black cut out fuses

There was something posted recenetly abiout this phenomena - maybe on eefix or somewhere like that. Basically, they said that some of the older phenolic fuseholders were beginning to break down and become conductive, albeit only slightly. The proffered advice was to informn the DNO and let them make the judgement as to whether or not a new grey plastic cut out replacement was required.

yes, and that's why I asked if it was a contact probe - if it had been, then significant current could have been flowing through the slowly oxidizing resin, and it would then be quite nasty. Sounds like this was not doing that, this time at least.

The saving grace is that the degradation of the resin is quite slow and it takes many years to go from a barely perceptible leakage to something that should be a real concern.

In the meantime non contact probes which measure field, can be triggered just because the field is unusually high.

Mike. 

yes, and that's why I asked if it was a contact probe - if it had been, then significant current could have been flowing through the slowly oxidizing resin, and it would then be quite nasty. Sounds like this was not doing that, this time at least.

The saving grace is that the degradation of the resin is quite slow and it takes many years to go from a barely perceptible leakage to something that should be a real concern.

In the meantime non contact probes which measure field, can be triggered just because the field is unusually high.

Mike.