Measurement Of High Resistance

Measurement of high resistance can be done by the methods such as Direct defelection method, Loss of charge method, Megaohm bridge, and megger method.

Contents

Measurement of high resistance

The methods that can be adopted for measuring resistances of the order of 0.1 MΩ and higher are:

Direct deflection method
Loss of charge method
Megohm bridge
Megger method

These are discussed as follows.

Direct Deflection Method

Figure (a) shows an arrangement for measurement of high resistance in cables having metallic sheath by direct deflection method.

The leakage current I_L is carried by guard wire wound on the insulation and therefore does not flow through the galvanometer as shown.

Figure (b) shows the arrangement for measurement of high resistance in cables without metal sheaths.

The ends of the cable are immersed in water in a tank. The water and the tank then form the return path of the current.

The insulation resistance of the cable is, R = V/R

Loss of Charge Method

Figure shows the circuit for loss of charge method of measuring high resistances.

In this method, the unknown resistance is connected in parallel with capacitor and electrostatic voltmeter.

The capacitor is charged initially to a voltage V and then allowed to discharge through the resistance R. The voltmeter reading is v.

The terminal voltage during discharge is given by,

The insulation resistance is thus,

Megohm Bridge

Figure shows the circuit for the Megohm bridge.

The circuit is completely self-contained and includes inbuilt power supplies, amplifiers, bridge members, and an indicating instrument. It has range from 0.1 MΩ to 10⁶ MΩ.

The accuracy is usually within 3% to possible 10% above 10000 MΩ. Sensitivity of balancing at a high resistance is obtained by usage of adjustable high voltage supplies of 500 V to 1000 V.

The use of a sensitive null indicating arrangement such as a high gain amplifier with an electronic voltmeter or a cathode ray oscilloscope can also be used for the purpose.

Megger Method

Figure shows Megger arrangement for measuring high resistances. The current coil is the same as that in PMMC instrument.

V₁ and V₂ are the two voltage coils and V₁ encloses the annular magnetic core.

The voltage coil V₁ is in weak magnetic field when the pointer is at infinity and so this coil exerts lesser torque.

The torque exerted by V₁ increases as it moves into a stronger field.

This torque becomes maximum when it is under the pole face of the magnet and under this condition the pointer will be at zero of the resistance scale.

In order to modify the torque in the voltage circuit, another coil V₂ is placed in the circuit.

This coil is located in such a way that it can move from infinity to zero. Coils V₁ and V₂ combined function like a spring of variable stiffness, such that.

It is very stiff near zero when the current in the current coil is very small due to the presence of unknown resistance R_x which is very large.

As a consequence, the low resistance portion of the scale is compressed and high resistance part of the scale opens up.

The voltage range for measurement using a Megger circuit can be controlled by varying the series resistance R (to R′or R′′) connected with the current coil.

The test voltages can be varied as 500, 1000 or 2500 V and can be supplied using generator G.