Getting Down to Earth 25electrode, the earth shells are of greater surface area and therefore of lower resistance. Now, assume that you have three rods driven into the earth some distance apart and a voltage applied, as shown in Fig. 10a. The current between rods 1 and 2 is measured by an ammeter; the potential difference (voltage) between rods 1 and 3 is measured by a voltmeter.If rod 3 is located at various points between rods 1 and 2, preferably in a straight line7, you can get a series of voltage readings. By Ohm’s Law (R = E/I) you can determine the earth resistance at any point measured. For example, if the measured voltage E between rods 1 and 3 is 30 V and the measured current I is 2 A, the resistance of the earth R at that point would be 15 Ω. The series of resistance values can be plotted against distance to obtain a curve (Fig. 10b). Note that as rod 3 is moved away from rod 1, the resistance values increase, but the amount of increase gets less and less until a point is reached where the rate of increase becomes so small that I can almost be considered constant (20 Ω in Fig. 10b). The earth shells between the two rods (1 and 3) have so great a surface area that they add little to the total resistance. Beyond this point, as rod 3 approaches the earth shells of rod 2, resistance gradually picks up. Near rod 2, the values rise sharply.Now, let’s say that rod 1 is our earth electrode under test. From a typical earth-resistance curve, such as Fig. 10b, what is the resistance to earth of this rod? We call rod 2 current-reference probe C and rod 3, potential-reference probe P (simply for convenience in identification). The correct resistance is usually obtained if P (rod 3) is placed at a distance from the center of the earth electrode (rod 1) about 62 percent of the distance between the earth electrode and C (rod 2). For example, in Fig. 10b, the distance D from the earth electrode to C is 100 ft. Taking 62 percent of this distance, we get 62 ft. From Fig. 10b, 7 Actually, current can exist in other paths between the two fixed electrodes, so that rod 3 could be (and might have to be) located at other than along a straight line.
