Methods of measuring Earth Resistance

Methods Of measuring Earth Resistance:

To measure electrolytic resistance IS 3043:2018 suggests following methods
1. Fall of potential method (or) 3 pin method
2. Fall of potential 61.8 percent test
3. Fall of potential – Slope test
4. 90°/180° Test

Fall of potential 61.8 percent test:

Measurement of Earth Electrode Resistance

Three electrodes potentiostat method is 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 current electrode and potential across the other two electrodes is measured. Based on the values of current injected and potential measured, the resistance value can be calculated by

ohm’s law R = V / I

R – Resistance of test electrode in Ohms
V – Reading of the voltmeter in Volts
I – Reading of the ammeter in Ampere

IS 3043 suggests fall of potential 61.8 percent test to measure the earth electrode resistance for the small electrode system like LV installation and 11 kV ground mounted distribution substation and pole mounted transformer.

Ground mounted distribution substation:

Ground mounted distribution substation

The ground mounted distribution substation comprises of pin type insulators, Air Break switch, lightning arrestor and distribution transformer. All those equipments and their metal structures should be earthed properly in order to ensure safety of human beings and equipment.

Measurement of Earth Electrode Resistance using 61.8 percent test:

Here also three electrode potentiostat test procedure will be used but the positioning distance of probes should be changed. The current probe should be connected in straight line in order to achieve reliable readings.

Measurement of Earth Electrode Resistance using 61.8 percent test:

IS 3043 suggests that for small earth electrode system the distance between earth grid and current probe should be 50 m and the voltage electrode shall be placed at 61.8% of 50m which is 31m. The measurement arrangement is shown above.

The distances should be increased when the earth electrode system having medium size of electrode. In order to confirm the accuracy, the voltage probe shall be moved to 50% percent and 70% of current probe distance and the corresponding readings are also tabulated.

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.

Fall of Potential – Slope test:

IS 3043:2018 suggests Fall of Potential - Slope test to measure the earth electrode resistance for the large earth grid system where the electrical centre of the earth system is unknown. The electric field in the soil at substation and transmission line tower site conditions will be very high. Hence, placing the spike outside the influence of such fields might not be possible at some conditions and hence IS3043 suggests this method for such site conditions.

Earthing in substation:

Substations are one of the major parts of electrical network. Any abnormality in substation will lead to isolation of all the areas which are under the control of that substation. IS3043:2018 has explained about the earthing for such substations under Clause 25.

In general, earthing installations will be required at power stations and substations for:

1) The neutral points of each separate electricity system which has to be earthed at the
power station or substation.
2) Apparatus framework or cladding or other non-current carrying metalwork associated
with each system, for example, transformer tanks, power cable sheaths.
3) Extraneous metalwork not associated with the power systems, for example boundary
fences, sheaths of control or communication cables.
The earth mat serves as a common ground to all the metallic structures and devices located
in that substation. Since it forms the large earthing grid its resistance value will be
measured by slope test.

Earthing in transmission tower:

The transmission tower comprises of insulator, the transmission lines and other components of tower. All those equipment and metal structures should be properly earthed by methods like using earthing electrodes of rod / pipe, burying the conductor, using counterpoise wires.

Since the tower is immersed on the soil we need to take tower foot resistance along with earth electrode resistance in order to avoid touch and step potential. We can use slope method for measuring the earth electrode resistance value

Procedure to measure:

Earth electrode resistance tester should be connected to the earth electrode rod under test. Let the current probe be placed at a distance D from the earth electrode under test and potential probe shall be placed at 0.2, 0.4, 0.6 times of distance D of current probe.

Measured resistance will be noted as R1, R2, R3. By using this formula we can find
rate of change of slope (µ).

90°/180° Test:

IS 3043 recommends this test to check the true resistance value obtained from slope test and 61.8% test.

The current probe C2 should be place at a specific distance (D) from earthing system and voltage probe P2 should be placed at an angle between 90° and 180° to the current probe. Different values shall be recorded by varying the distance of potential probe.

For example, let the current probe is placed at a distance ‘D’ then the voltage probe shall be placed at a distances of 0.2, 0.4, 0.6 times the distance of current probe. The number of readings must be taken and should be plotted on a curve

This method shall be used to verify the values obtained from the previously explained methods.





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