Insulation resistance test on a single cable

[Jupiter01]

I’ve seen some online videos which show an insulation resistance test being done on a single 2.5mm T&E cable where one of the end is clamped by the MFT and the other end is hanging loose. Would such a test not always fail as the loose end will have some exposed copper next to the copper of the other wires?

 

[RandomGrinch]

Not if the cable has a good clean cut!
I may be wrong, but I think the maximum arc distance for 500V DC is 0.05mm, when the insulation thickness on 2.5 T&E is about 0.8mm.
So I believe an IR test of several kV may be needed to fail a 2.5mm T&E with a clean cut?
The length of the cable, humidity and contamination etc. may also effect the results!

 

[EFLImpudence]

Do the test with no cable.

How far (or near) do the lead clips have to be to show an unacceptable test result.

 

[ericmark]

I have done insulation tests using higher voltages, the spy holiday detector was used on pipe lines to test the rap, I think around 2500 volt was used, with an earth wire that looked like the inner of a speedo cable, and a wand and coil which was walked up the pipe, it was 1980 in Algeria where I used these, over 40 years ago, so can't remember all details, know I wreaked an AVO Mk8 working on them, it blow the germanium diode.

But with domestic the insulation tester only uses 500 volt.

 

[conny]

We used to test HV motor coils with 13,000 volts. You could smell the ozone if the insulation broke down. Lovely crackling sounds as it happened.

 

[Jackrae]

We used to test HV motor coils with 13,000 volts. You could smell the ozone if the insulation broke down. Lovely crackling sounds as it happened.

Ah the fun we had in days gone by doing Hi-Pot testing

 

[RandomGrinch]

When I first started PAT'ing (many years ago!), my department was still using ancient Clare testers and performing 3kV flash tests on class II appliances!
25A earth bond tests can also be entertainingly misused, for welding and destroying components on PCB's/motherboards!