Cable Management
Electrical earthing – do you know the requirements?
Jake Green, group technical engagement manager, Scolmore Group, takes a look at earth electrodes and the relevant code of practice associated with their application.
Earth electrodes
BS 7430: 2011+A1:2015 is a code of practice for protective earthing of electrical installations. This standard is closely linked to the requirements of BS 7671, where relevant, and both should be considered when dealing with earthing requirements.
BS 7430 provides recommendations on meeting the requirements for earthing of electrical installations. The three areas include, protective earthing of low voltage installations, the interface between LV and HV substations (11 kV / 400 V), and protective earthing and changeover switch arrangements for generators. Earthing of a system (or equipment) requires a physical connection to the general mass of Earth. The connection should have resistance such that any means of protection will operate as designed. 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 Ωm), whilst granite has a resistivity in the region of 1000 Ωm. The presence of low volumes of rainfall will also have a significant impact on soil resistivity. For example, clay soil in locations having a normal rainfall exceeding 500 mm per year has a typical resistivity in the region of 5 – 20 Ωm, whilst in locations having a rainfall less than 250 mm will have a resistivity range of 10 – 100 Ωm. An earthing system should be of high integrity and robust construction. The use of an Earthing system using earth electrodes relies on the resilience of the earth electrode(s) for the safe functioning of the protection system within an installation.
The types of earth electrode can vary. Regulation 542.2.2 of BS 7671 recognises seven permitted types of earth electrode, including earth rods or pipes, tapes or wires, earth plates and the like. The most commonly selected earth electrode is the rod.
Application Earth electrodes are to be (or may need to be) 30 | electrical wholesalerJune 2023
ewnews.co.uk
“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.”
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