Megger

MEGGER

The Megger is an instrument used for measurement of high resistance and insulation resistance. It consists of a hand driver D.C. generator and direct reading ohmmeter. The simplified diagram of electrical connections of the instrument is shown in figure 6.18.

Construction

• The moving system consists of three coils known as current coil (deflection coil), pressure coil (control coil) and (compensating coil).

• These coils are mounted rigidly and pivoted to control shaft and they are free to rotate.

• The current coil is connected through Resistance 'R' to protect the coil in case of short circuiting.

• The compensated coil is provided to give better scale proportions.

• Help Compensatory coil is generally accommodated in the current coil itself.

• Hence these two main coils, current coil and pressure coil are connected in parallel across the small generator mounted in the instrument.

• Both these coils are in the field of special permanent magnet provided in the instrument itself.

• The guard ring is provided to shunt leakage current over the test terminals without passing through the current coil of the instrument.

Working Principle of Megger

• The handle of the megger is rotated after connecting the terminals across the resistance to be measured.

• Voltage is generated. The generator housed in the megger develops voltage of 250V, 1000V and 2500V depending upon the megger.

• The control coil produces a constant torque since it is series with a constant resistance. The current coil produces a torque proportional to the resistance to be measured.

• The polarities of the coil connections are such that the torque produced by them are in opposition.

• Then the external resistance is infinitely high, no current flows in the current coil.

• At that time the moving system aligns such that the pressure coil is perpendicular to the magnetic axis.

• The pressure coil exerts an resorting torque to balance when both the torque are equal.

• The position of the pointer in moving system is marked to indicate the resistance in megaohms.

Application of Megger

• Megger is used to measure in insulation resistance.

• Megger is modified and constructed as bridge megger to test the insulation of any equipment incorporating with the device wheatstone bridge facilities for measurement of resistance of wide range.

• It can be used to find out cable faults.

• With modifications a Special Instrument "Ductor ohm meter" can be used for measurement of low resistance of a few ohm down to one microhm.