Weird fault that no-one can work out the issue.

[Raptor0014]

Evening all.

Been at a property the last couple of days that had suffered damage due to a surge. I got the power back on all circuits apart from the upstairs lights as it immediately trips.

Long story short I broke the circuit down and re-added room by room to narrow down the problem area, as no matter what test I did there were no apparent faults. Yet turn the lights on and the circuit trips. I ramp tested the RCD and that tripped at 24mA. The earth leakage with all the circuits on and under load with the particular RCD was 6mA.

The light in question is a weird GX38 CFL that sits in an alcove. But I removed it and wired a brand new pendant in and tried that. Immediately the circuit trips. The light works when wired in place of another light on the circuit. So the light itself isn’t faulty.

I removed all working loads in the circuit again
and had just this one light wired, circuit still trips.

I removed all other circuits from the RCD and had just this one light wired to the switch to rule out cumulative earth leakage and again circuit trips.

I tried the light and the pendant wired directly to the switch and both work fine.

So the problem seems to be a peice of cable approx 5m long that is buried in 2 walls and runs under the floorboards across the landing.

I wondered if the problem was maybe rodent damage on just one conductor that was giving a fault to earth via the rad pipes. But testing between the rad and the conductors gives open circuit. So it’s not that either.

I spent 20mins on the phone to technical support and they were at a loss as to the fault.

I did a R1+R2 and RN+R2 to check continuity. All fine.

I IR tested at 500v L-N, N-E and L-E and all gave >999MΩ

So there is a problem with the cable it seems. But testing it is not showing what it is.

At the moment I’ve left the suspect piece of cable between the load side of the switch and this one light disconnected. I’m going back in a couple of weeks to change the CU as the owner now wants a full RCBO surge protected unit. When there the only thing I can think left to do is run a new 5m long piece of cable directly from the switch to the light and see if the circuit still trips.

If it doesn’t trip then problem solved. But it still doesn’t make sense that any test I do shows no apparent problem. If it still trips then I haven’t a clue what the fault is.

So over to you knowledgeable folk. I’d appreciate any advice as it’s a real head scratcher and I can’t think of anything else.

 

[timhoward]

Fun!
Have you tried continuity between the Neutral for this circuit and every other neutral in the CU?

 

[cireland]

Evening all.

Been at a property the last couple of days that had suffered damage due to a surge. I got the power back on all circuits apart from the upstairs lights as it immediately trips.

Long story short I broke the circuit down and re-added room by room to narrow down the problem area, as no matter what test I did there were no apparent faults. Yet turn the lights on and the circuit trips. I ramp tested the RCD and that tripped at 24mA. The earth leakage with all the circuits on and under load with the particular RCD was 6mA.

The light in question is a weird GX38 CFL that sits in an alcove. But I removed it and wired a brand new pendant in and tried that. Immediately the circuit trips. The light works when wired in place of another light on the circuit. So the light itself isn’t faulty.

I removed all working loads in the circuit again
and had just this one light wired, circuit still trips.

I removed all other circuits from the RCD and had just this one light wired to the switch to rule out cumulative earth leakage and again circuit trips.

I tried the light and the pendant wired directly to the switch and both work fine.

So the problem seems to be a peice of cable approx 5m long that is buried in 2 walls and runs under the floorboards across the landing.

I wondered if the problem was maybe rodent damage on just one conductor that was giving a fault to earth via the rad pipes. But testing between the rad and the conductors gives open circuit. So it’s not that either.

I spent 20mins on the phone to technical support and they were at a loss as to the fault.

I did a R1+R2 and RN+R2 to check continuity. All fine.

I IR tested at 500v L-N, N-E and L-E and all gave >999MΩ

So there is a problem with the cable it seems. But testing it is not showing what it is.

At the moment I’ve left the suspect piece of cable between the load side of the switch and this one light disconnected. I’m going back in a couple of weeks to change the CU as the owner now wants a full RCBO surge protected unit. When there the only thing I can think left to do is run a new 5m long piece of cable directly from the switch to the light and see if the circuit still trips.

If it doesn’t trip then problem solved. But it still doesn’t make sense that any test I do shows no apparent problem. If it still trips then I haven’t a clue what the fault is.

So over to you knowledgeable folk. I’d appreciate any advice as it’s a real head scratcher and I can’t think of anything else.

That's a cracker.
Just for the sake of clarity, when testing to earth, did you leave the earth in the bar?
Does it go with a bang or merely trip?

 

[littlespark]

Any electronics on the circuit that could be confusing the tester? You say you disconnected loads, what about dimmers or smart switches?

High resistance short like a fried rat?
Testing with you mft not giving enough juice to read as a fault.

Try 1000v if tester does it.

 

[plugsandsparks]

Have had to do that once in 12 years, lol - basically run a temp cable to bypass the suspect cable to prove the installation is stable. Sooner you than me nowadays

 

[Raptor0014]

Fun!
Have you tried continuity between the Neutral for this circuit and every other neutral in the CU?

No I haven’t as I removed every circuit bar the light circuit in question when testing for cumulative leakage.

All the other circuits were tested individually as part of checking everything after the surge. But I’ll give that a test when I go back to change the CU in a few weeks.

It’s been a real head scratcher. Good job I’m bald as I’d have scratched it all off by now anyway.

 

[Raptor0014]

Any electronics on the circuit that could be confusing the tester? You say you disconnected loads, what about dimmers or smart switches?

High resistance short like a fried rat?
Testing with you mft not giving enough juice to read as a fault.

Try 1000v if tester does it.

There are some Hive lights etc. but no dimmers. Plus I disconnected every switch and wago’d the links between for the common as it’s all looped through the switches. They’re all metal so took them out the equation as they’re earthed but just to make sure nothing spurious.

My tester will do 1000v so will give that a try next time.

 

[Raptor0014]

That's a cracker.
Just for the sake of clarity, when testing to earth, did you leave the earth in the bar?
Does it go with a bang or merely trip?

It just trips the RCD not the MCB. And no the earth was removed from the bar to rule out parallel paths.

The piece of cable was also removed at both ends when I test the suspect piece.

 

[cireland]

It just trips the RCD not the MCB. And no the earth was removed from the bar to rule out parallel paths.

The piece of cable was also removed at both ends when I test the suspect piece.

Could be the parallel path causing the trip.

 

[timhoward]

When there the only thing I can think left to do is run a new 5m long piece of cable directly from the switch to the light and see if the circuit still trips.

You've proved a pendant directly on the switch works, and the original fitting works directly on the switch, so logically adding some fresh copper in the middle will work.
(You might want to consider terminating the feed to this switch, sending a new feed to the fitting in the loft from somewhere else and adding a quinetic switch.)

High resistance short like a fried rat?
Testing with you mft not giving enough juice to read as a fault.

Try 1000v if tester does it

If he's testing clear at 500v I don't see any value in sending 1000v through it, it's not going to suddenly jump from millions of ohms to under 7666.6 ohms surely?