Measurement of Resistance | Different Methods to Measure Resistance | Measurement of Resistance by MEGGAR

Measurement of Resistance | Different Methods to Measure Resistance | Measurement of Resistance by MEGGAR

Measurement of resistance by Different ways which all the methods are described in this article.

Methods Of Measuring A Resistance

The resistance may be measured by the following methods which all the methods are mentioned below:

(1) Ammeter voltmeter method

This method is mainly applicable to medium resistances and the accuracy is moderate or around 1%.

(2) Ohmmeter

This is an instrument used for directly indicating the value of the unknown
resistance connected across its terminals.

It provides a very quick but moderately accurate means of resistance measurement.

(3) Meggar

This is an instrument for measuring very high resistances such as insulation resistance of the cable.

(4) Wheatstone Bridge

This is the best and the commonest method of measuring medium resistances and measures resistance with a good degree of accuracy.

(5) Kelvin's Double Bridge

This is one of the best method for measurement of low resistances.

(6) Loss of charge method.

This method is particularly suitable for measuring the insulation resistance of a cable because the cable itself may be used as a capacitor provide its capacitance is known.

(7) Murray loop test

This is one of the methods available for location of a ground or a short circuit fault on a cable.

(8) Varley loop test

This is another method for location of faults on cable and is a modification of the Murray loop test.

MEGGAR

Resistance Measurement using MEGGAR

Resistance Measurement using Meggar

Meggars or megohmmeters are instruments which measure the insulation resistance of
electric circuits relative to earth and one another.

A meggar consists of an e.m.f.source and a voltmeter.

The scale of the voltmeter is calibrated
in ohm such that  kilo ohms or megohms, as the case may be.

In measurements the e.m.f.of the selfcontained source must be equal to that of the source
used in calibration.

Below Figure shows diagram of a meggar whose readings are independent of the speed of the self-contained generator.

Circuit Diagram of Meggar

The moving system in-corporates two coils 

1.current coil and 

2.pressure coil 

Which is mounted on the same shaft and placed in the field of a permanent magnet 90° apart.

The generator energises the two coils over
separate wires.

Connected in series with one coil is a fixed resistance R1or several different resistances in order to extend the range of the instrument.

The unknown resistance R is connected in
series with the other coil.

The currents in the coils interact with the magnetic field and produce opposing torques.

The deflection of the moving system depends on the ratio of the currents in the coils and is
independent of the applied voltage.

The unknown resistance is read directly from the scale of the instrument.

The accuracy of measurement is unaffected by variations in the speed of the generator
between 60 and 180 r.p.m.