N/E fault IR=0 - RCD won’t hold

[B3nMar]

Hi all,
After some options/opinions on fault finding N/E fault on ring final as fairly newish to being out alone.

Customer wants to upgrade from rewirable CU to RCBO so fairly old wiring but no signs of cable breakdown or anything totally messy on inspection. This circuit basically holds everything inc. boiler etc but exc. lights and cooker.
Ring continuity tests are slightly messy but well within limits.

IR testing L/N & L/E = approx 150M, however N/E = dead short @ 0M

Unplugged everything, isolated all FCUs that I can find but still no luck. Temporarily tried the circuit on a RCD to check I hadn’t missed any loads or just being an idiot but it only holds for a minute or two before it trips confirming a fault.

My thoughts are to now break down the legs of the ring and go hunting the fault similar to finding a continuity fault.

Any one have any other options or go to places to start that might speed up the process??

Cheers in advance.

 

[DPG]

Were all the ring conductors disconnected when you did the IR test? (I know the CPCs should technically be left connected during this test, but we're talking fault finding here).

Out of interest, what does it read if you test using the low Ohms range of your tester?

 

[timhoward]

Is there an immersion heater?
Also, checking an obvious question, you had the Neutral's disconnected at the CU when you did the test showing 0 N-E ?
(just double checking you aren't measuring the TNCS link, or indeed another circuit could have the fault)

Otherwise - yes - break ring somewhere and see which half it's on.

 

[B3nMar]

Is there an immersion heater?
Also, checking an obvious question, you had the Neutral's disconnected at the CU when you did the test showing 0 N-E ?

Yes, however it was isolated at FCU.

And yes, N legs disconnected and CPC’s tried both in & out of the bar.

 

[timhoward]

Yes, however it was isolated at FCU.

Some cheaper FCU's are single pole by the way, so wouldn't isolate the N
I'd say you are generally unlikely to find one but it's a possibility.

 

[DPG]

Sounds like it's going to need the ring circuit splitting.

I'd still test it on the low Ohms range though to see what actual resistance you are talking about here. Remember the MR range will show 0R but it probably won't actually be that.

 

[timhoward]

As @DPG says, if it can be detected on a low resistance continuity test it might speed things up. Leaving the CU ends disconnected and measuring at each socket until you find the two lowest L-E readings. In theory it's between them if neither of them are spurs.

Best of luck. You'll get there in the end. If you reach an understanding of the problem in under 2 hours you'll have done well!

 

[IzzyS]

If it's a conventional ring just make an educated guess where the mid point is and split it there, and test each leg from the CU. That should rule out half the ring. Split the faulty leg at its mid point, test again. Keep going till you run out of places to split it and you will have hopefully found the fault, or at least be close. It's the most efficient way, known in other fields as a "binary search".

If on the other hand it's a DIY disaster zone like the one I had last week, which was a figure of 8 bodge with spurs off spurs off spurs you're in for hours of fun. Eventually traced it to a choc block plastered into a (now slightly damp) wall.