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other points, causing changes in earth resistivity. Graded soil aroundconstruction sites or buried objects such as pipes can cause such localizeddeviations. Therefore, you want to get some degree of flatness or levelingoff of your curve to make such a variation easily noticeable. At the sametime, remember that the resistance will rise again in the electrical field ofthe current probe, so measurements in this area are to be avoided.As a practical example of this effect, consider the case illustrated in Fig.14. This shows two earth resistance curves for two locations of C. Curve Awas obtained when C was 100 ft from the earth electrode; Curve B whenC was 700 ft away. Curve A shows that C was too close to the earthelectrode; Curve B shows the desired tendency toward leveling out ofthe measured resistance. The 62 percent distance gives resistance valuesnearly the same in this case since the earth resistivity is fairly uniform.Simplified Fall-of-Potential Test: The preferred test method is to alwaysgather sufficient data to plot the actual curve of resistance vs. distance.In the event that this is impossible, a simplified test might be used witha compromise on accuracy. This procedure is similar to that outlinedunder Fall-of-Potential Method as described in IEEE Standard No. 81 (seereferences), but you start with P midway between the earth electrodeand C.This reading with P at 50 percent of the distance from the earth electrodeto C is noted as R 1 . Reference probe P is then moved to a location 40percent of the distance to C. The reading at this point is noted as R 2 . Athird reading, R 3 , is made with P at a 60 percent distance. The average ofR 1 , R 2 and R 3 is calculated as R A . You determine the maximum deviationfrom the average by finding the greatest difference between individualreadings and the average. If 1.2 times this percentage is less than yourdesired test accuracy, R A can be used as the test result. As an example ofthis technique, use the data from curve B in Fig. 14 as follows:R 1 = 58 Ω R 2 = 55 Ω R 3 = 59 ΩR A = 55 + 58 + 59 = 57.3 Ω3R A - R 2 = 57.3 - 55 = 4.0%R A 57.34.0% x 1.2 = 4.8%If your desired accuracy was 5 percent, 57 Ω (R A ) could be used as theresult. If the result is not within the required accuracy, probe C has tobe placed farther away and the tests repeated. This method can givesufficient accuracy but will always give values on the low side. (Seediscussion following with reference to Table II.)Fig. 14: Example of how C location affects the earth resistance curveSome Rules of Thumb on Spacing P and C: For testing a single earthelectrode, 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 undertest; P correspondingly can be placed about 62 to 78 ft away. If the earthelectrode system is large, consisting of several rods or plates in parallel, for32 1-866-254-0962 www.megger.com Getting Down to Earth 33