A megger is a portable device which is used for the measurement of high resistance. The component of megger are

  1. Permanent magnet
  2. C shaft
  3. Hand driven generator
  4. Resistances RC and RD

Two Coils-

  1. Control coil (Cs)
  2. Deflection Coil (Cd)

Working Principle-

As current provided by the hand driven generator flow through the coil C because of which the coil will tend to set itself at right angle to the magnetic field. The wire on the coil C is wound in such a way that the torque generated is counterclockwise. Coil D is wound with the help of wire in such a way that the torque generated in clockwise.

Megger method for measurement of high resistance

Initially, the test terminal is open, giving a very high value of resistance by pointing to the infinite point. In this condition there will not be any current flow in coil D. Thereby coil C will allow the motion of the pointer. When the test terminal is short circuited the current flow through the coil D and generated clockwise torque and the pointer of the ohm meter rest at 0Ω.

When a resistance with unknown value is connected between the test terminal, current from the hand generator flow through both coils. In this situation torque in coil C and torque in coil D comes in equilibrium and a particular value of resistance can be seen over dial. So in this manner we can find out the value of unknown resistance.

Resistance                  Torque

∞Ω                              ZC>ZD

0 Ω                              ZD>ZC

R Ω                              ZC=ZD


It is possible.

An external power supply is not required.

Also Read- Direct deflection method for measurement of high resistance

Sachidananda Dhar

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