On sites where soil resistivity is high, the earth resistance is lowered using deep driven rods, rods with increased diameter, multiple rods, or using chemical compounds or soil of lower resistivity as an earth electrode backfill. Tests have shown that increasing the diameter of an earth rod from 12.5 to 25 mm has increased the weight by 400%, increased its cost by 400%, but only reduced the earth resistance by 9.5%. Increasing the number of rods indefinitely also has little effect. The improvement in the earth resistance that can be expected from installing additional rod is: two rods 60%; three rods 45%; and four rods 35%. If soil resistivity survey indicates lower resistivity at greater depths then a logical choice is the deep driven rods. However if soil resistivity is uniform then the percent reduction in earth resistance per unit length of rod becomes exponentially less with each increment of length. The use of chemical compounds or soil of lower resistivity as an earth electrode backfill is a very effective method of dealing with high resistivity soil. Improvement in resistance values up to 20% has been recorded using chemical compounds as soil treatment agent.
The earth resistance of electrode has three components: the resistance of the electrode conductor, the electrode/soil contact resistance, and the resistance of the soil surrounding the electrode. Typically, the first two components are negligible compared to the resistance of the soil. Thus most analytical formulae for earth electrode resistance usually account for the resistance of the soil only. The resistance may be considered as being made up of a number of concentric shells of earth or soil of equal thickness. If the soil is assumed to be of uniform resistivity, then the resistance of a shell varies inversely as its cross-section and therefore beyond some distance from the earth electrode, the resistance of successive shells will no longer noticeably affect the total earth resistance. Some study has shown that at a distance of about 1.1 times the length of rod electrode from the electrode, the resistance is about 95 % of the total earth resistance and change in resistance becomes negligible. The area within this distance of the rod is the so-called effective resistance area. Measurements have also shown that about 90% of the total earth resistance surrounding an earth electrode is generally within a radius of 2 to 3 m from it.
A natural method of reducing earth resistance will be replacing all soil in the effective resistance area with soil of the lowest resistivity available. However this method may not in general make sense economically. For this to be realistic, it will be necessary to limit the backfill to the area very close to the earth rod and judged to be accounting for a substantial percentage of the total earth resistance. The area is referred to as critical resistance area and its radius as critical resistance radius. In this paper, the effectiveness of using a backfill in critical resistance area to lower earth resistance is demonstrated by field test data. One needs earth rod resistance as a function of distance away from the rod to apply this method of earth resistance reduction. An analytical formula for this is derived and validated with field test data.