Intermittent Ring fault! Any advice please.

kmulcahy1994 · Jul 9, 2019

Hello all, I was hoping for some advice on this ring fault I have come across. It has been intermittent for about 2 months now.

It is a converted church to a children and family centre. Installation is within 10-15 years. TN-C-S supply.

The fault is on a 32A type C RCBO 30ma which supplys 12 DSSO in dado trunking. The run is about 30-40 metres. Wired in 2.5 t+e.

The only equipment plugged in is x4 computers and x4 monitors. X2 photocopier machines and x1 shredder. All equipment is PAT tested.

It has tripped around 6 times in the past 2 months. The tests I have carried out are end to end and the values I am getting are:

r1:0.57
rn: 0.57
r2:0.69

These are fine and with figure of 8 tests comply aswell. I have then carried out IR tests which are around 45-60 meg. I have split the ring and found the incoming legs.

RCD tests pass on x1 and x5 and also ramp test is showing it trips at 25ma.

However today I tested IR again with everything disconnected and had short circuit across all conductors. I then split the ring and tested IR again on both legs and they pass. The only suspect thing is where the cables come from above the boiler room into the kitchen (behind the units) there is mouse bait and also strawberry jam used for bait however the jam looks like it follows the cables into the trunking.

The only thing I have not tried is testing for earth leakage, although I do not have the tester for that. Could anyone help by suggesting anything else I can do? Thank you!

James · Jul 9, 2019

can you move the circuit off the rcd?
then split the ring and change it to 2 x 20A rcbo's
it will show if you have an intermittent cable fault by tripping the rcbo on that circuit only and leaving the other half of the old ring powered up.
if it is due to an accumulation of leakage from the IT equipment then all will be ok and nothing will trip.

kmulcahy1994 · Jul 9, 2019

James the Spark1976 said:
can you move the circuit off the rcd?
then split the ring and change it to 2 x 20A rcbo's
it will show if you have an intermittent cable fault by tripping the rcbo on that circuit only and leaving the other half of the old ring powered up.
if it is due to an accumulation of leakage from the IT equipment then all will be ok and nothing will trip.

I did this over a month ago, as one leg was showing a fault on it so i split the ring and put the good leg on a 20A rcbo radial, however it still tripped a couple weeks later. The run is 30-35Metres anyway so wouldent want to leave it on a radial permanently as would need to upgrade to 4mm. I then went back a month ago, tested both legs are they passed the IR so i put back on the 32A. And now as of yesterday thats when it tripped once again. I asked the client when it first tripped to record times and what was plugged in... did i receive any of that information... no. The frustrating thing is that the readings im getting one time are fine then the next are showing a fault...

James · Jul 9, 2019

think about maybe using 16A rcbo's for testing the circuit.
use 2 and split the ring, this will guide you into what area to look at, you can then move the point where you have split the ring and eventually the breaker that trips will change from one to the other. this may help in finding the source of the problem.?

Lucien Nunes · Jul 9, 2019

The frustrating thing is that the readings im getting one time are fine then the next are showing a fault...

This is typical of a chewed or punctured cable where there's a tiny airgap between the conductors, large enough to not flash over during the IR test but small enough to short out when anything expands with heat or other minor disturbance happens.

Have you tried testing it at 1000V?

kmulcahy1994 · Jul 9, 2019

Lucien Nunes said:
This is typical of a chewed or punctured cable where there's a tiny airgap between the conductors, large enough to not flash over during the IR test but small enough to short out when anything expands with heat or other minor disturbance happens.

Have you tried testing it at 1000V?

Thanks for the reply mate, i have not tried testing at 1000v yet. Will be removing the kitchen units tomorrow where the cables enter as thats where they have had rats before...

Charlie_ · Jul 9, 2019

Keep clean
Weil’s Disease is one of the nastiest things out there

kmulcahy1994 · Jul 10, 2019

UPDATE! Found the fault. Tested between neutral and earth, however left cpcs in earth bar to see if it was shorting on another circuit or pipes. Tester showed dead short. Traced cables and found they run behind the boxing in for rad pipes. Took off rad cover and there it is in front of my eyes the leg that had been squished.... therefore shorting on to the neutral.... replaced leg of ring and re tested. All good. Must of been plumber who chsnged the stats on the rad cover and squashed the cable wheb putting the cover back. The reason it was tripping every so often was because there was a play centre cabinet for the kids and everytime the kids would push the cabinet it would push the rad cover into the neutral. IR reading are 10megaohms which is not that great but the cable run they have used goes under the floor after the boxing in. All is good now, thanks for everyones help! (Also attatched some pictures)

kmulcahy1994 · Jul 10, 2019

Lucien Nunes said:
This is typical of a chewed or punctured cable where there's a tiny airgap between the conductors, large enough to not flash over during the IR test but small enough to short out when anything expands with heat or other minor disturbance happens.

Have you tried testing it at 1000V?

Thank you, disturbance was the kids unfortunately

telectrix · Jul 10, 2019

looking at those pics.are you sure wet-pants didn't nail the cable after you'd questioned his parentage?

Risteard · Jul 10, 2019

kmulcahy1994 said:
UPDATE! Found the fault. Tested between neutral and earth, however left cpcs in earth bar to see if it was shorting on another circuit or pipes.

I don't mean to be unkind, but why did you test without the cpcs connected to the means of Earthing before? It is a requirement of the Wiring Regulations that they be connected when conducting the test.

kmulcahy1994 · Jul 10, 2019

telectrix said:
looking at those pics.are you sure wet-pants didn't nail the cable after you'd questioned his parentage?

Quite possibly! Haha

Intermittent Ring fault! Any advice please.

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