EARTH ELECTRODE RESISTANCE MEASUREMENT TESTING METHODS

Objective:

After Installation, checks must be made on an earthing system to verify if there is any significant change in the resistance over a period of time or under different soil moisture conditions. Such checks will indicate if the earth electrode resistance to earth has been exceeded by changing soil conditions or aging of the system.

General Test Procedure:

It is preferable that the earth electrode to be tested is first isolated from the circuit it is protecting, so that only the earth is measured and not the complete system. When this is done, the circuits and equipment must be de-energized. If however this is not possible, the earth electrode should be duplicated so that when it is disconnected for test purposes, the other one provides the necessary circuit protection. The safety precautions essential when working near high tension systems where any unintentional Live earths may be encountered between the site earth and remote earths established for test purposes. A Live earth is one that carries current from the mains supply or could do so under fault conditions.

1.    Connect the Wires in the Potential, Current spikes after firmly sitting them in the ground at the distance of 5 & 10Mtrs from the earth electrode and connect it to the Earth Megger.

2.    Connect the Earth electrode under test to the Earth megger.

3.    Start the testing by pressing the earth megger on (3 terminal method).

4.    Check for any adverse conditions are indicated, the cause should be rectified before the test continues. If the conditions for the test are satisfactory, the display will stabilize, and the reading given may be accepted.

5.    The earth value should be less than 2 Ohms for package s/s, 11KV Ring Main s/s and Transformers.  

6.    If the High resistance indicated, the cause should be rectified. This may be caused by an open circuit, poor test lead connections, or excessive earth resistance of the soil near the electrode under test. Moistening the soil around the Potential spike, re-siting the spike in a new position or using more than one spike may solve the problem.


Measurement of earth resistance by 3 pole method









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