A Megger is a type of electrical test equipment used to measure the insulation resistance of insulators. It is also known as a megohmmeter, which is derived from the words "megohms" and "tester". The name "Megger" is actually a trademark of Evershed & Vignolas Ltd. The Megger consists of a DC generator or battery that produces a high voltage, and an indicating pointer attached to two coils, Coil A and Coil B, which constitute one instrument. The test terminals at X and Y are used to measure insulation resistance. The Megger is designed to measure high quantities of insulation resistance, such as between the windings of a transformer or motor, or between the conductor in a cable and the conduit or sheath enclosing the cable.

The Megger is operated by hand-cranking the generator, which produces a voltage that causes the pointer to deflect. The deflection of the pointer increases with the increase in voltage in the external circuit and also decreases with the increase in current. The Megger is used to identify potential problems with insulation, such as damage or degradation.

Some advantages of using a Megger include its high accuracy compared to other instruments, and its ability to measure high resistance values. However, there are also some disadvantages to using a Megger, such as the need for two people to operate a hand-driven Megger, and the fact that accuracy can vary with cranking.

In summary, a Megger is a type of electrical test equipment used to measure insulation resistance. It consists of a DC generator or battery that produces a high voltage, and an indicating pointer attached to two coils. The Megger is operated by hand-cranking the generator, and is used to identify potential problems with insulation.