Measurement of High Resistance - Engineering notes

High Resistance:

• Resistances of 100 kilo ohms and above are usually termed as high resistances.
• Measurement of high resistances are required for determination of insulation resistance of components and built up electrical equipment of all types volume, resistivity of a material, surface resistivity and resistance of high resistance circuit elements.

Measurement of High Resistance:

1. Three wires are required to represent a high resistance. 
2. The third, guard terminal, G is utilized to reduce the errors because of leakage currents caused by insulation.
3. The methods are used for the measurement of high resistance are

• Loss of discharge method
• Direct deflection method
• Mega ohm bridge resistance method
• Megger

 

1. Loss of charge method:

 

Schematic diagram of  Loss of charge method 

2. Direct deflection method:

Schematic diagram of Direct deflection method

This method is suitable for the measurement of volume resistivity, surface resistivity of any insulating material available in sheet form. A guard terminal surrounding resistance terminal is connected to the battery side as a micro-ammeter. The guard terminal and the resistance  terminal are almost at the same potential and hence there is no flow of current between them. The leakage current IL which would otherwise flow through the micro-ammeter , bypasses the micro-ammeter. The micro-ammeter indicates the current IR only. The resistance value is determined by the readings of voltmeter and ammeter as, 

R = V / IR ohm

3. Mega ohm bridge resistance method:

Schematic diagram of Mega ohm bridge resistance method

By connecting the guard terminal to one of the terminals of galvanometer. We can make the insulation resistance not effects the value of high resistance to be measured.

AG, BG are the insulation resistances of the order of 1Mohm.

4. Megger Method:

This the most commonly used method for measurement of high resistance. The essential parts of the megger are as shown in the fig below. It consists of a hand driven d.c generator and a direct reading ohmmeter. Permanent magnets provided field for both. The moving element consists of three coils, one current coil, potential or pressure coil and compensating coil. The coils are mounted over a rigid shaft and are free to rotate over a C-shaped stationary iron core. The coils are connected to the circuit by means of flexible leads or ligaments. E and L, are test terminals. The current coil is connected in series with the terminal L. The series resistance R' protects the current coil in case the test terminals are short-circuited and controls range of the instrument. The pressure coil, in series with a compensating coil and  protection resistance R is connected across the generator terminals. Compensating coil is provided for better scale proportions and to make the instrument astatic.

 

Schematic diagram of Megger method

When the current from the generator flows through pressure coil, the coil tends to align itself at right angles to the permanent magnet field. When test terminals are open i.e, at infinite resistance no current flows through the current coil and pressure coil governs the motion of the moving element causing it to move its extreme counter-clockwise position, the point under this condition is marked infinite resistance. Current coil produces clockwise torque on moving element. When the terminals L and E are short circuited, the current flowing through current coil is large enough to move the pointer to its extreme clockwise position, marked zero. For any resistance connected between L and E, the opposing torque of the coils balance each other so that pointer comes to rest at some intermediate point on the scale.