Earthing Theory

Earthing Theory An earthing system basically provides a drain path to all kinds of faults that may develop in an electrical system comprising of power circuits, control circuits, EMP and statics. It also provides the reference voltage for functional earth. These faults may be caused by surges or transients from indirect lightning strikes, or spikes, or harmonics, or faults emanating from within your own electrical system. The nature of these faults is varied with respect to frequency, amplitude and time. Therefore, the earthing system of any installation needs to be thoughtfully designed to handle all such faults effectively. Generally an earthing installation consists of an elaborate system of earth grid comprising of earth rods or electrodes, tapes or strips, and pipes that are connected to the electrical earth terminal. The entire grid acts as a carrier of such faults. However, the grid by itself is not capable of discharging theses fault currents into the ground. It is the electrical resistance property of the surrounding ground that helps to absorb and dissipate the fault currents from the carrier grid it to the earth mass. Hence, while designing an earth grid, one should consider the following points in great detail. 1. The grid should be capable of conducting such fault currents for a given time duration. 2. The neighboring groundmass should be capable of absorbing such fault currents and dissipating it to the larger earth mass. IEEE 80-2000, IEEE 665, IEEE 142, IEEE 81-993, BS 7430, IS 3043 are the standards which provide detailed procedures for designing such a grid. If the soil resistivity is high, the penetration of the charges into the earth mass would be poor. The concentration of charges in the vicinity of the earth grid will thus be high and create high voltage. Further, in a high resistive soil, the concentration of charges dips steeply as you move away from the earth grid. This results in a steep drop in voltage. On the other hand, if the soil resistivity is low, the penetration of the charges into the earth mass will be quicker and hence, concentration of charges around the grid will be low. Thus in a low resistivity soil, as you move away from the earth grid, there is a steady and very small drop in voltage. The presence of high voltage in the vicinity of the grid & its neighboring areas is a matter of concern and a serious hazard to the safety of human life and livestock. Therefore, while designing the earth grid and its various components, we need to be cautious of the following parameters: • • •

Step Voltage: It is the difference in voltage between the right & the left foot when a person is standing in the vicinity of the grid. Touch Voltage: It is the difference in voltage between the point at which a man stands & the object he touches, which is connected to the grid. Mesh Voltage: Maximum touch voltage within a mesh of a ground grid.

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Grid Potential Rise: The maximum potential a grid may attain relative to a distant grounding point, which is at the potential of the remote earth.

Human heart fibrillates if more than 0.116 ampere current flows through it. In an earth grid, the difference in step / touch voltage can cause such a current flow. Hence, every grid must be designed to have a maximum allowable ‘step potential’ & maximum allowable ‘touch potential’. Thus any earth grid should necessarily have the following parameters. • The earth grid should have the minimum cross section to carry the maximum fault current for particular time duration without fusing. • The actual step potential should be lower than the allowable step potential. • The actual mesh potential should be lower than the allowable touch potential. To design an earth grid meeting all the above parameters is basically an art of selecting the right cross section of the grid conductor, the right geometry of laying the earth grid, treating the soil artificially to lower the soil resistivity & selecting the material of the earth grid conductors. Therefore, before taking up the design of an earth grid, it is very important to determine the anticipated type of fault, the level of fault, the duration of fault, as also to map the soil resistivity of the area where the grid is proposed to be laid. It may be noted that earthing efficiency is much better if larger surface area of grid is in touch with larger ground mass. Hence, increasing the length of the earth grid is better than increasing the crosection of the grid. Thus thinner & deeper electrodes will give better results than shorter & wider electrodes in unilayer soil. It is also observed that lower soil resistivity reduces the earth grid size. The grid thus becomes more compact and economical. Thus in situations where soil resistivity is high, or in installations where the grid resistance is desired to be very low, it is always desirable to treat the soil with artificial treatment compounds in order to bring down the soil resistivity. Today there are many artificial soil treatment compounds available in the market that provides maintenance free earthing solutions. TEREC +, Conductive Gels, Bentonite & various salts can be used as artificial treatment compound. TEREC + provides a wholesome cure to reduce the soil resistivity permanently by enhancing its ionic properties, dispersion properties, hygroscopic properties, expansion properties, diffusion properties etc. TEREC+ was brought to India recently by 172 years old French company, Duval Messien and has since been tested for critical applications in defense, nuclear science, information technology industries, commercial establishments etc. TEREC+ has recently been approved for grounding the world’s tallest structure, ‘the Burj Tower’ in Dubai.

Proper Earthing •

Provides proper reference voltage for all electronics.

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Ensures all your protection device to function properly and appropriately trip the circuit saving you from electrical fire.

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Drains all fault current at the earliest.

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Does not allow any dangerous voltages to appear at the time of fault saving you from Heart attack.

You may contact SGI Engineers Private Ltd. ([email protected]) for further information, or for getting your existing earthing design audited, or to develop an earthing design for your new establishment, including drafting of tender enquiry.