Earthing is the process of connecting the current carrying and non-current carrying metal parts of an electrical system to dissipate the currents to the ground. The main objective of earthing is to ensure the safety of operating person & equipment, reference for the system voltage, dissipating the impulse lightning current and discharge the static charges.
The major components of earthing are
1) Earth Electrode
2) Earth enhancing compound or Back fill compound
3) Terminal connector
Earth enhancing compounds (EEC) or Back filling compounds (BFC) are highly conductive compounds which are filled around the earth electrode by replacing the existing high resistance soil to provide low earth resistance value and improve the performance of earthing system.
In areas with soils of high resistivity, multiple earth rods, even in large number, may sometime fail to produce an adequately low resistance to earth. The alternative is to reduce the resistivity of the soil immediately surrounding the earth electrode.
The greater part of the fall in potential occurs in the soil within a few feet of the electrode surface, since it is here that the current density is highest. To obtain a low overall resistance the current density should be as low as possible in the medium adjacent to the electrode, which should be so designed as to cause the current density to decrease rapidly with distance from the electrode. Hence, using highly conductive earth enhancing compounds around the earth electrode is an effective way of achieving low earth resistance value.
Since the materials are used directly on soil, the impact of these materials on environment is very high and it may pollute the land and ground water. Hence, special care should be taken on selecting the materials for BFC. The materials should have the following characteristics.
Considering the importance of earth enhancing compounds in providing low earth resistance and the impact that the materials can create on environment, IEC 62561-7 specifies the requirements and tests for earthing enhancing compounds producing low resistance of an earth termination system. The tests are,
• Leaching test
• Sulphur determination test
• Determination of resistivity test and
• Corrosion test
BS7430 explains the procedure for calculating the earth resistance value in the presence of Earth Enhancing Compound. The formula for finding the earth resistance (R) is,
From the above calculation, we can find that, the earth resistance value can be reduced by using earth enhancing compounds which have very low resistivity value and diameter of the material used.