Insulation Resistivity of Cable

The insulation just experiences the voltage. It cannot "know" what current is flowing in the cable, as it is an insulating material.

Thank to "PWSlack" for your support, but when the cable in operation the current flow thought the conductor it make the conductor warmer than no load (maximum 90 degree celcius) there temperature shall be effect to the cable insulation and It make the resistivity of insulation drop down, respectively. that why i want to know the allowance value of insulation resistivity of cable in Full load operation.

Yes! I agree with you! The cable insulating material has a negative temperature co-efficient of resistance and hence, when the cable conductor "warms-up" while carrying current, the insulation too warms up and hence its insulation resitance would definitely go down. But, that should not be a cause for concern, as the insultaion resistance itself will be in tens or hundreds of megaohms and a marginal reduction in the same due to heat should not matter.

Anyhow, let me search my library for a direct answer to your question and if I find an answer, I will reply.

dear,

There is no load resistivity test, as at full load, temp. rise max. 70 deg, C, you can calculate the same.

70 Deg for PVC insulated only (90 deg for XLPE insulated) as per IEC, what do you mean "calculate the same" ?. as we discuted above, at full load operation the insulation resistivity the differ from no load operation due to the rising of conductor temp ?. thank you.

Power cable insulation is rated for a specific termparature rise (usually either 90C or 105C) above ambient (usually 20C for underground or 40C for in air). The insulation resistance (NOT resistivity!) is rated for the voltage class, and does not vary enough under those conditions to be of concern across the normal operating temperature range.

It is up to the engineer to assure that the proper size cable is used, with correct derating for the installation conditions, so that the temperature does not continually exceed the specifications (it can for limited lengths of time for emergency operation). If the loading would cause your operating temps to be too high, you need to specify a larger gauge cable.

The standards probably say no-load insulation resistance test because you are going to test your cable off-line (not energized with load connected, but only energized by the test set). I don't know of anything that directly tests insulation resistance while a cable is under load and in operation, except that it keeps the electrons in.