Got a call from a former neighbour the other day... "all my electrics have gone off!" A quick over the phone diagnosis suggested the socket circuits RCD was had tripped and would not reset even with all the downstream MCBs off.
So I went to investigate. Sure enough, it was tripped, and would not reset. So having enquired if he had been nailing anything to a wall recently (his attempts at DIY have varying outcomes, the last time he had a problems like this was when nailing down loft floorboards through the nice new lighting circuit cables, and then later filling a light switch back box with wet plaster). He denied all knowledge.
It was protecting three ring circuits, so I disconnected each pair of neutrals in turn, and tried it again. Still would not reset. Dropped thee live bus bar out of the bottom and the neutral flylead to the split load neutral bus bar *still would not reset*. So I decided the RCD was knackered, and realised I forgot to bring a spare. Went home, came back with another, wired it in, and the bugger still tripped. This time however turning off the kitchen sockets circuit cured the problem. First problem fixed.
So since the only things not unplugged in the kitchen were the cooker and the washing machine, I turned off the cooker isolator. Problem found.
Combination cooker - gas hob, lekky oven. Figured, bet I know what that is - knackered element. Stuck an insulation resistance tester between L & E on the plug and got a reading of 20k ohms. Had the back off the cooker, and disconnected the element (and like a dope actually removed it from the oven before testing it), and found that was fine and it was still leaking like an electric sieve without the element. Disconnected the fan, no change, The oven light? Nope. Running out of things to disconnect now... In the end I disconnected the wires connecting the back of the flex inlet to the rest of the cooker. Still leaky! This was getting silly. Disconnected the live wire of the flex from the the flex inlet, still 20k with it dangling in free space. Cut the moulded plug off the end of the wire, eureka! Sent him up to the shop to procure a plug while I resembled the cooker[1]!
Then the storey becomes clear... I was explaining to the lady of the house how it must be that moisture has somehow got into the plug. Ah, she says, perhaps it happened when I was washing down the wall above the cooker this morning!
So two and a half hours titting about to find a damp plug! (and an RCD that had obviously got a bit over sensitive in its old age).
[1] I had to marvel at his brass neck.... He returns with a plug with 8" of two core flex dangling out of it... I asked where on earth did he find that? Turns out he went to the local hardware shop (traditional arkwright style affair), looked at all the nice new plugs on display and baulked at the £2.50 they wanted for them. Said to the nice girl behind the counter, have you got any plugs. She pointed him at the display. "Oh no, not those, they are expensive, haven't you got any cheap ones?". She obviously took pity on him, vanished into the back of the shop and returned with a slightly used example and relived him of 50p! (probably left the boss wondering why his anglepoise is not working!)