One common method of measuring resistances ranging 0–1000 MΩ is by using meggers or insulation-resistance testers. This is the usual ohmmeter with a battery used for voltage source.
Resistance is directly displayed on the front panel digital display. A range of voltages can be selected while testing.
This instrument is used to measure very high resistances, such as those found in cable insulations, between motor windings, in transformer windings, etc.
Normal multimeters do not provide accurate indications above 10 MΩ because of the low voltage used in the ohmmeter circuit.
Meggers can apply a high voltage to a circuit under test and this voltage causes a current if any electrical leakage exists. This makes it useful as an insulation tester.
Some laboratory test meters have a built-in high-voltage source. The high voltage permits accurate high-resistance measurement, but such meters are usually not portable.
The megger is essentially a portable ohmmeter with a built-in high-voltage source. The built-in high-voltage source may be derived from a magnet-type DC generator or battery.
In a DC generator-type megger, a hand crank is used to turn the armature to produce voltages up to 500, 1000, and 2500 (depending on the model used).
An electronic battery-operated type of instrument is popular because it is light, compact, and can be held and operated in one hand, i.e., there is no generator to turn.
High-testing voltage is produced by an electronic circuit, which uses an internal battery as an energy source.