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.
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Ground Testing FAQS
