32 www.megger.comother points, causing changes in earth resistivity. Graded soil around construction sites or buried objects such as pipes can cause such localized deviations. Therefore, you want to get some degree of flatness or leveling off of your curve to make such a variation easily noticeable. At the same time, remember that the resistance will rise again in the electrical field of the 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 A was obtained when C was 100 ft from the earth electrode; Curve B when C was 700 ft away. Curve A shows that C was too close to the earth electrode; Curve B shows the desired tendency toward leveling out of the measured resistance. The 62 percent distance gives resistance values nearly the same in this case since the earth resistivity is fairly uniform. Simplified Fall-of-Potential Test: The preferred test method is to always gather sufficient data to plot the actual curve of resistance vs. distance. Fig. 14: Example of how C location affects the earth resistance curve
