Earth ground resistance tester

An earth ground resistance tester is a portable instrument that measures the resistance to earth of a grounding electrode or ground system. You can use it while the electrode is going in and again in periodic tests to see how the resistance changes over time. Full-featured testers support four test methods: soil resistivity (with stakes), fall-of-potential (with stakes), selective (one clamp plus stakes) and stakeless (two clamps only). For the fall-of-potential test, first disconnect the electrode from the site and connect the tester to it. Then drive two stakes in a straight line away from the electrode; a spacing of 20 meters (65 feet) is normally enough. The tester drives a known current between the outer stake and the electrode, measures the voltage drop at the inner stake, and calculates the resistance with Ohm's Law. If the electrode is still tied in parallel or series with other rods, the reading is the total of all of them. In the stakeless clamp-on test, the tester clamps over the ground being measured, and the test current returns through the other grounds connected in parallel. This works best where the return-path loop resistance is low compared with the ground being tested. Learn the stakeless method properly before relying on it, because misuse gives wrong or misleading readings. Trending clamp-on results over time can give early warning that ground resistance is getting worse. Some models do more than measure resistance. For example, one three-terminal unit measures ground resistance from 0.01 to 2000 ohms, measures earth voltages up to 100 V, and with an optional clamp measures ground current.