I am trying to visualize a burr paring back the insulation rather in the same way that one might do with a penknife.

That seems a bit unlikely inside a properly formed bend. I take lyledunn's point, but in practice, do singles in conduit fail the IR test? Has anybody in here ever had to unpack the conduit and then found some stripped cable?

After initial verification, IR tests only have value when fault finding.

Unless monitoring IR,  e.g useful for critical systems like flood pumps.

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

The insulation could be stripped bare, gnawed by rodents or burned to a crisp and still the IR test could show an off-scale reading. 

I'm not sure about the burned to a crisp situation - I would have thought the insulation (especially if of the PVC variety) would carbonise and so leave a detectable path to anything earthy in the vicinity. Admittedly that's probably not a useful case for initial verification where the installation is yet to be energised.

+1 for detecting damp ingress - I once got <100kΩ which turned out to be a plastic (polythene?) choc block in a bulkhead fitting that had been saturated with condensation.

I hope everyone remembers that IR tests these days need to be done with a connection to true earth as well, not just a disconnected c.p.c..

   - Andy.

yes I advocated that in college a few years back and was informed by the Tutor that such "true" earths etc need not be connected. He was an NICEIC approved contractor too so that might have been their take on it also. Anyway they eventually caught up with my idea to connect such earths, preferably all at the same time as you might find in a real life situation so I felt vindicated.

I also argued the one about the ring final R1 + R2 crossover test not being "exactly the same" all round but actually "substantially the same" with a difference of about 6% I think. 6% of a small number makes the answer a very small number if you consider probable ring lengths and also likely measurement accuracy/errors but none-the-less I stuck to my objections of the word "exactly" and they eventually stopped using it.

Us old gits know a thing or two! (Sometimes -LOL)