Resistances of the order of 0.1 M ohm and upwards are classified as high resistances.
These high resistances are measured by a portable, instrument known as megger. It is also used for testing the insulation resistance of cables.
Principle of Operation:
It is based on the principle of electromagnetic induction. Fig. shows the construction of megger.
Construction:
It consists of a permanent magnet that provides the field for both the generator G and ohmmeter. The moving element of the ohmmeter consists of three coils viz. current or deflection coil, pressure or control coil, and compensating coil.
These coils are mounted on a central shaft which is free to rotate over a stationary C-shaped iron core.
The coils are connected to the circuit through flexible leads called ligaments which do not produce a restoring torque on the moving element, consequently, the moving element takes up any position over the scale when the generator handle is stationary.
The current coil is connected in series with resistance R1 between one generator terminal and the test terminal T2.
The series resistance R1 protects the current coil in the event of the test terminals getting short-circuited and also controls the range of the instrument.
The pressure coil, in series with a compensating coil and protection resistance R2, is connected across the generator terminals.
The compensating coil is included in the circuit to ensure better scale proportions. The scale is calibrated reversely means the normal position of the pointer indicates infinity while full-scale deflection indicates zero resistance.
Working:
When the current flows from the generator, through the pressure coil, the coil tends to set itself at right angles to the field of the permanent magnet.
When the test terminals are open, corresponding to infinite resistance, no current flows through deflection coil.
Thus the pressure coil governs the motion of the moving element making move to its extreme anticlockwise position.
The pointer comes to rest at the infinity end of the scale.
When the test terminals are short-circuited i.e. corresponding to zero resistance, the current from the generator flowing through the current coil is large enough to produce sufficient torque to overcome the counter-clockwise torque of the pressure coil.
Due to this, the pointer moves over a scale showing zero resistance.
When the high resistance to be tested is connected between terminals T1 and T2 the opposing torques of the coils balance each other so that pointer attains a stationary position at some intermediate point on the scale.
The scale is calibrated in megaohms so that the resistance is directly indicated by the pointer.
The guard ring is provided to eliminate the error due to leakage current. The supply to the meter is usually given by a hand-driven permanent magnet d.c. generator sometimes motor-driven generator may also be used.
Applications:
The megger can be used to determine whether there is sufficiently high resistance between the conducting part of a circuit and the ground. This resistance is called insulation resistance.
The megger can also be used to test continuity between any two points. When connected to the two points, if the pointer shows full deflection then there is electrical continuity between them.