This is another test performed on a dead circuit only. The objective is to check for insulation of cables or a power circuit. The device used to check integrity of insulation is known as an ‘Insulation-Resistance Tester’.

Generally, this is used during the installation of high-voltage power cables and terminations. Below is a general motor circuit is shown with breaker, fuses, and overload relay.

To check insulation of the circuit (excluding motor), disconnect the power supply by opening the breaker.

Then, isolate the motor from the circuit through terminals T1, T2, and T3. First checking insulation resistance between earth and T1, then earth and T2, and finally earth and T3 checks insulation resistances of conductors, as well as other devices. If the insulation resistance of any branch shows zero or a very low reading, then it can be concluded that there is an insulation failure.

This test is also used in fault finding, to check for earthed motors or cables and for checking insulation failure of conductors. Individual phases of three-phase motor winding can be insulation-tested only if all six leads of the winding are brought out. 

The winding being tested should be connected to the tester’s output with the other two windings connected together and to the earthed frame of the motor. Where only three leads are available, the insulation of the machine winding as a whole can only be tested with reference to the earthed frame of the motor.

These insulation testers are also called Meggers and have a built-in energy source (either DC generator or battery) to produce test voltages of rating 500 V DC or more. This is required since the electrical circuit to be tested applies voltage of different ratings.

For example, when the insulation resistance of HV cables is checked, 1000 V minimal voltage is applied, whereas for a domestic circuit 500 V is sufficient for testing. Testing on a live circuit requires extreme caution and should be restricted to LV circuits.

Precautions should be taken to prevent inadvertent contact of the technician with live parts.

The probes and tools must be insulated with minimum exposure of conducting parts. This will minize inadvertent bridging of two terminals which are at different potentials which can cause a short-circuit and arcing leading to burn injuries to the technician.

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