SOLUTION

The object of an earth electrode is to provide a low resistance path to fault currents that may cause injury to the operating staff or damage or the equipment. The currents will dissipate safely when properly conducted to earth via earth electrode. There are three components of the earth resistance;

(1) Ohmic Resistance of the electrode materials and connections to them,

(2) Contact resistance between the electrode and the soil surrounding it,

(3) Resistance of the earth immediately surrounding the grounding electrode or resistivity of earth, which is often the most significant factor.

The grounding electrodes are usually made of a very conductive metal (copper or copper-clad) with adequate cross-sections so that the overall resistance is negligible.

The resistance between the electrode and the surrounding earth is negligible if the electrode is free of paint, grease, or other coatings, and if the earth is firmly packed.

The only component remaining is the resistance of the surrounding earth. The electrode can be thought of as being surrounded by concentric shells of earth or soil, all of the same thickness. The closer the shell to the electrode, the smaller it’s surface; hence, the greater its resistance. The farther away the shells are from the electrode, the greater the surface of the shell; hence, the lower the resistance.

The distance at which this effect occurs is referred to as the effective resistance area and is directly dependent on the depth of the grounding electrode.