- Reaction score
- 8,188
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.
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.