Earth Resistance Value - Measurement Practices
Er. Saravana Krishnan Dharmaraju
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Introduction:
In the previous post, we discussed the impact of spacing between the electrodes in the combined earth resistance value for multiple earth electrodes in parallel connection. In simple words, if the earth electrode dimensions and the number of electrodes in parallel connection are the same if we change the spacing between the electrodes the combined earth resistance value will change.
Based on the formulae provided in IS3043:2018 and BS7430:2011, we can conclude that the combined earth resistance value depends on the spacing between the earth electrodes.
The links for the previous articles are as follows for further reference.
In this article, we shall discuss the procedure for measuring the earth resistance value as per IS3043:2018.
Measurement of Earth Resistance Value:
As we know, the resistance associated with earthing is electrolytic in nature and we can't measure the earth resistance value with a multimeter. We have already discussed the impact of soil resistivity on earth resistance value in our previous article. Since the surrounding soil plays an important, in measuring the resistance value also we need to inject the current through the soil.
The three electrode potentiostat method shall be used to determine the electrolytic resistance of a system. In this method, there will be three electrodes.
A - Test electrode; B - Potential electrode; C - Current electrode.
Current will be injected from the current electrode and potential across the other two electrodes is measured. The resistance value shall be calculated with the ratio of potential measured to the current injected. Since the resistance value depends on the potential drop between the potential and test electrodes, this method is generally termed Fall of Potential method.
To measure electrolytic resistance, IS 3043:2018 suggests the following methods under Fall of Potential method.
1.?Fall of potential - 61.8 percent test
2.?Fall of potential - 61.8 percent test using clamp
3.?Fall of potential - Slope test???
4.?Fall of potential - 90°/180° Test
Among these methods, the "Fall of potential - 61.8 percent test" is commonly used for measuring the earth resistance of the electrodes.
Fall of Potential - 61.8 Percent Test:
In this method, the potential spike will be placed at a distance of 61.8% of the distance of the current spike from the test electrode.
For example, If the current spike is placed at a distance of 50m from the test electrode, the potential electrode has to be placed at 31m (61.8% of 50m). From the above image, we can observe that the current injected will be flowing in the soil for almost 49.9m and it will be flowing in the compound and rod for only 100mm (0.1m).
If the earth electrode resistance of an individual earth electrode needs to be measured without disconnecting it from the earthing network, instruments with CT clamp tester shall be used. The CT will detect the current returning through that particular earth electrode.
Conclusion:
Since the resistance associated with earthing is electrolytic in nature, the resistance value shall be measured by injecting the current through the soil. Fall of Potential - 61.8 Percent Test shall be used to measure the earth resistance value.
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