Electrical Considerations in Establishing Working Clearances for Energized Line Maintenance

Abstract

The minimum clearance requirements for live line work, initially published in 1968 as "Recommendations for Safety in Live Line Work", was based on rod gap data from 13 sources. Since that time much has been learned about the parameters affecting the electrical behavior of air, both alone and in the presence of insulation, especially for large gaps. It has been shown that there is a critical wave shape associated with different gap geometry, which raises questions about the use of rod gap critical flashover data as the basis for working clearances. There are also concerns over the effect of atmospheric conditions on clearances and how to apply the correction factors developed to correct for "non-standard" atmospheric conditions. To establish safe working clearances, the maximum electrical stress that can appear at the work site must be determined and the distance required to withstand that stress under all working conditions. This paper discusses various types of voltage stress that can appear at the work site, as well as the factors that affect work site insulation strength. If all worst case conditions are used simultaneously to determine working clearances, recent work would suggest that clearances would be increased over those required by the present Guide. However, the experience to date with both HV and EHV circuits indicates that there is no significant safety problem with the working clearances being used by operating utilities.