Since we bought our house (2 semi detached cottages knocked into one) we've had problems with the RCD tripping at random times.
The tripping seemed to coincide with wet weather mostly.
I then painted the outside of the house and the occurrences became less frequent, whereas before we were lucky to go a couple of weeks without incident we went a few months.
Recently it started happening again and I've narrowed it down to one half of the downstairs lighting circuit, it trips the main RCD whenever I de-isolate at the mini circuit breaker.
Due to the wet weather theory stated before I put it down to some lead flashing -that had come loose (roof of a 'lean-to' downstairs) and water ingress into the circuit somehow.
I've had this repaired now (couple of days ago) but the problem persists, is there any effective way I can narrow it down to one part of the circuit? Am I right to suspect water ingress? Nothing has changed on the circuit, there is a light in the downstairs toilet, this was my main suspect as we have to go outside to get to it (but it is integral to the house) and part of the suspected leaking roof is above it. I disconnected the wires to the switch but the RCD still tripped.
I was tempted to wait until it dried out but I'm getting fed up now.
ok first thing to check is any external light, if you have any make sure water is not getting into the fitting.
You spoke of the outside toilet fitting sounds like its your culprit, have a proper check of the fitting and the cabling to see if there is any tracking across live to earth or neutral to earth, if there is change the fitting after you have fixed the roof
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