On 12/11/2011 17:13, David WE Roberts wrote:

"crb" wrote in message
...

Checked all the wires for 240V a/c - nothing.

Interesting. How did you measure the 240v - between what and what?

Try measuring again between each wire in turn and a known good earth,
both with the CB engaged and tripped.


Interesting thought - however I can't test with the CB engaged because
it always trips.
For those reading this, please note that the fault manifests with no
light fitting attached and no switch attached.
Just bare wires out of the wall, and the two cables (power and switch)
clearly seperated.
So it cannot be a fault in the light fitting or switch.
The fault is inwards of the power cable coming from the rest of the
downstairs lighting circuit.

Could it be your fixing screws were longer than the previous ones?
Perhaps the cable is routed in such a way as a screw has damaged it. If
that were the case, at least the fault would be very close to the light.

Another possibility is the MCB is faulty. What happens if you disconnect
the circuit wires from the top of the MCB - will it reset then?


--
Cheers,

John.

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