In article ,
Rob writes:
Hi,

Electrics are supposed to be simple. but perhaps it's me who is
simple.

We have a light fitting in the kitchen with 4 halogen bulbs. It is
controlled by 2 switches in parallel - one normal and one dimmer. It's
been working fine - every so often a bulb goes, trips the RCD, gets
replaced, and all continues working.

This time when one blew, it tripped the RCD, I reset the RCD and now

I suspect you mean MCB?

the light doesn't work at all. All other lights on the circuit work
OK. I have tightened all the wiring in the switches, and also the
fitting. A multimeter across the two contacts in each bulb housing
shows there is 240V getting to each one when the switch is on. I have
replaced every bulb with brand new ones. But still no light.

Multi-meters are not good for measuring mains voltage. A tiny
capacitive leakage in a length of cable can result in mains
voltage readings, when there is in fact no connection at all.

I have no idea where to go from here - surely if power is getting to
the bulbs then there can't be a wiring issue? Having said that I don't
know if the fact there's 240V there means there's enough *power*
there. I also noted that when the switch is off there is still about
25V across the contacts, but have no idea if this is normal.

I suspect the lamp which failed arced across, drawing high current,
and blew the dimmer (after all, it tripped the MCB instantly, which
probably means it passed over 30A momentarily). It might have only
blown a fuse in the dimmer, but IME it is likely to have blown the
triac (the key semi-conductor component).

--
Andrew Gabriel