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When a ground rod is being tested against a known ground, the resistance of the known ground is so small that the total resistance in the test circuit is accepted as a measure of the ground rods resistance. Since the known ground may be some distance from the ground rod being tested, inspectors should carry with them up to 500-feet of building wire, type TW, number 14 AWG or larger, solid copper (or any other solid copper wire number 14 AWG or larger). To test against the known ground, connect a 24-volt aircraft battery, the ground rod being tested, a multimeter, and the known ground in series. Read the battery voltage and the milliamperes flowing through the circuit. Use the following formula to find the resistance, E equals volts, and I equals milliamperes:

      R = 1,000E

Take a second set of readings with the polarity of the battery reversed (reverse the flow of current in the circuit). Use the same formula to find the resistances. The average will approximate the true resistance of the ground rod.


Use this method of testing the ground rods resistance when there is no known ground against which to test. Drive two test stakes (other ground rods) into the earth near the ground rod being tested. Use R1 to equal the resistance in ohms of rod 1, R2 to equal the resistance of rod 2, and R3 the resistance of rod 3. Test and compute the resistance between each pair of rods (rods 1 and 2, rods 1 and 3, and rods 2 and 3) using the method and formula used to test against a known ground. Let A be the resistance between rod 1 and 2 (R1 + R2 =A). Let B be the resistance between rods 1 and 3 (R1 + R3=B), and C be the resistance between rods 2 and 3 (R2 + R3=C). Use these figures to solve the following equations for the resistance of the three rods:

    R1 = A +B -C     R2 = A + C - B
                  2                           2

    R3 = B + C -A

Not one of these resistances should be more than 10,000 ohms. If the resistance is higher, redrive and retest rods until resistances are below 10,000 ohms. If after continued tests the resistance is not below 10,000 ohms, remove the rod or mark it as shown in Chapter 2.

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