To measure the insulation resistance in wires and motors, electricians use megohm meters, also known as insulation megohm meters. It differs from the ohmmeter in that, unlike the ohmmeter, it can measure resistance over a million. This capacity is useful when measuring resistance at high voltages, because accurate resistance measurements require you to provide a higher voltage than the ohmmeter provides.

Step 1

Turn off all power supplies connected to the test cable.

Step 2

Disconnect the power cable.

Step 3

Disconnect the cable from the device to power it.

Step 4

Remove all nuts or tape used to temporarily cover the cable.

Step 5

Set up a safety barrier to prevent anyone from touching the bare copper wire at the end of the cable, as your megometer will provide high voltage during the test.

Step 6

Any wire that touches the megohm meter is used in the electrical system. For example, if the system you are testing includes a motor control cabinet, you may touch the wires in the cabinet itself because it is grounded.

Step 7

Touch the bare copper wire of the cable to be tested with another lead.

Step 8

Open and read the meter. Unless other problems are found, any reading between 2 and 1000 megabytes is generally considered a good reading. Less than 2 megohm indicates an insulation problem.