c. Treat the soil with chemicals if the ground resistance is still over 25 ohms after using the
methods described in paragraphs 1-3a and 1-3b. 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 1-15).
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 1-16).
Figure 1-15. Trench dug around grounding rod
Figure 1-16. Trench filled with rock salt