Describe with neat circuit diagram the measurement procedure of earth resistance for an installation.

Describe with neat circuit diagram the measurement procedure of earth resistance for an installation. 
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1 Answer

Answer :

Following procedure (Method) for testing of earth pit resistance with necessary diagrams.

1) Earth Tester : i) Three point method ii) Four point method

2) By Potential drop method

3) Water tap method

1) earth resistance measurement for Earth Tester

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* The earth tester has two coils named current coil and pressure coil.

* The three GI rods or iron rods (electrode) are embedded in the ground. The distance between the electrode no.1 and no.2 is kept 100 ft (30m). The connections for this electrode are made as shown in figure.

* Initially electrode no.3 is kept or embedded in the ground near to electrode no.2.

* The earthing pit connection is done to the rod no.1. The procedure for earth resistance measurement test is as below.

* Make the connection as shown in figure.

* Rotate the handle of earth tester near to 100 to 120 RPM and measure the first reading of earth resistance.

* Remove the rod no.3 and place at the distance of 90 ft from the rod no.1 and embed in the ground. Rotate the handle of earth tester at 100 to 120 RPM and measure the earth resistance.

* The same procedure is repeated and rod no.3 is kept at 80 ft., 70 ft,70ft,50 ft, 40 ft,30 ft ,2 0 ft, 10 ft and 0 ft, and by rotating handle of earth tester separate readings are taken.

* The graph is plotted between the earth resistance value and the distance between rod no.1 and rod no.3.

* The earth resistance of the earth pit should be which is specified by Indian electricity rule


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2) By Potential Drop Method: 

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* The connection are as shown in figure.

* In potential drop method external DC source battery or handle driven generator is used as DC source.

* The current flowing through the rod no.1 and rod no.2 current electrodes is measured

* AT same time voltage across the rod No.1 and rod No.3 is measured by apply the ohms law. = V /I is calculated. The procedure is as below

* Make the connections as shown in figure for observation no.1. Keep the rod no.3 near to the rod no.2 (at 20m distance) from rod no.1 measure the voltage and current, calculate resistance. 

* The rod no.3 is kept at position no.2 (23 m from rod no.1) and measure the voltage and current and calculate resistance.

* Keep the distance between rod no.1 and rod no.3 (17 m in the ground). Measure the voltage and current calculate the resistance.

* For the 3 observation 3 resistances are calculated the mean resistance of that is declared as earth resistance of that earthing pit.

* Due to external DC source there are chances of electrical shock so that skilled labours can be this test


3) By Water tap Method: 

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Earth resistance measurement test by water tap method is shown in figure. As per this figure the water tap should be of GI pipe which is embedded in the ground. The rod no.1 is not essential. The procedure is as below. 

* Make the connections as shown in figure

* The common link of C1-P1 is connected to the earthing pit and common link of C2-P2 is connected to the water tap.

* The distance between the water tap to earthing pit should be near to 20m.

* By rotating handle of earth tester at near about 100 to 120 rpm measure the earth resistance on that earth tester.

* That resistance is declared as earth resistance of that earthing pit. In this test the accuracy is less but electrical rods are not required.  

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