Cable Management
installed for: TT and IT Earthing systems, earthed generator sets, to conform to Regulation 411.4.2, swimming pools having a PME earthing facility, agricultural premises, mobile or transportable units, EV charging installations, and prosuming installations.
Where earth electrodes are installed, Regulation 542.2.1 requires that the earth electrode can withstand damage and take account of possible increases in resistance due to corrosion. The means of connection must be electrically and mechanically sound (526.1) and be accessible for inspection, testing and maintenance (526.3).
Earth electrode resistance Depending on the requirement of a particular regulation, the resistance of the earth electrode will have a maximum permitted value. For example, for a TT Earthing system, Regulation 411.5.3 details the conditions that must exist
There are many factors that will impinge upon the resistance of the connection to Earth. These will include the nature and properties of the soil within a location, rainfall, and the nature of the earth electrode. For example, light clay soils will have a relatively low resistivity (c 5 m), whilst granite has a resistivity in the region of 1000 m.
where an RCD is used for fault protection. This regulation requires the disconnection time to be limited to: 0.2 s for fi nal circuits (411.3.2.2) or 1 s for distribution circuits (411.3.2.4), and
Table 41.5 details the maximum earth fault loop impedances permitted for a range of values of RCDs.
However, due to the risks associated with drying and freezing conditions, BS 7430 references a note under Table 41.5 of BS 7671 referencing a value in excess of 200 Ω not being stable.
The earth electrode resistance is less a function of the diameter of the electrode and more to do with the depth that the electrode is driven, the ability of the rod to be driven into the ground without damage, and the number of electrodes connected in parallel.
Clause 9.5.3 in BS 7430 details how the resistance of rod electrodes may be calculated. Where rod electrodes are to be paralleled to reduce the resistance, rods will have to be separated by at least 6 m to gain the maximum benefit (Figure 14a BS 7430). The formulae to determine the resistance of a single electrode and parallel electrodes are detailed in clauses 9.5.3 and 9.5.4 of BS 7430. Clause 9 of BS 7430 details many more methods of connecting electrodes, and reference should be made to this standard.
Conclusion
Earth electrodes are a necessary requirement for many parts of BS 7671. BS 7430 details the recommendations needed to ensure earth electrodes are correctly selected and installed. Unicrimp, part of the Scolmore Group of companies, supplies all the necessary elements for the safe installation of rod type earth electrodes.
ewnews.co.uk
June 2023 electrical wholesaler | 31
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