The Megger is a precision instrument which can be used to measure the insulation resistance of wiring Installation, machines, equipment and apparatus and whatnot. The beggar is a permanent magnet moving coil type and instrument the permanent et magnet has pole pieces with a projecting horn shape. There are two coils in Megger. One is a defacing coil and the other is a control coil. Both the coils are connected in parallel across a hand-driven DC generator which will produce 250V, 500V, 1000V, 5000V and 10000 volts.
Working Principle of Megger:
Normally we use a 500-volt merger for the measurement of insulation resistance. The merger can’t be used in live line, which may cause a disturbance in the meter & may cause damage. It is not possible to measure the low resistance with the help of a merger, as its scale is graduated in mega.ohm (106 ohm). The range of low resistance is up to 1 ohm, so we are unable to measure the low resistance with the help of the insulation testing merger.
The central coil with control circuit resistance is connected across the hand-driven DC terminals. The current flows in this coil are proportional to the voltage applied to the electrical circuit being tested. When there is no current in the deflecting coil, the control coil takes up the position of neutral, at that position the torque is zero. The position of the pointer under this condition is at the end portion of the scale and it is marked ‘α’ indicating infinity resistance for the electrical circuit under testing.