Page 144 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Necessity of measuring of earth electrode resistance: The only way to ensure the acceptable value of earth
electrode resistance is to measure the resistance with the use of an earth resistance tester.
Earth resistance tester: It is an electrical measuring instrument used to measure the resistance between any
two points of the earth. It is also called as earth tester.
Principle: The earth tester works on the principle of the fall of potential method.
In this method the two auxiliary electrodes B and C are placed at a straight line (Fig 1).

An alternating current of Iamps magnitude is passed through the electrode A to the electrode C via the earth and
the potential across electrodes A and B is measured.
The resistance of electrodes B and C does not influence the measurement result.
This is achieved by placing the electrode C at a sufficient distance from A so that the resistance areas of A and
C are quite independent. A distance of above 15 metres between electrode A and C is regarded as sufficient
distance.

Construction and working of earth tester : The earth tester essentially consists of a hand drive generator which
supplies the testing current and a direct reading ohmmeter (Fig 2).
The ohmmeter section of this instrument consists of two coils (potential and current coils) kept at 90o to each
other and mounted on the same spindle. The pointer is attached to the spindle. The current coil carries a current
proportional to the current in the test circuit whereas the potential coil carries a current proportional to the potential
across the resistance under test.

Fig 1 & 2

Thus the current coil of the instrument acts as an ammeter in the fall of potential method and the pressure coil
acts as the voltmeter. Since the deflection of the ohmmeter needle is proportional to the ratio of the current in the
two coils, the meter gives resistance readings directly.
When DC is used in electrode resistance measurement the effect of electrolytic emf interferes with the measurement
and the reading may go wrong. To avoid this, the supply to the electrodes should be AC.

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CITS : Power - Electrician & Wireman - Lesson 20-25 CITS : Power - Electrician & Wireman - Lesson 20-25