Resistance Contribution v's DistanceAround an earth electrode the resistance of the soil is the sum of series resistances of virtual shells of earth propagating outward from the electrode. Resistance and Distance from an Electrode If the resistivity of the ground is r, the electric field is:īy integrating the electric field from the surface of the electrode to infinity, the voltage of the electrode is:Īpparent resistance R g, can now be found from: If the lower layer contains hard and rocky soil, then their resistivity increases with depth.Method - assume a current I, is injected into an electrode then at some point distant from the electrode, the current density is: The lower layer of soil has more moisture and lower resistivity. The resistance of the earth varies from layer to layer. The resistivity of the earth is also affected by the composition of some soluble salts as shown in the figure below. When the temperature is greater than 0✬, then its effect on ground resistivity is negligible, But at 0✬ the water in the soil starts freezing which increase their resistivity. The resistivity of the earth depends on its temperature. The resistivity of the soil may vary from 1 to 1000 ohm-m depends on the nature of the soil. The flow of current into the grounds depends on the resistivity of the soil in which the earth electrode is placed. The fault current which is injected from the earth electrode is passing away from the electrode in all directions shown below in the figure. The region around the earth in which the electrode is driven is known as the resistance area or potential area of the ground. The value of the earth resistance for different power stations is shown below Where V is a measured voltage between the voltage spike and I is the injected current during the earth resistance measurement through the electrode. Mathematically, the earth resistance is given as the ratio of the voltage and the current shown below. The resistance of the earthing electrode is not concentrated at one point, but it is distributed over the soil around the electrode.
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