By The Stationery Office
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Extra info for BS 7430:1998 Code of Practice for Earthing
1 General Earthing of generating plant is provided to limit the potential of current-carrying conductors with respect to the general mass of earth and is generally necessary as an integral part of the protection against shock by indirect contact. Generator protective earthing is achieved by connecting the frame of the generator, associated exposed-conductive-parts and extraneous-conductive-parts to a main earthing terminal. The earthing terminal or bar is connected to an independent earth electrode and, where appropriate, to other earthing facilities associated with the installation.
This is to avoid the possibility of the fence becoming live and thus dangerous at points remote from the substation, or alternatively giving rise to danger within the resistance area of the electrode by introducing a good connection with the general mass of the earth. 6 m below the surface of the ground. 1 Vertical electrodes For vertical electrodes, the fraction E of the potential arising on the electrode(s) which appears at a point P on the ground surface is estimated approximately by the following equation: where 32 vi L = ---ri n is the number of electrodes; L is the buried length of an electrode, in metres (m); ri is the distance of point P on the ground surface to the ith electrode, in metres (m) (ri is greater than the radius of an electrode); d is the diameter of the electrode(s), in metres (m); s is the spacing between the electrodes, in metres (m).
It should be capable of dissipating the electrical energy in the earth path at the point at which it is installed under any condition of operation on the system. Failure of an electrode to meet its purpose is fundamentally due to excessive temperature rise at the surface of the electrode and is thus a function of current density and duration as well as electrical and thermal properties of the soil. In general, soils have a negative temperature coefficient of resistance so that sustained current loading results in an initial decrease in electrode resistance and a consequent rise in the earth fault current for a given applied voltage.