A few times in recent weeks Mrs Poly has come home to find the power has tripped out. It is the main breaker (RCD protected 1990’s panel with RCB switches not fuses)). No individual circuit is tripped. It is infrequent/irregular (although so far has always been during the normal working day). It doesn’t happen every day. This makes identifying the fault by disconnecting things virtually impossible as we don’t know if a day without a problem is luck or the unplugged item.

I’m especially concerned as if we are away over the festive period the freezer will defrost (we did have the this happen in the summer whilst we were away, and it has never recurred until the last few week, and I never want to smell the fermenting contents of a freezer again! The prawns were particularly foul!). It will also make the GCH shutdown and in winter that could get really messy.

My working hypothesis is that rain was getting into an external light fitting or the (mains wired) doorbell. The last time it happened it had definitely been torrential rain, but today was not that wet, just really cold and damp. If it is rain water in one of them am I likley to see corrosion etc if I strip them down? If its not rain water then intuitively it must be something that switches on/off when nobody is in?

Is there anything a sparky can do to detect the fault other than trial and error?

A sparky should be able to trace it – maybe an earthing fault?

When this happened to me it was water in the outside socket. Once it had become damp it had arced and charred the plastic, which caused subsequent shorts. Once opened up it was very obviously the culprit. I’d say check everything external first – should be easy to do.

I’m assuming that by “main breaker” you’re referring to the RCD, and MCBs rather than RCBs??
Start by switching off anything outside or in the garage/shed that involves water. Outside lights, condenser dryers in the garage, etc. although I’d think (always happy to be proved wrong) that wet fittings wouldn’t reset easily

As molgrips suggests ^^ but be careful because you’ll be exposing conductors, switch the circuit off first and look but don’t touch. A logical process of re-energising circuits and appliances might isolate the problem, ideally ask an electrician to check for anything obviously faulty. An electrician can also diagnose the RCD if that’s at fault

Freezer motors can cause an inrush of current that may trip an RCD if there’s a problem, but is more likely to trip the local circuit breaker

we had similar for years in our newish house. Was fine for first 5 years from when it was built, then would trip a few times a week, particularly in winter, and bizarrely often when running a bath. This went on for 3 or 4 years and we couldn’t trace it, not could a few electricians. Ended up getting the whole panel replaced and it sorted it.

Tj – thanks, I guess that’s my question though can a sparky find a fault even when the fault isn’t bad enough to trip the earth device? I don’t really want to pay someone’s day rate for trial and error that might lead to nothing if it isn’t raining that day…

Molgrips – thanks obvious arcing would make life simpler! Sounds like a job for daylight this weekend.

Timba – thanks I’ve learned the hard way that 240v hurts so won’t be poking round live circuits! The problem with the systematic approach and reenergising the circuits is so far we have never been home when it has tripped, so it has several hours to dry out (if that is the issue).

We had the same a few months ago, I started a thread too

Never got to the bottom of it (intermittent fault, never while we were in so hard to track down, like yours). Had an electrician pop in (he was at the neighbour’s house) and took a look. He fiddled with the board a bit, declared it OK and said that it’d be a bastard to find.

Seems that did the trick and we’ve had no bother since then 🙂
(maybe something in the board was loose?)

last time we had some ‘random’ trip going on it turned out to be, indeed, damp related — shower pump / controller was under the floorboards, I assumed it was just dampness but it hadn’t been fitted properly and the seals had burst, water was flowing freely where it shouldn’t..

Sounds like the old earth neutral chestnut to me. Basically earth and neutral faults can disconnect the RCD. There doesn’t have to be much of a fault and this won’t take out the MCB (individual circuit protection) which typically works for a dead short or inrush of current or over current situation. Not saying that there isn’t a device or outlet somewhere with water or damp ingress so we’ll worth checking. Earth neutral faults can be found by taking neutrals out of the bar in the consumer unit until the offending circuit is isolated and the RCD can be re-set. It’s then a case of splitting down the installation into halves until you find the issue.
Best get a sparks in though.

When this happened to me it was water in the outside socket.

Very common, and easy to deal with, so worth checking before you call out an electrician if you can do stuff yourself.

Also look for damp in the wall behind the washing machine / dryer (where they plug in) – also very common.