Getting Down to Earth 23the electrode will be reached where inclusion of additional earth shells does not add significantly to the resistance of the earth surrounding the electrode. It is this critical volume of soil that determines the effectiveness of the ground electrode and which therefore must be effectively measured in order to make this determination. Ground testing is distinct when compared to more familiar forms of electrical measurement, in that it is a volumetric measurement and cannot be treated as a “point” property.Generally, the resistance of the surrounding earth will be the largest of the three components making up the resistance of a ground connection. The several factors that can affect this value are discussed in Section II on Earth Resistivity. From Section II, you’ll see that earth resistivity depends on the soil material, the moisture content, and the temperature. It is far from a constant, predictable value ranging generally from 500 to 50,000 ohm-cm6.Principles Involved in Earth Resistance Testing The resistance to earth of any system of electrodes theoretically can be calculated from formulas based upon the general resistance formula: R = ρ L Awhere ρ is the resistivity of the earth in ohm-cm, L is the length of the conducting path, and A is the cross-sectional area of the path. Prof. H. B. Dwight of Massachusetts Institute of Technology developed rather complex formulas for the calculation of the resistance to earth for any distance from various systems of electrodes (Reference 11). All such formulas can be simplified a little by basing them on the assumption that the earth’s resistivity is uniform throughout the entire soil volume under consideration.Because the formulas are complicated, and earth resistivity is neither uniform or constant, a simple and direct method of measuring earth 6 An ohm-centimeter (abbreviated ohm-cm) is defined as the resistance of a cube of material (in this case, earth) with the cube sides being measured in centimeters.
