Megger

Resistances of the order of 0.1 MQ and upwards are classified as high
resistances. These high resistances are measured by portable, instrument known as megger.
It is also used for testing the insulation resistance of cables.

It is based on the principle of electromagnetic induction. The Fig ` shows the
construction of megger.

Fig. 4.23 Megger

When a current carrying conductor is placed in a uniform magnetic field it experiences
a mechanical force whose magnitude depends upon the strength of current and magnetic
field. While its direction depends on the direction of current and magnetic field.

Construction

It consists of a permanent magnet which provides the field for both the generator G
and ohmmeter. The moving element of the ohmmeter consist of three coil viz. current or
deflection coil, pressure or control coil and compensating coil. These coils are mounted on
a central shaft which is free to rotate over a stationary C-shaped iron core.

The coils are connected to the circuit through flexible leads called ligaments which do
not produce a restoring torque on the moving element, consequently the moving element
takes up any position over the scale when the generator handle is stationary.

Tie current coil is connected in series with resistance R, between one generator
terminal and the test terminal 7. The series resistance R; protects the current coil in the
event of the test terminals getting short circuited and also controls the range of the
instrument. The pressure coil, in series with a compensating coil and protection resistance
R, is connected across the generator terminals. The compensating coil is included in the
circuit to ensure better scale proportions. The scale is calibrated reversely means the
normal position of pointer indicates infinity while full scale deflection indicates zero
resistance.