Getting Down to Earth 29placed as far from the earth electrode as practical; this distance may be limited by the length of extension wire available, or the geography of the surroundings (see Fig. 11). Leads should be separated and “snaked,” not run close and parallel to each other, to eliminate mutual inductance. Potential-reference rod P is then driven in at a number of points roughly on a straight line between the earth electrode and C. Resistance readings are logged for each of the points. A curve of resistance vs. distance, like Fig. 10b, is then drawn. Correct earth resistance is read from the curve for the distance that is about 62 percent of the total distance from the earth electrode to C. In other words, if the total distance is D, the 62 percent distance is 0.62D; for example, if D is 120 ft, the distance value for earth resistance is 0.62 x 120 or 74 ft.There are three basic types of the Fall-of-Potential test methods and a number of related test methods that will be described in the appendices.The types of Fall-of-Potential are:n Full Fall-of-Potential — a number of tests are made at different spaces of P and the resistance curve is plotted. n Simplified Fall-of-Potential — three measurements are made at defined distances of P and mathematical calculations are used to determine the resistance (to be described in more detail later).n 61.8% Rule — a single measurement is made with P at a distance 61.8% (62%) of the distance between the electrode under test and C.The related test methods tend to be more complex and sophisticated requiring many measurements and/or a great deal of math. These methods have been developed to help overcome the problems faced when testing large ground systems or when there is limited space. A list of these methods follows:n Intersecting curves method (Appendix IV, page 58)n Slope method (Appendix V, page 62)n Four-potential method (Appendix VI, page 65)n Star delta method (Appendix VII, page 67)
