Working Principle of Earth Resistance Tester

Working Principle of Earth Resistance Tester Points : Working Principle of Earth Resistance Tester To determine the resistance between the earth plate and the surrounding ground in the distribution systems is of utmost importance. The principle of this measurement is fall of potential method. The resistance area of an earth electrode is the area of soil around the electrode within which a voltage gradient measurable with commercial instruments exists. Shows the arrangement for the measurement. “E” is the earth electrode under test, and “A” is an auxiliary earth electrode positioned so that two resistance areas do not overlap. “B” is a second auxiliary electrode placed halfway between ‘ii” and “A”.

Then earth resistance, R_e = Voltage drop between E and B/current through earth path = V/I

To ensure that the resistance area do not overlap, the auxiliary electrode “B” is moved to position “B₁” and “B₂” respectively. If the resistance value determined are of approximately the same value in all the three cases, the mean of three readings can be taken as the earth resistance of the electrode. Otherwise the auxiliary earth electrode “A” must be driven in at a point further away from “E” and the above test repeated until a group of three readings obtained, are in good agreemnt.

To eliminate electrolytic effect, it is necessary to use alternating current source.

This test can be performed, with current at power frequency from a double wound transformer, by means of a voltmeter and an ammeter, the arrangement shown in fig or by means of an earth tester.