cables, bad connections etc. it can come down to a process of elimination

I know you are involved in the industry and know to be careful when working on these things

Stoner, the switches are metal and I had a look yesterday to see if I could figure out the problem.

It seems at the light fitting, the switched live from the switch is shorted to the neutral. It's not obvious where it is shorted but I belled them out with a multimeter.

As soon as the live is switched through to the fitting, the MCB trips. There are 3 rooms on this MCB and this is the only room that trips the MCB when you throw the switch ???

I'll have to get the spark back to check it out as I don't have the time to troubleshoot any more

Andrea B.

Just before you give up, you could try removing the fixing screws from the ceiling rose and see if it resolves issue. If so, pull wires down and remove damaged sections.

Lex Luthor

Andrea B. wrote: »

Just before you give up, you could try removing the fixing screws from the ceiling rose and see if it resolves issue. If so, pull wires down and remove damaged sections.

I removed the sitched live wire from the switch at the light fittng and its still showing a short to the neutral at teh light fitting. Can only think its shorted somewhere along the run from the switch. Not good

Michael Collins

Did you test it from the light fitting itself or did you disconnect the wires from the fitting and test it directly? It would be very odd to be shorted along the run to the switch.

Lex Luthor

Michael Collins wrote: »

Did you test it from the light fitting itself or did you disconnect the wires from the fitting and test it directly? It would be very odd to be shorted along the run to the switch.

I disconnected the switched live wire from the switch and tested between that broken wire and the neutral in the light fitting and got a short...