Biddle MEGGER - Page 54

FIGURE 46. Ohmmeter system of the ‘‘Major’’ Megger Insulation Tester.
OS = ohmmeter scale A = deflecting coil
P = pointer C = fixed C-shaped iron core
R = control coil L = ligaments
Β΄ = astatic part of control coil M = permanent magnet
Variations in Generator Voltage
The calibration of the “Megger” ohmmeter is unaffected by
the applied voltage, within plus or minus 20% of normal—or
between 400 and 600 volts in a 500-volt instrument. This is
because the two ohmmeter coils 4 and B (Figure 45) receive
current from the same source. Any change in the applied volt-
age will affect both coils in the same proportion, and therefore
the pointer will take the same position for a given resistance
under test. Consequently the calibration is unaffected by the
exact speed at which a hand crank may be turned or by the
exact strength of the permanent magnets.
Guard System
Figure 45 shows how the Line terminal is guarded by a
metallic guard washer which prevents errors due to leakage
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