By The Stationery Office
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Additional resources for BS 7430:1998 Code of Practice for Earthing
C. c. potentials are picked up by the electrodes if there is a traction system in the area. Both of these forms of interference can be eliminated by testing with alternating current at a frequency different from that of the interfering power currents and their harmonics. This is usually achieved by using a frequency of about 60 Hz to 90 Hz. © BSI 05-1999 If the source of power for the measurement is a hand driven generator, the frequency of the measuring current can be varied to obtain the best result.
When generation is by a number of sets which may operate in parallel, protective earthing of the generator frames and associated metalwork follows the same lines as that described for one machine. However, the method of system earthing for the windings is influenced by the magnitude of the circulating currents, commonly third harmonic, which can flow in the neutral/earthing conductors. g. zig-zag wound machines). Alternatively one of the following arrangements may be used. a) A neutral earthing transformer connected between the phases and earth.
Modern jointing techniques do not ensure electrical continuity of metallic pipes. g. for oil, gas, compressed air or drainage, should be bonded to the protective conductors but not used as the sole means of earthing. Where, in an existing installation, a service pipe has been used as the sole earth electrode, another means of earthing should be provided. 13 Earthing cathodically protected structures Cathodic protection is normally applied to wholly or partly buried ferrous structures in order to counteract electrolytic corrosion.