Peter Walsh, PE
The National Electrical Code requires a supplementary grounding electrode, but it has an exception. Paragraph 250.53(A)(2) says that a single grounding rod, pipe or electrode shall be supplemented by an additional electrode. The exception allows the single electrode if the first one has a resistance to ground of 25 ohms or less. How can you determine that?
Many intuitive measurements do not give you the right answer. Measuring the ohms with a traditional ohmmeter is not accurate. Ground currents between ground connections can decrease or increase the measurements. The ohmmeter has to attach to two locations, one of which is the ground under test. Where should the other lead attach? Different attachment locations can give different results.
A solution is to use a clamp-on ground resistance tester. It operates without disconnecting wires and is suitable for most installations. The operation is based on the principles of circuit analysis to automatically calculate the true ground resistance.
An example is the AEMC Model 3711 clamp-on ground resistance tester. This instrument comes with a 25-ohm test calibrator and simple instructions. Rapid and accurate testing is possible on systems with multiple grounds.
Measuring ground resistance is different from testing soil resistivity, which is often a requirement of cell tower construction. Soil resistivity is a measure of the soil itself, not the connection of the grounding system to the soil. Soil resistivity is measured by the fall-of -potential method. This method requires three or four widely spaced ground connections for accuracy. Measuring soil resistivity is more complex and time consuming than measuring ground resistance.
Photo courtesy of AEMC