Lighting tripping

[flinty]

The upstairs lights at my parents house have developed a problem. The consumer unit has an RCD on it and it tripped out.
It is fine on every circuit until you switch the MCB for the upstairs lights on then the main RCD will trip.
As nothing has been done to the lighting circuit I guess the most likely cause will be a rodent of some kind that has gotten into the loft and had a nibble at a cable.
If so whats the best way to trace the faulkt or is there another likely senario?

Thanks in advance.

 

[fattony]

Is it possible the roof has leaked or any kind of water damage, you say nothing has been changed recently on the circuit so we could be looking at old cables etc - could you estimate the age of the installation?

Assuming there is no obvious signs of water damage you could try removing light bulbs and start metering out the circuit with a simple multimeter but a insulation resistance test will tell you so much more, unfortunately I'd bet you dont have one.

 

[flinty]

Actually I do have one.. and water damage doesn't seem likely. The age of the wiring would be about 20 years.