This wisdom is culled from the voluminous writings of W. Fred Knorr;
a grounder par excellence, a good friend and great mentor.
Grounding is an every day term. It is a common term, but unfortunately there is lot of confusion about its true meaning. The primary function of a ground is to increase safety for life and property. Its secondary function is to improve the operation of a system and the continuity of service. This is particularly important with utility systems. Usually these two functions are synonymous and cannot be differentiated between.
Good grounding is actually an art, not a science. It's not a subject that can be mathematically computed to the final degree. It is subject to the changing environment depending on the soil composition, moisture in the soil and ambient temperature.
The earth is an excellent conductor because of its size. It has unlimited conductance capacity but its resistivity varies from approximately 2 ohms to 3,000 ohms and sometimes even greater. Basically the soil is made up of silica and alumina, both excellent insulators. It is also composed of various soluble salts and moisture.
The basic relationship for computing a ground is given by the relationship-resistance (R) equals Þ, the resistivity times a factor length, divided by area, or R=Þ*L/A.
Actual field measurements show that 90% of the total resistance is within a radius of 10 feet of the electrode. This is called a "relative cone of earth". This is important to remember because too often, one of the errors made in using an electrode driven into earth is driving them too close together. In effect using many electrodes but not obtaining a good, economical, effective ground.
Resistivity of the earth is strongly dependent on water content. When we have approximately 20% moisture content (or greater) the earth resistivity runs fairly constant. When we have moisture content below 20%, then we have extremely high resistivity in the soil. In fact, if we took red clay soil with 10% moisture, the resistance would be 30 times greater than that same soil with 20% moisture content. The variation of moisture in the soil, by tests, averages about 10% in the dry season and approximately 35% in the wet season. This gives a yearly average somewhere in the neighborhood of 16-18%.
Earth resistivity decreases as the temperature increases from below freezing to normal ambient temperature. Below 32° F, the earth freezes, and the resistivity climbs extremely high. This is from data based on testing red clay soil with 18.6% moisture content.
Because of the forgoing facts, the depth of an electrode in the earth is one of the most important factors affecting the resistance. The basic reasons for this are:
As stated previously, the earth is composed of insulating material, soluble salts and water. The accepted theory is the deeper the penetration into the earth, the greater will be the concentration of salts and moisture; and the less fluctuation of these elements will be encountered.