Different means of achieving low earth or ground resistance are to drive ground rods, bury wire, copper strip, metallic plate etc. In this article, formula for a single ground wire or pipe buried parallel to earth’s surface is discussed. Buried wire electrodes also known as trench electrodes makes good contact with earth and is an intricate part of any large ground grid system.
For a single ground wire or pipe buried in ground, equation for effective earth or ground resistance [Rg] is given by the following equation [Ref 1]:
The following calculator can be used to calculate earth or ground resistance for a single horizontally buried electrode in ground. Typical resistivity values as provided in IEEE Std. 80 can be used or user can input soil resistivity data directly in to calculator. Since the conduction in soil is largely electrolytic, the amount of moisture and dissolved salts affects the resistivity of soil. Without actual measurement, accurate characterization of a particular soil as ‘wet organic soil’ or ‘moist soil’ is hard to define.
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Picture shows a buried bare copper ground wire cad welded on to a steel post. Buried ground wire is part of substation ground grid system and the connection to steel post is for equipotential bonding.
Read: Single point grounding in power systems
From the equation above, it can be seen that for earth resistance (Rg) of wire electrode to be low, the following parameters play role.
Soil Resistivity [ρ]: Low soil resistivity yields reduced earth resistance. Relationship between soil resistivity and electrode resistance is linear.
Length of Electrode [L]: Doubling the electrode length will reduce the resistance roughly by ~ 40%. However, extremely long conductors are not recommended due to poor transient performance. Instead, ‘crows foot’ (star) arrangement or a buried grid (mesh) provides better performance.
Diameter of Electrode [d]: Increasing the conductor diameter only has very limited influence on earth resistance. Typical buried conductors are either 2/0awg [70mm2] or 4/0 AWG [120 mm2].
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Buried wire electrodes are integral part of substation ground grid layout. Substation ground grid or ground mesh consists of many such buried electrode conductors that are joined together by irreversible connections such as cad weld. In a large substation there may be many locations at which fault current may be injected making ground grid (mesh) the perfect choice. Ground grids are often supplemented by having ground rods at periodic intervals for even better performance. Ground grid using buried conductors help to stabilize ground potential rise during fault condition. Buried ground grid conductors are often covered with a layer of crushed stones to increase surface resistance and improve the touch and step potential performance.
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Often buried wire electrodes are arranged in some pattern such as star formation (crow’s foot) for improved surge impedance performance and to achieve overall lower earth resistance when compared to a single long conductor.
Reference [1]: Power system grounding and transients; A.P. Sakis Meliopoulos