strange one

[tbs1983]

Hi there, plumber phoned me during a boiler fit in a house saying RCD had tripped (nothing to do with boiler circuit), but I suspect he's disturbed something.
It seems to hold for 10 mins or so then trips.
I have reached a point where ive found the approx mid point of the ring, disconnected both legs and with both legs disconnected at db, I'm getting clear IR.
but here's the strange bit- if I iR across the neutral of one leg and the earth of the other leg at the db, i get 0.6 Mega ohms. same reading if I do it on the N & E of the opposite legs.
Any ideas???? I'm baffled!

 

[Tim]

Fault Rectified !!! This evening I disconnected each leg of the ring, plus the spurs from jb's. IR'd every individual leg, every single one clear. Reconnected one leg at db and 1st socket, carried out ramp test and got 26mA. connected the next socket, ramp tested and got 26mA. so on and so on until after connecting another leg the ramp tester would not trip, gave >51mA as a reading. left this leg out of the ring, reconnected everything else, ramp tested 26mA and plugged in the loads, happy days. Rewired faulty leg and bobs your uncle problem solved.

What I've learnt from this fault -
1. I need to buy a ramp tester as I would never have found the fault otherwise.
2. I hate faults !
3. Insulation resistance alone doesnt always identify a faulty cable (although this one is still baffling me why the leg IR'd clear yet it was causing the fault)
.

Thank you for all the help and advice on this one.

I had almost identicle IR readings to you a couple of weeks back.
I got .5Meg between leg 1 line and Leg 2 Earth with all sockets connected but fault vanished when you disconnect all points.
When i connected the earths back in the db and retested the fault was back.
Took a while to find but turned out to be a screw through the line that was touching a bonded water pipe so the fault only appeared when the earths were connected in the db. Spent a couple of hours with a wander lead to finally trace the offending cable. Luckily it was roughly center of the ring so i converted it to 2 16a radials as they didnt fancy me chasing their newly decorated walls.

Tim

 

[Hawkmoon]

Perhaps boiler fixing screws have pierced neutral of ring cable in wall - not sure that would account for all your symptoms but with N-E fault after boiler installation, I would have considered it (along with everything else).

 

[Sharpend]

1. test at 250V. 2. test at 500V. 3. shove 1000V up it. 4. get the hammer out. nothing hard there. :behead:

I always thought your replies were rather professional, now I'm convinced you're an expert!!!!!