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A megohmmeter, or megger as it is more commonly known, is an electrical test instrument designed to test extremely high resistances by producing a DC (direct current) voltage of between 300 and 15,000 volts. A megohmmeter produces a high voltage, low current DC charge which allows for testing resistances commonly encountered in electric motor winding or cable insulation tests. Megohmmeters produce this high voltage via battery powered internal circuitry or a manually operated generator.
Testing electrical equipment, machinery, or installations for winding, ground, or insulation resistance with a regular ohmmeter may not be accurately achieved due to the extremely high resistances common in these applications. Resistances in these cases may range from several megohms to several million megohms and require a test voltage far higher than that utilized by smaller ohmmeters. A megohmmeter uses DC voltages ranging from 300 to 15,000 volts to accurately measure these very high resistance values. These voltages are supplied at very low current ratings and are typically not dangerous to the megohmmeter user.
There are two basic types of megohmmeter: a battery operated variety and one that makes use of a hand crank or motor operated generator. Both megohmmeter variants are capable of conducting accurate insulation resistance testing on installations and equipment with resistances of several teraohms (1,000,000 megohms). Battery operated megohmmeters employ specialized internal circuitry to convert the low battery voltage to the higher test voltage. These instruments are typically smaller and lighter than the generator versions and offer the benefits of single button, one-handed operation and the choice of multiple operational voltages. The downside of battery operated megohmmeters include short battery life and that they generally produce only a maximum of 5,000 volts.
Generator megohmmeters rely on a small internal generator to produce the required high test voltages. These generators are usually hand operated using an external crank handle but may be fitted with an internal motor drive. These instruments can produce voltages ranging from 300 volts to 15,000 volts and require no battery replacement. One detrimental characteristic of using this type of instrument is that the operation requires two hands, thereby necessitating the use of clip-on leads or the assistance of a second person. They are also typically bulkier and supply a single test voltage.
The high voltages produced by these instruments should be kept in mind at all times when testing electrical equipment. Megohmeter test voltages should not exceed the operating voltage of the equipment being tested by too large a margin as this may cause irreversible damage. Although the test voltages are supplied at very low amperages, care should also be exercised at all times to prevent electrical shocks when working with a megohmmeter.