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!

 

[jbcom41]

so am i

 

[telectrix]

has Mr. wet-pants got something wet?

 

[Rockingit]

My advice would be to go looking everywhere the wet head has been and try to second guess what he might have disturbed. If the boiler is wired on another circuit entirely, what about things like thermostats, valves and so on? You might have a cross over somewhere between the two circuits.

 

[tbs1983]

Its nothing to do with the boiler circuit because the old immersion circuit has been used for combi boiler and its currently on the non RCD side (although I will be transferring it to RCD once this fault is rectified). There are no wired stats just a remote wireless one.
I had to leave the circuit disconnected over the weekend, and set up some temp ext leads for them, all other circuits held fine all weekend .
There are a couple of spurs off the ring
but all seem to megger clear.
I just cannot understand how im getting this short across two seperate sides of the ring.

 

[Rockingit]

I just cannot understand how im getting this short across two seperate sides of the ring.

How sure are you that you've pulled the middle of the ring? Most likely explanation for a reading that low is normally something that you haven't found yet like a neon or something plugged in.

If you get your r1 / r2 values at the DB then you can work out how long your circuit is using whatever resistance values suit the cable in question, then if you do the same (using r1r2) at your break point both sides you'll see how far off you are.

BUT....first principles say if it worked before, and it's not been messed with by you, then whatever DID get messed with by Mr wet head is what remains.

 

[tbs1983]

going to borrow a ramp tester tomorrow off a mate and see what joy that brings me.

 

[tbs1983]

a neon or something left plugged in would create low IR across L-N, not N-E.
Ive got both sides of the ring clear from disconnected socket back to disconnected legs in db, but when I megger opposite sides N-E (with the mid point socket still disconnected, thats when I get 0.6 Mega ohms. just can't see how this is possible, they are 2 completely separate runs (effectively 2 radials while disconnected).

 

[Rockingit]

a neon or something left plugged in would create low IR across L-N, not N-E.
Ive got both sides of the ring clear from disconnected socket back to disconnected legs in db, but when I megger opposite sides N-E (with the mid point socket still disconnected, thats when I get 0.6 Mega ohms. just can't see how this is possible, they are 2 completely separate runs (effectively 2 radials while disconnected).

Ah.... got you. Guess it's a case of testing leg by leg and see where you get to, but my money's on it being something simple!

 

[chrish_1977]

There must e a connection somewhere, disconnect the feed to the boiler and controls and see what happens, it's not tripping when the boiler is starting is it, or the vale,motor or any other part the plumber "hasn't" touched.