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.

 

[CrazyScozzese]

IR on the ring may be a red herring, if the boiler is on a seperate radial.

However, the boiler is new, so is the fault, and coincidence is not a friend of mine. I would try to eliminate the obvious. The boiler could be triping the RCD even if not fed from it. Usually this is an N to E issue. Also, bad joints and connections can spark, which can trip an RCD on an adjacent cct.

However, if disconnecting the ring stops the trip, does disconnecting the boiler do the same?

If the trip takes 10 mins, do a ramp test to see when the RCD trips with all on, none on, and one only of each cct on at a time. This way you will find the leakage of each cct, and it may point to the dodgy one.

Where was the boiler before, if it is on the immersion now? I presume there has been new bonding, any issues there?

Silly question, are you sure you have the N and E of each leg the correct way around?

 

[tbs1983]

its deffo nothing to do with boiler, its a combi boiler so no seperate valve, pump etc etc etc its just a straight feed from db to a 3A fused outlet, and 3 core flex into boiler. nothing else involved.
I think he's disturbed the ring cables somehow whilst putting his pipes under the floorboards, but what and where im struggling with!!!

 

[tbs1983]

good point about the old boiler before, I hadn't thought of that ! head too busy messing round with the ring, perhaps it was spurred off the ring even, will have a look tomorrow. going to ramp test it too so will hopefully get to the bottom of it.

 

[Sharpend]

Have you checked that the wet head hasn't snagged the flex when fitting it into the spur, perhaps nicked the neutral core on screw?

 

[Lucien Nunes]

I also think the 0.6 megohms is a red herring as far as the trip is concerned, although it might lead you to the actual fault by way of being across an unexpected combination of conductors. Something similar cropped up on a recent thread - to trip a 30mA RCD it would have to be a thousand times lower or thereabouts, as it's N-E.

However I have seen a very similar problem caused by a heating installer, who moved a ring point out of his way by jointing the cables in the old box with a 6-way choc block to two new runs of 2.5 leading the new position. But he crossed the CPCs of the two cables leading to odd results at the moved point when the ring was split for faultfinding. If the S/O at which you have split the ring had been moved in the same way, you might get the strange crossed-over low IR if the joint also happened to be faulty or damp, leaking between adjacent terminals which happen to be on different legs of the ring. That would be a prime candidate for also being the fault that causes the tripping.

Another possibility - have you tested the IR from the ring to any other circuits? Both suspect cores may be faulted to something else entirely, by way of a major snarl-up somewhere, and that would cause tripping.