### Steve Maurer, IME

In Part 1, we talked about the below-grade ground, or earthing. We looked at some of the approved grounding electrodes possible. This article concerns the installation of ground rods.**Ground rod installation**

There should be at least two of them, no closer than six feet apart. The lengths recognized are 8 feet (minimum) and 10 feet. The code states that the actual distance should be twice the length of the rod, assuming both are the same length.

Code requirements state that a minimum of 8 feet must be in direct contact with the soil and below the permanent moisture level.

Therefore, an 8-foot rod must be completely buried and should be 18 inches below grade. A 10-foot rod can stick out 6 inches above grade, while still maintaining the 8-foot requirement below permanent moisture level.

Rocky soil makes installation difficult. I don't know how many times I've seen installers get so frustrated that they consider cutting off the "excess."

Not only is this a Code violation, but it decreases the surface area of the rod contacting the soil. Using the simplified formula V=5 L3, where V is the volume of earth and L is the length of the rod, you can see that an 8-foot rod contacts about half the earth that a 10-foot rod does.

Cutting the rod further will seriously hamper its effectiveness.

A far better solution, both physically and electrically, is to auger the hole, place the ground rod, and then fill the hole with dirt. Often, this is better because more of the rod comes in contact with soil, rather than being hampered by rock that does not conduct electricity.

Speaking of dirt and rock, the quality of the soil should be tested for resistivity. There are specific calculations for the sphere of influence surrounding the grounding electrode. Instruments designed specifically for soil testing should be used.

Finally, the system must be connected to the ground with approved grounding conductors, and installed using approved methods and hardware.

The best advice is to contract an engineer or engineering firm that specializes in grounding system design. This is the best way to avoid code violations and safety issues.