Resistance
This check should be made with the grounding jumper between the water pipe or a good ground and the electrical system. A metallic cold-water system makes a good point for testing the resistance of an electrical grounding system. If 25 ohms to ground cannot be achieved, take the following actions:
Figure 1. Megohm meter
a. Drive additional 8-foot grounding rods into the earth and bond them together. The rods must be at least 6 feet apart (Figure 2).
Figure 2. Additional grounding rods
b. Drive grounding rods deeper, if necessary, to reach the water table (Figure 3).
Figure 3. Grounding rod reaching the water table
c. Treat the soil with chemicals if the ground resistance is still over 25 ohms after using the methods described above. Treat the soil by Digging a circular trench around the grounding electrode. The trench should be 1 foot deep and 9 inches away from the electrode (Figure 4).
Filling the trench with 50 to 100 pounds of rock salt (copper sulfate or magnesium sulfate may also be used) and then filling the trench with water. As the water goes through the salt into the ground, resistance should be lowered. Natural rainfall will continue the process, but you must replace the rock salt every two years (Figure 5).
Figure 4. Trench dug around grounding rod
Figure 5. Trench filled with rock salt
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