By The Stationery Office
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Additional info for BS 7430:1998 Code of Practice for Earthing
BSI 05-1999 14 Selection of an earthing conductor and connection to an electrode Selection of the material for an earthing conductor should take into account compatibility with the material of the earth electrode and, for a conductor installed in the ground, the corrosive effect of the soil. Information provided in clause 11 for electrodes is relevant to bare earthing conductors also. Aluminium or copper-clad aluminium conductors should not be used in contact with soil or in damp situations and in any case should not be used to make the final connection to an earth electrode.
BSI 05-1999 A calculated value of electrode resistance can be expected to be of only limited accuracy because of uncertainty as to the true value of soil resistivity and its homogeneity. Soil resistivity is dependent on moisture content and temperature as well as on soil constituents, so that it can vary seasonally and progressively due to hydrological trends such as changing water tables or continuous drainage. It is therefore advisable to check the resistance of an electrode both on installation and at regular intervals thereafter.
An accuracy of 2 % is more than adequate, and accuracies of the order of 5 % are usually quite acceptable. The best method of measurement is illustrated in Figure 11. A measured current is passed between electrode X, the one being tested, and an auxiliary current electrode Y. The voltage drop between electrode X and a second auxiliary electrode Z is measured and the resistance of the electrode X is then the voltage between X and Z divided by the current flowing between X and Y. The source of current and the means of metering either the current and voltage or their ratio are often, but not necessarily, combined in one device.