Meggers (or megohmmeters) are instruments which measure the insulation resistance of electric circuits relative to earth and one another.
A megger consists of an e.m.f. source and a voltmeter. The scale of the voltmeter is calibrated in ohms (kilo-ohms or megohms, as the case may be). In measurements the e.m.f. of the self-contained source must be equal to ·that of the source used in calibration.
Fig. 35 shows diagrammatically a megger whose readings are independent of the speed of theself-contained generator. The moving system incorporates two coils 1 (current coil) and 2 (pressure coil) mounted on the same shaft and placed in the field of a permanent magnet (not shown) 90° apart, The generator energizes the two coils over separate wires. Connected in series with one coil is a fixed resistance R1 (or several different resistances in order to extend the range of the instrument). The unknown resistance Rx is connected in series with the other coil. The currents in the coils interact with the magnetic field and produce opposing torques.
C = Crank
1,2 = Coils
P = Pointer
2 = Pressure/Voltage Coil
Rx = Unknown resistance
R1 = Fixed resistance
R2 = Safety resistance
Fig. 35. Circuit diagram of megger.
The deflection of the moving system depends on the ratio of the currents in the coils and is independent of the applied voltage. The unknown resistance is read directly from the scale of the instrument. (The accuracy of measurement is unaffected by variations in the speed of the generate between 60 and 180 r.p.m.).