Getting Down to Earth 33In the event that this is impossible, a simplified test might be used with a compromise on accuracy. This procedure is similar to that outlined under Fall-of-Potential Method as described in IEEE Standard No. 81 (see references), but you start with P midway between the earth electrode and C.This reading with P at 50 percent of the distance from the earth electrode to C is noted as R1. Reference probe P is then moved to a location 40 percent of the distance to C. The reading at this point is noted as R2. A third reading, R3, is made with P at a 60 percent distance. The average of R1, R2 and R3 is calculated as RA. You determine the maximum deviation from the average by finding the greatest difference between individual readings and the average. If 1.2 times this percentage is less than your desired test accuracy, RA can be used as the test result. As an example of this technique, use the data from curve B in Fig. 14 as follows: R1 = 58 Ω R2 = 55 Ω R3 = 59 Ω RA = 55 + 58 + 59 = 57.3 Ω 3 RA - R2 = 57.3 - 55 = 4.0% RA 57.3 4.0% x 1.2 = 4.8%If your desired accuracy was 5 percent, 57 Ω (RA) could be used as the result. If the result is not within the required accuracy, probe C has to be placed farther away and the tests repeated. This method can give sufficient accuracy but will always give values on the low side. (See discussion following with reference to Table II.)Some Rules of Thumb on Spacing P and C: For testing a single earth electrode, C can usually be placed 50 ft from the electrode under test, with P placed about 31 ft away. With a small grid of two earth electrodes, C can usually be placed about 100 to 125 ft from the electrode under test; P correspondingly can be placed about 62 to 78 ft away. If the earth electrode system is large, consisting of several rods or plates in parallel, for
