A megohmmeter is an electrical test instrument that produces a DC voltage of 300-15,000 volts to accurately measure high resistances in electric motor windings or wire insulation. There are two types: battery-operated and generator-powered. The test voltages should not exceed the operating voltage of the equipment under test.
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 that allows you to test resistances commonly encountered in electric motor windings or wire 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, grounding, or insulation resistance with a standard ohmmeter may not be accurate due to the extremely high resistances common in these applications. The resistances in these cases can range from several megohms to several million megohms and require a test voltage far in excess of that used 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 generally not dangerous to the user of the megohmmeter.
There are two basic types of megohmmeter: a battery-operated variety and one that uses a hand crank or motor generator. Both megohmmeter variants are capable of accurate insulation resistance tests 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 tools are typically smaller and lighter than generator-powered versions and offer the benefits of a single push button, one-handed operation, and a choice of multiple operating voltages. The disadvantage of battery-operated megohmmeters includes short battery life and that they generally only produce a maximum of 5,000 volts.
Generator megohmmeters rely on a small internal generator to produce the high test voltages required. These generators are usually manually operated via an external crank, but can be equipped with an internal motor drive. These tools can produce voltages ranging from 300 volts to 15,000 volts and do not require battery replacement. A detrimental feature of using this type of tool is that operation requires two hands, thus necessitating the use of clip cables or the assistance of a second person. They are also typically bulkier and provide a single test voltage.
The high voltages produced by these instruments should always be kept in mind when testing electrical equipment. The megohmmeter test voltages should not exceed the operating voltage of the equipment under test by too large a margin as this could cause irreversible damage. Although the test voltages are supplied at very low amperages, care should always be taken to avoid electric shock when working with a megohmmeter.
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