The megger, also known as the Mega-Ohm tester, is used to test the integrity of insulation between conductors. Meggers have around since 1905, and the name comes from the fact that the insulating resistance is expressed in the range of hundreds of megohms. The megger is a simple device but a lot of caution must be used to avoid any electrical shock when using one. The steps below explain how to connect and use a megger.
Disconnect the item being tested from other circuitry and the power supply if possible. It may not be possible to remove the device from the other device it is connected to, so in these cases and it is okay to have them connected.
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Connect one test lead of the megger to the insulation and the other test lead to the conductor. For example, if you are testing the insulation for an electrical wire, the outer covering of the wire is the insulator and the copper wire that lies inside the insulation is the conductor.
Rotate the crank until the slip clutch begins to slip. You will hear a click as you rotate the crank. The rotation powers the DC generator, which is connected to a meter. The connections between the test leads, the insulation, and the conductor are made when the DC generator turns on.
Note the reading from the meter. The readings will be in millions of ohms, also expressed as megohms. The ideal meter reading should be one megohm for each 1,000 volts of operating voltage. For example, if you are testing a motor that has a 2,400 operating voltage, the megger reading should be around 2.4 megohms. For lower operating voltage, the minimum reading should never be lower than one megohm.
Remove the conductor and the insulator from the test leads.