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Megohmmeter is a special type of ohmmeter used to measure the electrical resistance of insulators. Insulating components, for example cable jackets, must be tested for their insulation strength at the time of commissioning and as part of maintenance of high voltage electrical equipment and installations. For this purpose megohmmeters, which can provide high DC voltages (typically in ranges from 500 V to 5 kV, some are up to 15 kV) at specified current capacity, are used. Acceptable insulator resistance values are typically 1 to 10 megohms, depending on the standards referenced.
Resistance to be measured is connected across the terminals i.e connected in series with the deflecting coil and across the generator. When the current is supplied to the coils then they have torque in opposite directions.
If resistance to be measured is high, no current will flow through the deflecting coil, the controlling coil, will therefore, set itself perpendicular to the magnetic axis and hence set the pointer to the infinity.
If the resistance to be measured is small, a high current flow through the deflecting coil and the resulting torque sets the pointer to zero.
For intermediate value of resistance, depending upon the torque production , the pointer is set at a point between zero and infinity.
The hand driven generator is of permanent magnet type and it is designed to generate from 500 to 2500 volts.