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Old 11-Aug-2019, 2:48 AM   #43
GroundUrMast
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Join Date: Oct 2010
Location: Greater Seattle Area
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The traditional method of measuring ground rod resistance has been the "Three Point Fall of Potential" (TPFP) though there are alternate methods as well. The cost of specialized equipment with recognized certifications is likely well beyond the budget of most DIY'ers. Here is a somewhat technical description that make reference to The Institute of Electrical and Electronics Engineers in IEEE Standard #81, IEEE Recommended Guide for Measuring Ground Resistance and Potential Gradients in the Earth.

The TPFP provides a means of accounting or the variability of soil conductivity in the area near the ground rod under test, and also account for the resistance in the test connections. This method also avoids the hazards of breaking the connection of a live system from the ground rod... If there were fault current flowing in the grounding system, hazardous voltage could be present when the ground conductor is taken loose from the ground rod. (I've added a safety warning to my post #24 of this thread.)

Having rambled enough... If you simply drive a rod into the earth and then connect a multi-meter between the new rod and the existing electrical service ground, check for AC and DC voltage differences before checking resistance. You may find there is enough current through the earth due to any number of possible sources. Only if there is negligible voltage present can you then take a DC ohm reading that would provide an approximation of the new ground rod resistance. Though not likely, you can encounter hazardous voltage differences between different points on the ground, so don't allow yourself to touch both points at the same time.
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Last edited by GroundUrMast; 11-Aug-2019 at 3:21 AM.
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