Earth Resistance Value - Parallel Connection

Introduction:

In the previous post, we discussed calculating the earth resistance value for multiple earth electrodes in parallel connection. In simple words, the resistance associated with earthing is electrolytic in nature and the formula used for calculating the combined resistance values of resistors in electronics is applicable to earthing.

In addition to that we discussed about the calculation of earth resistance value for the different earth electrode arrangements as per IS3043:2018. From those formulae, we can conclude that the combined earth resistance value mainly depends on the spacing between the earth electrodes.

The links for the previous articles are as follows for further references.

Earth Resistance Value:

https://www.dhirubhai.net/posts/er-saravana-krishnan-dharmaraju-5901a475_electrical-earth-resistance-activity-7224024423576522752-h3RB?utm_source=share&utm_medium=member_desktop

Parallel Connection of Earth Electrode:

https://www.dhirubhai.net/pulse/earth-resistance-value-parallel-connection-dharmaraju-sumbc/?trackingId=udXYNbq3SXaLYZJoxGUJXg%3D%3D

In this article, we shall discuss about the impact of spacing between the earth electrodes in the combined earth resistance value.

Parallel Connection of Earth Electrodes:

The formula provided in clause 14.2.11.5 of IS3043:2018 shall be followed for calculating the combined earth resistance value of vertical electrodes in a hollow square configuration.

In this arrangement, the electrodes shall be installed along the four sides of a square. Hence, the total number of earth electrodes installed will always be a multiple of four.

The formula provided in clause 9.5.4 of BS 7430:2011 shall be used to calculate the combined earth resistance value of the electrodes installed in a straight line.

In this arrangement, the electrodes shall be installed in a straight line at equal intervals as shown below.

If we observe in both formulae, we can understand the importance of spacing between the electrodes.

For example, let us consider 5 electrodes of the same dimensions are installed in a straight line at three different spacing - 1m, 3m, and 6m among them. Let us analyse the impact of spacing on the combined earth resistance value.

Soil resistivity : 500 ?.m

Diameter of the earth electrode : 76mm or 0.076m

Length of the earth electrode : 3000mm or 3m

Number of earth electrodes in parallel : 5

1m Spacing:

3m Spacing:

6m Spacing:-

The summary of the calculation is as follows.

1) 1m spacing -> 44.13 ?

2) 3m spacing -> 31.54 ?

3) 6m spacing -> 28.39 ?

In all three cases, the dimensions of the earth electrodes are the same and the number of earth electrodes are also the same. However, the combined earth resistance values are different mainly because of the spacing between the earth electrodes. For 1m spacing, the combined resistance value is 44 ?, and for 6m spacing the resistance value is 28 ?. The difference in the resistance value is around 16 ?. This difference clearly explains the impact of spacing between the earth electrodes in the combined earth resistance value.

Comparison of actual method and wrong practice:

Wrong Practice:

The individual earth resistance value 76mm dia and 3m long earth electrode shall be calculated as follows.

The individual resistance value is around 116 ?. If we divide it by the number of earth electrodes, then the combined resistance value would be 23.2 ?.

If we compare this value combined resistance value (23.2 ?) with the combined resistance value for 1m spacing (44.13 ?), there will be a very huge difference. Even if we provide 6m spacing between the electrodes, the combined resistance value was found to be greater than the value obtained by this practice.

Hence we should not divide the individual earth resistance value by the number of earth electrodes to arrive at the combined earth resistance value.

Spacing between the earth electrodes:

IS 3043:2018 under clause 14.2.2 states that "In practice, this is satisfied by a mutual separation equal to the driven depth. Little is to be gained by separation beyond twice the driven depth."

Hence, it is recommended to maintain the spacing between the earth electrodes equal to the driven length of the electrodes. The impact will be lesser if we increase the spacing beyond twice the length of the electrodes.

From the above graph, we can observe that the combined resistance value of multiple electrodes in parallel connection mainly depends on the spacing between the electrodes and if the spacing increases beyond twice the length(6m) of electrodes the impact will be much lesser.

For a better understanding, you can refer to the following video that explains the impact of spacing between the earth electrodes.

https://youtu.be/TtYeW1PsXOE?si=GFOJ2M21FgX4qgUZ

Conclusion:

The resistance associated with earthing is electrolytic in nature and the combined earth resistance value mainly depends on the spacing between them. Even if the earth electrode dimensions and the number of electrodes are the same if we change the spacing between the electrodes the combined earth resistance value will change.

For more details related to earthing, contact CIKIT Electricals & Technologies Kamlesh Sundar Vidhya Selvaraj muthupriya cikit

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