M-C-B Tripping

Postby Phaedris » Fri Sep 26, 2008 3:54 pm


I recently moved three light switches in my house, the first two worked out fine, but after moving the third one the MCB keeps tripping and cannot be reset.
I have checked the wiring on the switch and its connected exactly the same as before I moved it.
The switch is earthed and the wiring is connected firmly
What could the problem be could the MCB need replacing?
Posts: 5
Joined: Fri Sep 26, 2008 3:18 pm


Simply Build It

Postby ericmark » Sat Sep 27, 2008 3:19 pm

It would be unusual for a MCB to become faulty just when you moved your light switch. I would expect if as you say all wires are as they were before that you have trapped a cable. It is common for the 3.5mm screw to damage a cable when switch plate is screwed to backing box but faults like you relate really do need a site visit to work out what is wrong.
Batteries are flat on my crystal ball.

Postby Phaedris » Sat Sep 27, 2008 3:53 pm

Thanks for the reply.
I just checked the wiring and none of them are trapped.
When I was moving the switch I noticed that it had a crack in it so I replaced it with a different one.
The replacement switch I installed was a 1 Gang 2 Way Light Switch and the original one was 1 Gang 1 Way Light Switch could I have maybe wired the switch up incorrectly and damaged the MCB or is that possible?
After it tripped the MCB and I couldn't get it to reset again I put the switch with the crack back in but it still didn't work.
I have 2 MCB's for lighting in my RCD could I swap the one I know works with the one that is tripping and see if that is the problem.

Additional information:
RCD is a GE Fix O Rail Classic IP30-5
MCB 0L-61 C10
Posts: 5
Joined: Fri Sep 26, 2008 3:18 pm

Postby ericmark » Sun Sep 28, 2008 1:39 pm

Well MCB does seem wrong I would have expected a B6 not a C10. If you have short circuited the MCB it could be damaged. With main isolator off will it reset? If not then likely faulty.
As for MCB size most ceiling roses are rated at 5/6 amp and although one is allowed up to 16 amp for a lighting circuit to use the max then all the fitting must also be rated at 16 amp which would be rare. Also as type C10 that means 100 amp will flow before it trips on short circuit. Rather than the 30 amp for a B6 type.
There seems to be a little more than meets the eye here and I would advise to get an electrician to check it out.

Postby Phaedris » Sun Sep 28, 2008 7:45 pm

Thanks again for all your replies

I just went and tried the MCB again and it worked the strange thing is that i didn't even touch the wiring it just worked out of the blue.
It wasn't working before even with the isolator off so I'm at a loss to why it's working now.
I have noticed that the light bulb's in my house blow quite alot too could this have something to do with the MCB type?
I'm getting a Electrician in to do some work in my kitchen so I'll get him to have a look at it.
Posts: 5
Joined: Fri Sep 26, 2008 3:18 pm

Postby ericmark » Mon Sep 29, 2008 1:34 am

No the MCB will not make bulbs blow. Where people have added loads of those little spot lights they do sometimes up rate the MCB which in some cases may be OK. But it rings alarm bells and if I see anything like that I realise there may be other alterations where the person doing them does not fully understand the implications so I am just a little bit more careful.

Display posts from previous
Sort by
Order by


  • Related Topics