So here's this year's story ! I thought I would get the PAT testing done on my equipment as I have put it off from January this year. This years mystery surrounds a pair of active subs. One failed the insulation resistance test and the guy doing the testing was kind enough to do the test again with several items to confirm the results and explain the result which was good. In order to return the amp module I removed it from the sub and was really surprised by what I found. The sub is dry on the inside and out with no signs of damp apart from on the heat-shrink insulation in two locations. The surrounding locations to the damp are completely dry and it almost appears the heat shrink insulation is sweating and was really damp - as in droplets of damp on it. There are two locations one is on the amp module where the insulation covers the spade connectors that carry the mains power, and the other is on the heat shrink insulation that covers the choke which is half way along the cable from the amp module to the sub.

I've dried everything off carefully and I suspect that's the cause of the insulation resistance test failure but I'm just wondering if anyone has any ideas why the damp would be attracted to this insulation or if it would sweat like this. I've never seen anything like this myself.