IEC method of calculation minimum approach distances for live working

Abstract

After 10 years and as many drafts, the proposed IEC standard 61472 is now in the final voting stage. The standard, when published, will define the IEC method of calculating minimum approach distances (MAD) for live working for use worldwide. The standard 61472 provides a method of calculating MAD for voltages between 1 kV and 800 kV AC. In common with other standards, the MAD is taken to consist of two components: an electrical component, DU, and an ergonomic component, DE. The component DU is calculated to minimize the risk of breakdown for all voltages expected at the worksite. The rationale for calculating DU is based on IEC Publications 71-1 and 71-2. The component DE depends on work procedures, training, workers' skill, type of construction, contingencies such as inadvertent movement and errors in judging distances. Guidelines for selection of DE are included in IEC 61472, but without giving specific values. Atmospheric conditions and specific details of the worksite (such as the effects of broken insulators and electrically floating conductive objects) are counted for through the application of correction factors. Guidelines for selecting the values for these factors are included. This paper provides an overview of the IEC method of calculating MAD, its background and theoretical foundations, and gives comparisons with the IEEE/ESMOL method, which is defined in IEEE Std 516-1995. The paper concentrates on systems voltage levels above 72.5 kV, and on phase-to-ground distances.