Bizarre variable IR problem at 250V

[SparkyChick]

So, what I've mostly found in situations like this is a pinhole somewhere in the insulation where a cable's been pushed back into a box and scraped on a sharp edge or clipped by a screw etc.

You run the test for along period (on the Megger MFT1731 you can just hold the test button to keep the juice flowing) and what is usually observable in such situations is the reading climbs to a nice high acceptable reading but then out of the blue it drops down to a much lower value before climbing back up again.

I've observed the same behaviour as you've described on many occassions and as I say, it's usually been a pinhole in the insulation of a conductor. It's not manifesting during your investigations because you've probably pulled all the conductors forward out of the box and thus the cable can't generate enough charge to breakdown the insulation/air gap that now exists, but when you put it all back together it's being pushed back in and it can develop a sufficient charge to flash over (the momentary drop of IR reading).

Unfortunately, if you've not made any changes to the circuit or accessories and it's only been found due to the RCBO tripping, it could be a real pain to find it.

 

[plugsandsparks]

There is always the mice issue, watching you and putting his/her paw across the wire just when you are about to leave. They are not as daft as they look. When i get desperate, if i think i have narrowed it down, you can always run a replacement cable across the floor to see if it holds. Only had to do this once or twice a decade , sometimes you cannot let it go and it becomes personal........

 

[pc1966]

So, what I've mostly found in situations like this is a pinhole somewhere in the insulation where a cable's been pushed back into a box and scraped on a sharp edge or clipped by a screw etc.

Some form of damage is my thought, but what is odd here is both L and N seem equally impacted, for usual scrapes, etc, I would be surprised if they got equal "treatment".

You run the test for along period (on the Megger MFT1731 you can just hold the test button to keep the juice flowing) and what is usually observable in such situations is the reading climbs to a nice high acceptable reading but then out of the blue it drops down to a much lower value before climbing back up again.

I've observed the same behaviour as you've described on many occasions and as I say, it's usually been a pinhole in the insulation of a conductor. It's not manifesting during your investigations because you've probably pulled all the conductors forward out of the box and thus the cable can't generate enough charge to breakdown the insulation/air gap that now exists, but when you put it all back together it's being pushed back in and it can develop a sufficient charge to flash over (the momentary drop of IR reading).

I could see it happen with sockets off and forward. Also without any noticeable movement, I would see it if linking two L, but not one L at a time.

My suspicion is the fault would flash over and with enough cable it would establish a big enough arc to allow enough fallow-through to make the MFT notice.

But what is odd is I could not persuade any of the partitioned for test segments to show anything odd even at 1kV. Maybe had I brought my Megger I would have seen some odd changes in the 200M-1000M region, but if it comes back I will be prepared.

Unfortunately, if you've not made any changes to the circuit or accessories and it's only been found due to the RCBO tripping, it could be a real pain to find it.

Alas, I suspect you are right

 

[pc1966]

There is always the mice issue, watching you and putting his/her paw across the wire just when you are about to leave. They are not as daft as they look. When i get desperate, if i think i have narrowed it down, you can always run a

Mice are one option, as in they might have nibbles both L and N over a segment, but most circuits seemed OK so far and no other evidence of an infestation.

replacement cable across the floor to see if it holds.

If it keeps happening (odd trip) then I might try that with one segment at a time. Hoping it is not several faults...

Will get an extra RCBO so if that happens I can partition in to two radials as well allowing finer detection of dodgy segments.

Only had to do this once or twice a decade , sometimes you cannot let it go and it becomes personal........

Hopefully it will be OK and not haunt me to become my MobyDick.

I guess Captain Ahab could get cream for that these days...