Working Principle of Megger Test

• Voltage for testing produced by hand operated megger test by rotation of crank in case of hand operated type, a battery is used for electronic tester.

• 500 Volt DC is sufficient for performing test on equipment range up to 440 Volts.

• 1000 V to 5000 V is used for testing for high voltage electrical systems.

• Deflecting coil or current coil connected in series and allows flowing the electric current taken by the circuit being tested.

• The control coil also known as pressure coil is connected across the circuit.

• Current limiting resistor (CCR and PCR) connected in series with control and deflecting coil to protect damage in case of very low resistance in external circuit.

• In hand operated megger test electromagnetic induction effect is used to produce the test voltage i.e. armature arranges to move in permanent magnetic field or vice versa.

• Where as in electronic type megger test battery are used to produce the testing voltage.

• As the voltage increases in external circuit the deflection of pointer increases and deflection of pointer decreases with an increases of current.

• Hence, resultant torque is directly proportional to voltage and inversely proportional to current.

• When electrical circuit being tested is open, torque due to voltage coil will be maximum and pointer shows ‘infinity’ means no shorting throughout the circuit and has maximum resistance within the circuit under test.

• If there is short circuit pointer shows ‘zero’, which means ‘NO’ resistance within circuit being tested.