Measuring the resistance of the earth electrode.



It is very important to measure the resistance between the grounding rod and “true Earth”. Every ground rod must prove to have a low resistance to the earth. Different methods are used to test and confirm soil resistivity to ensure that it meets NEC 250.56 requirements.

The most common methods used in the industry are listed below.
1)       3-point or Fall of potential Method,
2)       4-point Method or Wenner method,
3)       Clamp-On Method.

3-Point Method – This is the most common method used when measuring the earth's resistance of a ground rod. It is used to effectively measure the ability of the ground rod to dissipate energy from a site. 
Three electrodes used in this test. the three electrodes are; The ground rod Under Test, The Voltage electrode (P) and the Current electrode(C)
The ground rod under test must first be disconnected from the earth bus, then the auxiliary test electrodes would now be driven into the ground in a straight line away from the rod under test. 
The spacing of the auxiliary electrodes from the rod under test would vary depending on the 3 point Method type you choose to use. It could be the 62% Method where the inner electrode (voltage electrode) is positioned at 62% of the total distance between the ground electrode and the outer electrode (current electrode) or it could just be the normal method where the electrodes are spaced equally. A spacing of 20 m is normally sufficient.


 After the arrangement of the electrodes and connecting them as shown in the image above, The ground resistance tester generates and injects a known current between the Ground rod under test and the current electrode(C2), then the ground tester now measures the voltage drop between the Ground rod under test and the Voltage electrode(P2). 
Using ohms law V=IR and R = V/I the ground resistance tester now calculates R as Voltage measured btw Voltage electrode and rod under test/Current injected btw rod under test and the current electrode








4-Point Method - This method is commonly used before the design of an earthing system as it gives a detailed knowledge of soil resistivity allowing for a more elaborate grounding system to be designed if the readings are poor.
In this method, four electrodes are used they must be driven into the earth in a straight line, at the same depth and equal distances apart.
Since the test is a measure of the soil resistivity, a 10m rod rests the resistivity of the soil at 10 m, a 6m rod test the the resistivity of the soil at 6m and so on and so forth so it is recommended that the ground rods be placed as deep as possible into the earth because moisture content, salt content, and temperature of the soil more stable at deeper strata.

Finally, the distance between the four test electrodes should be at least three times greater than the length of the test rods. So if the depth of each test rod is 10m make sure that the spacing distance between electrodes is at least 30m.

The four electrodes consist of two current electrodes (C1, C2) and two voltage electrodes(P1, P2) and the final arrangement should look like the image below


The ground resistance tester generates and injects a known current between the current electrode (C1) and current electrode (C2), then the ground tester now measures the resulting voltage between the Voltage electrode (P1) and the Voltage electrode (P2).
Using ohms law V=IR and R = V/I the ground resistance tester now calculates the Resistance value. This value is used to calculate the soil resistivity.
The resistivity of the soil (ρ) is calculated using the following formula



Where a = spacing of the electrodes or distance between the electrodes in cm
       R = resistance value reading measured by the ground tester

 The SI unit of resistivity is the Ohm-meter (Ω-m) and this value tells us how much the soil resists or conducts electric current. There is no fixed value for an acceptable soil resistivity value but when we get the value for our soil resistivity, This value is used for design and calculation of the earth electrode using a Nomograph. 

 The soil resistivity value would help us make decisions on the number of ground electrodes to driven into the earth, the diameter of ground electrode to be used and the depth of ground electrode needed to achieve specific resistance value example 10Ω  between the grounding rod and true earth.

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