Electronic Electrical Devices | Electronic Electrical Components
The megger is a portable instrument used to measure insulation resistance. A simple megger consists
of a hand-driven DC generator, or a voltage supply and a direct reading ohm meter. A simplified circuit diagram of
the instrument is shown below.
The moving element of the ohm meter consists of two coils, A and B, which are rigidly mounted
to a pivoted central shaft and are free to rotate over a C-shaped core (C on the illustration below). These
coils are connected by means of flexible leads. The moving element may point in any meter
position when the generator is not in operation.
As current provided by the hand-driven generator flows through Coil B, the coil will tend to set
itself at right angles to the field of the permanent magnet. With the test terminals open, giving
an infinite resistance, no current flows in Coil A. Thereby, Coil B will govern the motion of the
rotating element, causing it to move to the extreme counter-clockwise position, which is marked
as infinite resistance.
Coil A is wound in a manner to produce a clockwise torque on the moving element. With the
terminals marked "line" and "earth" shorted, giving a zero resistance, the current flow through
the Coil A is sufficient to produce enough torque to overcome the torque of Coil B. The pointer
then moves to the extreme clockwise position, which is marked as zero resistance. Resistance
(R1) will protect Coil A from excessive current flow in this condition.
When an unknown resistance is connected across the test terminals, line and earth, the opposing
torques of Coils A and B balance each other so that the instrument pointer comes to rest at some
point on the scale. The scale is calibrated such that the pointer directly indicates the value of
resistance being measured.