Specifications

92
aluminum ‘C’ former of peculiar shape. The pressure coil is connected across the sourced
through resistor of 0.1 MW as shown in Fig. 7.6.
Fig. 7.6 - Inside a Megger
When the generator handle is rotated at a speed of 160 rpm, current flows through a pressure
coil (Electronic megger develops 1000 V with dry cell batteries without generator and handle)
and sets up magnetic field which tends to be plane of that of main magnetic field due to which
the pointer attached on the axis of moving system deflects on the ‘infinity’ (the high megohms).
The current coil completes its circuit, through series resistance and external resistance under
test and through the shunt which divides the current coil circuit to increase deflection for small
insulation resistance. A guard system is introduced to prevent surface leakage from high potential
lead within instrument and at the specimen. If there is current in external circuit due to imperfect
insulation resistance of the specimen, a current will flow through the current coil in such a
direction that it turns the moving system towards the right of zero scale. Thus the deflection of
pointer is dependent on external circuit resistance, and independent of magneto generator voltage,
as same voltage is fed both to the controlling pressure coil and the current cool. In certain
design of megger, the magneto generator is driven through a clutch arrangement controlled by
a centrifugal governor, so that voltage can not rise above a definite maximum, when the crank is
turned fast enough, a constant testing voltage is obtained.
Sample Insulation testing:
Fig. 7.9 - Insulation test of floors and
walls for non-conducting location
Fig. 7.7 - Insulation test to earth
Fig. 7.8 - Insulation tests between phases