Mega is used for measuring the insulation resistance. The parts of Mega are as shown above. The armature of the generator is rotated by the hand-driven cranked lever. The clutch mechanism is designed to slip at a predetermined speed. This facilitates the generator to maintain a constant speed and hence the constant voltage while testing.
The two coils A and B constitute a moving coil voltmeter and an ammeter. Both are combined to form one instrument. The hot terminal of the equipment whose insulation resistance has to be measured is connected to the testing terminal X. The terminal Y is connected to the body of the equipment which is generally grounded.
When the crank handle is rotated, a voltage is generated in the generator. The generated voltage is applied across the voltage coil A through resistance R1. When the terminals X and Y are free initially, no current flows through coil B. The torque produced by the coil A rotates the moving element to show infinity.
While testing, the terminals X and Y are connected across the terminal and body of the machine for measurement. Now, the current passes through the deflecting coil B. The deflecting torque produced by coil B interacts with the torque of coil A and rotates the moving element to indicate the resistance value.
Voltage generated by this instrument is around 500 volts. Megas are available to generate 1000 volts, 2500 volts and 5000 volts also. High voltage megas are either motor operated or power operated.