Several companies manufacture earth testers, also known as earth ground testers. The main use of these instruments is to test the resistance of grounding rods. Grounding rods placed in the soil protect a home's wiring system by directing electrical surges into the ground. Over time, grounding rods corrode and become less effective. Making occasional checks with an earth tester lets you know your home's grounding is still sound.
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Locate the grounding rods outside your home. Look for a metal rod inserted into the ground with a wire lead running from it to your home. The rods are typically located near the foundation of your home. Disconnect the grounding wire from the rod.
Insert two probes of the earth tester into the ground in a parallel line to your grounding rod. Place one probe 65 feet from the grounding rod and the second probe 33 feet from the first probe (98 feet from the grounding rod).
Clip the wires of the earth tester onto the rods. The red wire connects to the outer probe, the white wire connects to the inner probe, and the blue (or green) wire to the grounding rod.
Turn on the earth tester and press "Start." Digital earth testers will provide a reading of resistance. Record the number.
Disconnect the probes and move the inner probe three to nine feet closer to the grounding rod. Test the resistance again. If the result is within 30 percent of the first reading, your test is done. If not, move the probes further from the grounding rod until you obtain two readings within 30 percent of each other. Make a note of the distances you used for your readings.
Perform the resistance tests again at least annually. If the resistance increases more than 20 percent from your original readings, your grounding rod should be replaced.
Metal objects in the ground will affect your readings. Place the probes in a parallel line that isn't near other metal. The probes should be placed in moist soil. The 65- and 98-foot placement of the test probes is for grounding rods placed nine feet deep. If your grounding rod is 6 feet deep, place the probes at 49 and 65 feet from the rod. If your grounding rod is 18 feet deep, place the probes at 82 and 131 feet.