Hmm. a perfect cable will pass IR test, but so will one with the insulation removed completely, so long as there is a small air gap between the bare metal and the thing(s) that it is not meant to touch.

I can stick a nail into twin and earth and find the live core, and drive it right through into a bit of dry wooden batten, and so long as I only find the live core, and am not earthing the nail at the time of the test, that too will pass the IR. If the wood is wet it may show something, but if it does or not will rather depend on what else the wood is in contact with.

An IR test is not a lot of use for reliably finding damaged insulation and this sort of thing. It is great for checking if pyro  has got wet, and things where damp has got into underfloor heating pads,  but is most unlikely to find truly dry cable damage that is not a dead short, and that could be just as well detected with a buzzer and battery tester.!

If that weakness is not taught these days  or is not obvious then perhaps there should be a comment to this effect in the GN.

Skinning a plastic insulated cable remains a pretty much  perfect insulator until  the few last tens of microns are removed.

If bored, try IR testing a single turn of cling film at 1000V on the test clips - if you go gently and do not puncture it, it too will pass an IR test at giga-ohm plus ;-) you need surprisingly little dry insulation to fool a meter.

Mike.

edited for grammer

Hmm. a perfect cable will pass IR test, but so will one with the insulation removed completely, so long as there is a small air gap between the bare metal and the thing(s) that it is not meant to touch.

I can stick a nail into twin and earth and find the live core, and drive it right through into a bit of dry wooden batten, and so long as I only find the live core, and am not earthing the nail at the time of the test, that too will pass the IR. If the wood is wet it may show something, but if it does or not will rather depend on what else the wood is in contact with.

An IR test is not a lot of use for reliably finding damaged insulation and this sort of thing. It is great for checking if pyro  has got wet, and things where damp has got into underfloor heating pads,  but is most unlikely to find truly dry cable damage that is not a dead short, and that could be just as well detected with a buzzer and battery tester.!

If that weakness is not taught these days  or is not obvious then perhaps there should be a comment to this effect in the GN.

Skinning a plastic insulated cable remains a pretty much  perfect insulator until  the few last tens of microns are removed.

If bored, try IR testing a single turn of cling film at 1000V on the test clips - if you go gently and do not puncture it, it too will pass an IR test at giga-ohm plus ;-) you need surprisingly little dry insulation to fool a meter.

Mike.

edited for grammer