Measurement of High Resistance - Use of Guard Circuits

There are several electrical equipments that possess high resistance. Some of the equipment like the insulation resistance of cables and machines, high resistance circuits like vacuum tubes, leakage resistance of the capacitor, the surface resistivity of the material, and insulation resistance of the transformer windings have resistance in the order of hundreds and thousands of megaohms.

The measurement of such high resistance using simple circuits is very difficult. Because of this resistance, the value of the current in the circuit will be very small. It is very difficult to sense such small currents in the circuit. Thus, due to this reason, several difficulties arise. They are as follows,

Difficulties in the Measurement of High Resistance :

  • In order to increase the current magnitude in the circuit, very high voltage is supplied to the instrument (galvanometer or micro-ammeter). But such high voltage is harmful to the instrument. Thus, proper precautions must be taken in order to avoid damage to the microammeter or to the galvanometer.
  • Whenever high resistance is to be measured, there is always a time delay between the application of voltage and the observation of the reading. This time delay takes place because the insulation resistance always requires a certain amount of time for charging and absorbing currents when the voltage is applied to it.
  • The electrostatic effect can also cause errors in the measurement i.e., the electrostatic effect gives rise to stray charges in the measuring circuit, which may cause errors in the measurement.
  • Due to the presence of leakage current in the circuit, errors are produced, whose magnitude is equal to the magnitude of measuring current.

Use of Guard Circuits :

In order to minimize or eliminate the errors, guard circuits are used. These guard circuits are mainly used to eliminate the errors, which are caused by leakage currents. The circuit for measurement of high resistance is as shown in the figure below.

Measurement of High Resistance

In the figure above, the leakage resistance of the resistance to be measured is in parallel. The microammeter carries the current IA = IL + IR i.e., it carries the current, which is equal to the sum of leakage current (IL) and the resistance current (IR). Hence the value of resistance will be equal to (IL + IR/V) instead of IR/V, due to which we get an error.

Due to the presence of leakage current (i.e., an additional current), the reading obtained by the measuring instrument is not true reading and hence the error in the measurement. Therefore, to obtain the true reading, a guard circuit is used. The guard circuit for measurement of high resistance is as shown in the below figure.

Measurement of High Resistance

In the above figure, it can be seen that, to the resistance terminal block, a guard terminal is attached, which encloses the complete resistance. Due to this guard terminal, the leakage current finds a separate path and hence the value of resistance under measurement will be equal to which is true value. Thus, by this guard terminal, the error due to leakage current is eliminated.

Wheatstone bridge can be used efficiently to measure the medium value of resistance. It cannot be used for very high values of resistance of the order of 1MΩ because of the reduction in the sensitivity to unbalance caused by the high resistance values.

Also, the high resistance measurements are inaccurate due to leakage over the insulation of the bridge arms. Hence, in order to overcome these problems, a guard circuit is used in the Wheatstone bridge to measure high resistance. The below shows the Wheatstone bridge with a guarded terminal.

Measurement of High Resistance

Methods for Measurement of High Resistance :

Also, there are various methods used for measuring high resistance. Some of them are,

  • Loss of Charge Method
  • Direct Deflection Method
  • Megohm bridge Method
  • Megger

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