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Ground Rod Resistance Measurement

December 13th Fact-of-the-Day

The resistance between two ground rods spaced farther apart than their length can be measured to obtain a general indication of their likely ability to safely conduct lightning induced currents to earth. However, resistance measurements taken soon after ground rod installation are inaccurate, because the soil is not yet in good contact with the rods. It is necessary to wait six to eight weeks in average soil to obtain a reliable measurement. The measured resistance will be twice the earth contact resistance of one rod if the rods are identical. Earth contact resistance below 5 ohms is excellent. 5 to 10 ohms is good. 10 to 15 ohms is poor. Grounds with more than 15 ohms resistance are of little value in reducing damage from lightning induced currents. ©2004 Martek International All rights reserved.


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