Getting Down to Earth 7Dr. Frank Wenner of the U.S. Bureau of Standards (now NIST) developed the theory behind this test in 1915 (see reference pg. 76). He showed that, if the electrode depth (B) is kept small compared to the distance between the electrodes (A)1, the following formula applies:ρ = 2π ARwhere ρ is the average soil resistivity to depth A in ohm-cm, π is the constant 3.1416, A is the distance between the electrodes in cm, and R is the Megger earth tester reading in ohms.In other words, if the distance A between the electrodes is 4 ft, you obtain the average earth resistivity to a depth of 4 ft as follows:1. Convert the 4 ft to centimeters to obtain A in the formula: 4 x 12 x 2.54 cm = 122 cm2. Multiply 2 π A to obtain a constant for a given test setup: 2 x 3.14 x 122 = 766Now, for example, if your instrument reading is 60 Ω, the earth resistivity would be 60 x 766, or 45,960 ohm-cm.There are other methods for measuring soil resistivity such as the Schlumberger method. However, the Wenner method is the most popular in the electric power industry.Fig. 1: Four-terminal method of measuring earth resistivity1B = 1/20A is generally recommended
