Getting Down to Earth 37How to Improve Earth ResistanceWhen you find that your earth electrode resistance is not low enough, there are several ways you can improve it:n Lengthen the earth electrode in the earth.n Use multiple rods.n Treat the soil.Effect of Rod Size: As you might suspect, driving a longer rod deeper into the earth, materially decreases its resistance. In general, doubling the rod length reduces resistance by about 40 percent. The curve of Fig.15 shows this effect. For example, note that a rod driven 2 ft down has a resistance of 88 Ω; the same rod driven 4 ft down has a resistance of about 50 Ω. Using the 40 percent reduction rule, 88 x 0.4 = 35 Ω reduction. By this calculation, a 4-ft deep rod would have a resistance of 88 - 35 or 53 Ω — comparing closely with the curve values.Fig. 15: Earth resistance decreases with depth of electrode in earth (Source: Ref 19)
