38 www.megger.comYou might also think that increasing the electrode diameter would lower the resistance. It does, but only a little. For the same depth, doubling the rod’s diameter reduces the resistance only about 10 percent. Fig. 16 shows this relationship. For example, a 10-ft deep rod, 5/8 in. in diameter, has a resistance of 6.33 Ω; increasing its diameter to 1-1/4 in. lowers the resistance o to 5.6 Ω. For this reason, you normally only consider increasing the rod diameter if you have to drive it into hard terrain.Use of Multiple Rods: Two well-spaced rods driven into the earth provide parallel paths. They are, in effect, two resistances in parallel. The rule for two resistances in parallel does not apply exactly; that is, the resultant resistance is not one-half the individual rod resistances (assuming they are of the same size and depth). Actually, the reduction for two equal resistance rods is about 40 percent. If three rods are used, the reduction is 60 percent; if four, 66 percent (see Fig. 17).When you use multiple rods, they must be spaced apart further than the length of their immersion. There are theoretical reasons for this, but you need only refer to curves such as Fig. 18. For example, if you have two rods in parallel and 10-ft spacing, resistance is lowered about 40 percent. If the spacing is increased to 20 percent, reduction is about 50 percent.Fig. 16: Diameter of a rod has little effect on its earth resistance Curve A, from Ref. 19 Curve B, average of Underwriters Laboratories tests at Chicago Curve C, average of Underwriters Laboratories tests at Pittsburgh
