What is Done During Insulation Resistance Measurement?

The measurement of insulation resistance is a common routine test performed on all types of electrical wires and cables. Its objective is to measure the ohmmic value of the insulation under a direct voltage of great stability, generally 50, 100, 250, 500, or 1000 VDC. The ohmmic value of the insulation resistance is expressed in megohms (MΩ). To conform to specific standards, the insulation resistance test can be performed under voltages up to 1500VDC. Due to the stability of the voltage source, it is possible to adjust the test voltage by steps of 1 volt.

The stability of the voltage is critical; a non-regulated voltage will drop sharply in presence of a bad insulation which will cause an erroneous measurement.

After the required connections are made, you apply the test voltage for a period of one min. During this interval, the resistance should drop or remain relatively steady. Larger insulation systems will show a steady decrease, while smaller systems will remain steady because the capacitive and absorption currents drop to zero faster on smaller insulation systems. After one min, read and record the resistance value

Selection of IR Testers (Megger):

Voltage Level	IR Tester
650V	500V DC
1.1KV	1KV DC
3.3KV	2.5KV DC
66Kv and Above	5KV DC