Measurement of Earth Resistance
The potential fall method is used for determining resistance between the earthing plate and the surrounding ground in the distribution system. 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.
E = earth electrode under test, and
A = an auxiliary earth electrode positioned so that two resistance areas do not overlap.
B =second auxiliary electrode placed halfway between E and A.
An alternating current of steady value is passed through the earth plate from E to A and the voltage drop between E and B is measured.
The earth’s resistance,
To ensure that the resistance areas do not overlap, the auxiliary electrode B is moved to positions B1 and B2 respectively. If the resistance values determined are approximately the same value in all three cases, the mean of the three readings can be taken as the earth resistance of the earth electrode. Otherwise, the auxiliary earth electrode A must be driven in at a point further away from E and the above test is repeated until a group of three readings obtained are in good agreement and the use of the alternating current source is necessary to eliminate electrolytic effect.
The test can be performed, with current at power frequency from a double wound Transformer, by means of a voltmeter and an ammeter or by means of an earth tester.
The earth tester is a special type of megger that sends ac through the earth and dc through the measuring instruments. It has got four terminals P1, C1, P2, and C2 outside. Two terminals P1 and C1 are shorted to form a common point which is connected to the earth electrode under test. The other two terminals C2 and P2 are connected to the auxiliary electrodes A and B respectively. The value of the earth resistance is indicated by the instrument directly when its handle is turned at a uniform speed.