Getting Down to Earth 31Minimum Distance for C: Consider Fig. 13 which shows earth shells around the earth electrode and reference probe C. In Fig. 13a, C is so close to the earth electrode that the earth shells seriously overlap. Then you don’t get the leveling off of measured resistance as P is moved away from the earth electrode; the shells of C add to the shells of the earth electrode so the resistance keeps increasing.In Fig. 13b, C is placed farther away. Then the measured resistance levels off enough and at the 62 percent distance it is very close to the actual earth resistance. The reason for having C farther away is to get assurance that the 62 percent value is “in line” with other values on the curve. The value could only be wrong (assuming there are no measuring mistakes) if the soil conditions at the 62 percent point vary from conditions at Fig. 13: Effect of C location on the earth resistance curve
