Getting Down to Earth 13As will be covered in Section II, the other main reason for measuring earth resistivity is to design earth-electrode systems for electrical power systems, lightning arresters, and so on. The measured resistivity values are used in standard engineering formulas that calculate factors like number and depth of rods necessary to achieve a required ground resistance, thus reducing the amount of trial and error in the installation of an effective ground. Earth resistance varies directly with earth resistivity and it is helpful to know what factors affect resistivity.The curves of Fig. 4 illustrate several worthwhile points. They show the expected change in earth resistance (due to resistivity changes) over a 1-1/2 year period; they also show that the deeper electrode gives a more stable and lower value. We conclude that the moisture content and temperature of the soil become more stable at greater distances below the earth’s surface. Therefore, the earth electrode should reach a deep enough level to provide:n Permanent moisture content (relatively speaking).n Constant temperature (below frost line; again, relatively speaking).Fig. 4: Seasonal variation of earth resistance with an electrode of 3/4” pipe in stony clay soil. Depth of electrode in earth is 3 ft for Curve 1 and 10 ft for Curve 2 (source: Ref. 9, page 76)
