The insulation resistance of cable is measured using a Megohmmeter. This is a simple, nondestructive method of determining the condition of the cable insulation to check for contamination due to moisture, dirt, or carbonization.

Insulation resistance measurements should be conducted at regular intervals and test records kept for comparison purposes. A continued downward trend is an indication of insulation deterioration even though the resistance values measured is above the minimum acceptable limit.

The readings must be corrected to a base temperature (such as 20°C) for a valid comparison. Keep in mind that insulation resistance measurements do not give the measure of total dielectric strength of cable insulation or weak spots in the cable.

When overpotential testing cable, its common practice to conduct the insulation resistance measurement first and then proceed with the DC overpotential test if acceptable readings are achieved. After DC overpotential test is completed, the insulation resistance test is done again to assure that the cable has not been damaged by the hi-pot.

Polarization Index is another insulation resistance test method that evaluates insulation quality based on the change in Megohm value over time. After voltage is applied, the IR value is read at two different times: usually either 30 and 60 seconds (DAR), or 60 seconds and 10 min (PI).

"Good" insulation will show a gradually increasing IR value over time. When the second reading is divided by the first reading and the resulting ratio is called the dielectric absorption ratio (DAR) or polarization index (PI).