The megohmmeter, or megger, is a measuring device that tests high electrical resistances. Typically these measurements are made on electrical wires and motor windings to test the insulation value of the wires. The prefix "meg" describes a numerical value of 1,000,000. In the case of an ohmmeter that value is also described in ohms. A megger will read a wire or motor windings in Megohms, where 1 Megohm is equal to 1,000,000 ohms. In order to read such a high resistance value the meter must be capable of generating a very high voltage. The smallest value that a megohmmeter can supply is 1,000 volts. Some hand-crank meggers can supply as much as 10,000 volts or more by a small generator supplied inside the meter.
Remove all power from the wires or circuit being tested. There can be absolutely no voltage connected to the circuit when these high-voltage tests are being conducted.
Uninstall all wires that are to be tested. This means that any wire that is to have the insulation value checked, must be completely disconnected on both ends of the circuit. The bare copper ends are sometimes covered by a wire nut or a piece of tape. This protects equipment or someone from being shocked. In the case of testing motors, all wires feeding the motor must also be completely disconnected.
Connect one of the megger's leads to the electrical frame or earth ground of the electrical system. In testing motor windings, this lead will be connected to the actual metal frame of the motor.
Attach the other lead from the meter to the bare copper end of the wire or one of the motor terminals. Check to be sure the other end of the wire being tested is in free air or covered by an insulator such as a piece of tape or wire nut.
Turn the meter on or begin to crank the generator handle. It may take 2 to 5 seconds for the high voltage to build inside the wire or motor windings.
Read the meter. A reading of greater than 999 megohms is a near-perfect resistance reading for a wire or new motor. Resistance readings less than 1.5 megohms may present problems in old wires or used motors. In most cases, any reading in between these values may be fine, unless trouble has been experienced in these circuits.
Continue to test other wires or the remaining motor terminals. Once the meter's high-voltage source has ceased, the wires or motor leads are safe to handle.
Caution must be exercised when using a megohmmeter as serious damage and injury to parts and personnel can occur from improper use.
Tips and warnings
- Caution must be exercised when using a megohmmeter as serious damage and injury to parts and personnel can occur from improper use.