Combined HV-LV earthing of substations in the built environment

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Session 7 Paper 1, University College London, 16-17 April 2015

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Earthing and bonding are important safety requirements for Electrical systems within the built environment. Increasingly buildings are equipped with distribution substations stepping down from high voltage (11kV or 6.6kV) to normal utilisation voltages (400V/230V). Prior to the issue of BS EN 50522:2010, the common practice for allowing the earthing installations of the high and low voltage systems to be bonded together, was to prescribe an overall combined earth resistance of 1Ωor less.

The aim of this paper is to discuss two situations; the first being where an earth resistance of less than 1Ωis not scientifically proven to meet the stress voltage safety requirement, the second demonstrating that in some other cases it is an over-design.