The Concept of Sphere Of Influence When Determining Grounding Electrode Efficiency

When determining how efficiently grounding electrodes discharge electrons into the earth and important concept is the “sphere of influence”. The sphere of influence is the volume of soil throughout which the electrical potential rises to more than a small percentage of the ground potential rise of the ground electrode, when that electrode discharges current into the soil. The greater the volume, compared with the volume of the electrode, the more efficient the electrode. Long electrodes, such as grounding rods, are the most efficient. The surface area of the electrode determines the ampacity of the device, but does not affect “the sphere of influence”. The greater the surface area, the greater the contact with the soil and the more electrical energy that can be discharged per unit of time.

Fig 1: Sphere-of-Influence of an Earth Electrode

Figure 2: The formula for calculating the volume of soil

A simpler version is used when the above formula is modified by rounding Π (pi) down to to 3 and cross canceling to get the formula: V=5L3

In this example, a single 10-foot driven rod would utilize 5,000 cubic feet of soil, where as a single 8 foot rod would utilize about half the soil at 2,560 cubic feet.