How to Use a Megger Insulation Tester

A megger, or a megohmmeter, uses a high voltage to test the insulation value of wires. A megohm is equal to 1,000,000 ohms of resistance. This high value of resistance measurement ensures that the wire's insulation does not have any breaks along the length of wire. In order to measure the high resistance value, a high voltage is placed on the wire conductor--in some cases, as much as 15,000 V. Most battery-type meggers generate a voltage from 1000 to 3000 V.

Things You'll Need

• Electrical hand tools
• Plastic wire nuts

Instructions

1. Remove all electrical power from the wire that is being tested. Use the appropriate electrical hand tools to isolate the wires from any portion of the circuit. Due to the complexity of some electrical circuits, various tools maybe required to isolate the wire that requires testing.

2. Remove all electrical connections from the wire. Expose both ends of the wire by revealing the bare copper connection point. Cover one end of the wire's copper connection with a wire nut. This will insulate the conductor from accidentally shorting to ground and giving a false reading.

3. Attach the positive (+) end of the megohmmeter to the other end of the bare copper connection on the wire being tested. Connect the ground wire from the meter to the metal ground of the electrical system. Typically, this point will be the metal conduit or the main electrical panel box.

4. Switch the meter to the "on" position. Most megohmmeters have a charge button located on the face of the meter. Press the charge button and observe the meter. It generally takes from three to five seconds for the meter to charge the wire to the full high voltage.

5. Read the meter. A reading on the face of the meter that indicates greater than (>) 999 meg is a perfectly insulated wire. A reading below (<) 1.00 meg may indicate a problem with the wire. A reading between these two resistances will generally operate with no problems to the electrical circuit.

Tips & Warnings

• Exercise extreme caution when using a high voltage megohmmeter. The high voltages can damage certain electronic equipment and give you a severe shock.

References

• Use a Megohmmeter

Resources

• Read this Article in Spanish
• Test Equipment Forum