Insulation resistance questions 2

[Gyo]

Does the length of a cable has any effect on the insulation resistance?
When an insulation resistance value of a cable is given as 10 G-ohm.meter, does it mean for a 10m cable, the IR is 1Gohm?
Why are there different test voltage for an IR test? Will using a different level of test voltage affect the IR reading?

 

[burnt2x]

Insulation resistance of cable is inversely proportional to its length. The longer the cable, the more parallel paths to the ground, hence, the smaller the insulation resistance.

 

[electricpete]

When an insulation resistance value of a cable is given as 10 G-ohm.meter, does it mean for a 10m cable, the IR is 1Gohm?

Yes, that’s the only logical way to interpet it.
(10 G-ohm meter) / (10 meter) = 1 G-ohm
However it’s not standard terminology in electrical testing to my knowledge.

Why are there different test voltage for an IR test? Will using a different level of test voltage affect the IR reading?

If we checked the same specimen with many different voltages, we should find relatively constant insulation resistance at low voltages, but as we approach breakdown the insulation resistance can start to decrease.

Typically for healthy insulation specimens, the insulation resistance will be constant all the way up to industry standard test voltages. However weak insulation specimens may show up as lower insulation resistance at standard industry standard test voltage than at some lower voltage.

Using high enough (industry standard) test voltage gives us better chance of identifying weak insulation. But not so high as to break down healthy insulation.


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