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The earthing is done
1 - to ensure that no current carrying conductor rises to a great potential with respect to general mass of earth than its designed insulation and
2 - to ensure that that potential of non – current carrying metal work does not rise to a value such that a person may get shock on touching it.
The supply system neutral point is earthed at power station or substation to serve the former purpose whereas the latter purpose is served by the regulations which specify that all non-current carrying metalwork of high voltage lines shall be permanently and efficiency connected to earth.
For this purpose a continuous earth wire shall be run and connected to earth at four points in equal wire shall be run and connected to earth at four points in every 1.6 km, the spacing between the points being as nearly equal as possible, or alternatively, the metal work shall be connected to an effective earthing device at each individual support