It was a strange one spesh. The fault was random intermittent tripping on the rcd main switch. Old Wylex metal clad rewireable cu. So, no rcd replacement available, would need a new board. It stated with a 'global' IR test (L+N to CPC) which returned suspicious readings so tested each circuit and it turns out to be sockets (always is isn't it). So I start going round the circuit trying to hunt down the low reading, 3. something Meg Ohm. Finally trace it to a length of cable that goes from the kitchen to a shed (not actually a shed, more a breeze block extension with polycarb roof. The kitchen is single story with pitched roof. Spoke to the guy and he says cable runs in a stud wall up to roofspace and that the front 2 rows of tiles can slide back. So off the tiles come, I find the cable topside. A little lube and away we go. Managed to replace. Put everything back together, thought I had cracked it. Went to power up and 'click' rcd wont hold (it wasn't tripping when I got there).
So, I start all over again, this time readings 50MegOhm. Spent most of day where I thought I was getting close to fault, they it would just disappear. So put everything back together and trip again. Obviously this job comes with an amount of responsibility in that we can't really leave them with no power. So I keep digging. Also, circuit had open Neutrals. So, I find the neutral issue, I find a cross polarity socket and a few other bits.
One socket in particular was a ****** to 2nd fix as cable tight as hell. So had it in connector to continue checks, at one point everything was working. I took the blocks off, put socket on, trip!.grrrrrrrr
Take socket off..............trip!
It really was strange. Each leg measured >200. I had the ring split and each half would measure 115 and 155 respectively so I'm assuming it's partly down to degrading. Put the ring together and it's now 70 odd MegOhm. I work out how low an IR would need to be to trip 30ma rcd
230 / 0.03 = 7666 : 7.6Meg, so It should run.
I finally got it to hold. Told her to keep me updated and explained that if she found herself not able to get it hold then unplug sockets fuse and plug and plug an extension into cooker switch (yay for cooker switch with 13a sockets) to keep fridge and freezer running. Said we could try a new cu before going down the rewiring route...
See what happens I guess
