Megger has become the generic description for a high voltage, low current insulation tester. The word is short for megohm-meter.

The original Meggers were manufactured by Evershed & Vignoles Ltd., Acton Lane Works, Chiswick since 1889. They were taken over by AVO Ltd. in 1895. The word is now the registered trade mark of Megger Group Ltd

Meggers were also made in the USA by James G. Biddle.

Older types have a small built-in generator. The generator is hand-driven, through gearing and a centrifugally controlled clutch slips at a predetermined speed so that a steady test voltage is obtained.

Modern types work with batteries and circuitry to generate the voltage required.

Both types can output test voltages of 100, 250, 500, 1,000 or 2,500 VDC, depending on model.

Although any Ohmmeter or Multimeter may appear capable of similar measurements, only a Megger type instrument can test the quality of the insulation at or above its operating voltage.

Choice of test voltage is normally determined by the operating voltage of the circuit under test; usually twice the operating voltage is sufficient.
Two basic insulation tests are possible: Insulation to ground and insulation between conductors.

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