Electrical fault in the rain only

Our house has an extension that has its own consumer unit wired off of the main unit in the hallway behind. When we have a certain amount of rain (generally heavy), the extension CU trips out. I have narrowed it down to the socket ring as the lighting ring will stay on. We have two external supplies, one going to an outside light and another goes up to a garden shed.Both these supplies are on a fused switch spur, which I have removed the fuses from to see if it is either of these causing the trip but no joy. I have had all of the socket fronts off to see if there is any signs of moisture or dampness getting in directly somehow, but all are dry. I've checked the loft space to see if anything is leaking directly onto any cables up there but all is ok.
My assumption is that water is getting into the cavity somehow and collects or reaches somewhere causing the short. We have lived in the property for 6 years and this has only started happening in the past six months with no works carried out that might have caused it. Would an electrician be able to pin point where a problem is occurring without basically ripping everything out? (I appreciate most do not have X-ray vision!)
Many thanks in advance.
Andy

heatcraft said:

Both these supplies are on a fused switch spur, which I have removed the fuses from to see if it is either of these causing the trip but no joy

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It will probably be water ingress in the outside light and/or shed. By removing the fuse, you only are isolating the line (live) leg of the circuit. To test it you need to isolate the line and the neutral. A fused switch spur should do this if you just turn the switch off. If that solves the problem then your issue is outside, in the wet, somewhere.

heatcraft said:

Would an electrician be able to pin point where a problem is occurring without basically ripping everything out? (I appreciate most do not have X-ray vision!)

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If you resort to an electrician, he will have a device called an Insulation Resistance tester. With this, he/she should be able to pinpoint the problem without too much,if any, disruption.

Even though you have removed the fuses, if the switch is ON on a fused spur, the neutral will still be connected. A short between neutral and earth will cause an RCD to trip.

Anything else outside that could be letting moisture in?

Water getting into a cavity and causing a short isn't a usual fault, I have to say, though still very, very possible.

Definitely had the fuse switch spur switched off, it was the first thing I did as I suspected the shed supply. There are no other outside supplies and I've been around everywhere mastic/filling anything looking susceptible.

Unless your FCUs are double pole then you're not effectively isolating the external supplies and thus can't eliminate them as the source of the fault.

As said in the previous 4 posts

Depending on your competence level there are ways to DIY- quickest and easiest is an electrician with the appropriate test gear, they'll be able to narrow the problem down safely in minutes (rather than DIY methods which are less safe, more time consuming and more potential for damage to wiring).

oldbutnotdead said:

Unless your FCUs are double pole then ....

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Maybe I've just been 'lucky', but I haven't seen a switched FCU which did not have a DP switch for very many years, if ever.

Kind Regards, John

JohnW2 said:

oldbutnotdead said:

Unless your FCUs are double pole then ....

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Maybe I've just been 'lucky', but I haven't seen a switched FCU which did not have a DP switch for very many years, if ever.

Kind Regards, John

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There's plenty of the cheapies are not labelled or described as such, the ones marked DP tend to be more expensive. I've not actually put a meter across one of the not-specified, must try that tomorrow.....

Many thanks guys, I completely isolated the fuse spur to the shed (which also had a second supply to an unused outside lighting set up!) then removed the fore mentioned lighting supply. Result! Calm is restored...

There you go. You should be able to locate it through a process of elimination (although measurement with a test meter is the better way!)

oldbutnotdead said:

JohnW2 said:

Maybe I've just been 'lucky', but I haven't seen a switched FCU which did not have a DP switch for very many years, if ever.

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There's plenty of the cheapies are not labelled or described as such, the ones marked DP tend to be more expensive. I've not actually put a meter across one of the not-specified, must try that tomorrow.....

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I certainly agree that many of them (even expensive ones) are not labelled as having DP switches - but I think that, if you get your meter out, you'll find that most, probably all, are. A good few sockets have DP switches these days but, again, many of those that do are not labelled as such.

Kind Regards, John

Interesting- ran out of tomorrow today so it's on the list for tomorrow . And re DP sockets- using a load I acquired (from a school being demolished) and was pleasantly surprised to find them all marked DP on the rear.