Technical reference pp267-296 05/09/2012 12:27 …

1 Furse, Wilford Road, Nottingham, NG2 1EB Tel: +44 (0)115 964 3700 Email: Web: standardsInstallation of a well designed earthing system isa fundamental requirement for all structures andelectrical systems (at all voltages).Effective earthing safeguards people from risk ofelectric shock, in that hazardous-live-parts shall not beaccessible and accessible conductive parts shall not behazardous live , and ensures a low impedance route toearth for currents in the electrical system, under bothnormal and fault number of national and international standards havebeen published which define earthing system designparameters for structures, electrical equipment andsystems, including:l BS 7430:2011 Code of practice for protectiveearthing of electrical installationsl BS 7354.

2 1990 Code of practice for design of highvoltage open terminal stationsl IEEE Std 80:2000 IEEE Guide for safety in ACsubstation groundingl ENA TS 41-24 Guidelines for the design,installation, testing & maintenance of mainearthing systems in substationsThe design, specification, inspection and periodictesting of earthing systems should follow the guidanceand recommendations provided by these 7430:2011 - Protective earthing ofelectrical installationsBritish Standard BS 7430 provides guidance onearthing of general land-based electrical installationsin and around buildings in the UK, including:l Low voltage installation earthing andequipotential bonding for general, industrial and commercial buildings, locations with increased risk, rail systems etcl The interface between low voltage and highvoltage substationsl earthing of generators and Uninterruptible PowerSupplies (UPSs)

3 Supplying low voltage installationsBS 7430 defines the elements for creating anappropriate earthing arrangement for a low voltageinstallation, including a main earthing terminal,protective conductors, earthing conductors and circuit protective conductors, and the use of earthelectrodes to dissipate currents to the general mass of the earthing arrangement through the useof equipotential bonding measures to cover exposedand conductive metal parts is further recommended toprotect against step and touch voltages, and to removerisk of dangerous classes of low voltage electrical installation aredefined within the standard - TN-S, TN-C, TN-C-S, TTand requirements for earthing these lowvoltage installations are defined in the IET WiringRegulations, BS 7671:2008(+A1:2011).

4 The earthing arrangement should be sufficientlyrobust to ensure it lasts the lifetime of the installation,and be protected from mechanical damage andcorrosion so that it remains capable of carrying themaximum expected current, under both normal andfault 7430 therefore defines selection parameters for theearthing arrangement, the size and material forconductors, earth electrodes etc, and makes clear theneed for careful consideration of site conditions (soilcomposition and resistivity).Taking actual measurements at the site is important togauge the expected effectiveness of the earthingarrangement, and guidance is provided for measuringresistance calculations for earth plates, earth rods, ringconductor and foundation earth necessary in high resistivity areas or on rockyground, treatment of the soil through use of an earthelectrode backfill is recommended to improve earthcontact earthingBS 7354, IEEE std.

5 80 and ENA TS 41-24 reference therequirements for earthing of design and specification of an appropriateearthing arrangement for substations is essential toprovide a low impedance path for earth, fault, andlightning currents to earth, and to protect personnelon site from potentially fatal step and touch standards provide guidance on (but not limited to):l Maximum permitted step and touch voltagesl Methods for calculating earthing system designl High voltage earth electrode selection, includingtype, material & sizel Switching and busbar arrangementl Equipotential bonding l Insulation co-ordinationPrimary to these standards is limiting earth potentialrise (EPR)

6 Under earth fault conditions so that step andtouch potential limits are not exceeded, and earthresistance remains as low as , use of an earthing grid consisting ofhorizontal cross-bonded earthing conductors isrecommended, with additional earth rods where thesite includes low resistivity layers beneath the earth rods mitigate seasonal variations in earthgrid resistance at the grid s burial power earthing solutions have been specified for many installations worldwide. For moreinformation, or to discuss a particular requirement,please contact reference pp267 -296 05/09/2012 12:27 Page 286