Circiut Fault

[MikeU]

I have a CU containing a GE 30mA RCD and 6 MCB's (3 ring mains, 3 lighting). This morning the RCD tripped for no apparent reason. As usual, I tripped all the MCB's, reset the RCD then the MCB's one at a time. Upon resetting the final lighting MCB, the RCD tripped again. Eureka, I thought. I made sure all the lights were switched off and tried again but to no avail. As far as I can tell, nothing on this circuit is switched on but the RCD will not reset with the MCB on.

Nothing has been done to this circuit for nearly a year, when I installed a new halogen light fitting. What is the best way to troubleshoot the circuit?? I do have an analogue multimeter.

 

[gavonto]

Does this lighting ciruit consist of any that may supply outside lights?

 

[MikeU]

No, internal lighting ony, bedrooms, shower room, stairs etc

 

[plugwash]

you say a new halogen light fitting? do you by any remote chance mean an outdoor floodlight?

 

[MikeU]

No, it was an internal fitting with 4 40w halogen capsules.

 

[gavonto]

Any chance of removing all lamps from that circuit, & then trying to reset the mcb.

 

[MikeU]

Yes, no problem. You'll need to educate me though. Given that all the lights are switched off, what am I testing by removing the bulbs?? (I'm assuming I've got an earth fault, hence the RCD tripping.)

 

[gavonto]

Has the halogen fitting have any internal components, ie ignitor, ballast, or is it a 240v halogen bulb?.

 

[MikeU]

No, the bulbs are mains voltage. (Crompton THG940F to be exact!) Two of them blew together about 10 days ago....

 

[andy]

test the insulation. ideally use an insulation tester, altho if there is a big fault then a multimeter will show it

 

[gavonto]

Is there only one metal fitting on that circuit ( the ones with the halgens on it)?

 

[MikeU]

Yes, it's a four-bulb fitting

 

[MikeU]

Sorry, Gavanto, I mis-read your question. No there are two metal fittings ans one metal-faced wall switch. Everything else is plastic.

 

[andy]

Sorry, Gavanto, I mis-read your question. No there are two metal fittings ans one metal-faced wall switch. Everything else is plastic.

are they earthed? (i hope they are)

 

[gavonto]

ok, disconnect both fittings,check the wiring behind them for signs of overheating, ( make sure that you terminate the cables into connector blocks), then try switching your mcb on. If it stays on, & your rcd doesn`t trip then bobs your uncle, if not, then I`m afraid you`ll have to trace the fault with an insulation tester, good luck