Measurement of overhead line electrical clearances of importance to the safety of persons

There are currently no international standards for the determination of withstand distances to ensure safety of personnel from power lines during their normal operation. The safety distances that are used within each country are generally specified in a deterministic manner by the utilities themselves or by their trade association. Experience has proved these safety distances to be adequate. The distances to be used in live line working have been considered by the IEC and a standard has been issued on the method of calculation. This standard uses the latest information from IEC 71 (on insulation co-ordination) and includes knowledge for Cigre WG 33-07. It is recognised by IEC TC 11 that there is a need for an international standard to determine electrical safety distances. The method is consistent with the IEC method for the determination of the Live Line Minimum Approach Distances and a Committee Draft (reference number 11/138/CD) has been circulated for national comments. The approach taken in this Committee Draft is similar to that of the live line document, but recognises that additional overvoltages on the power system can occur (in particular overvoltages associated with lightning and circuit energisation and reenergisation). The work described here is supportive of the approach being taken by the IEC TC 11 Project and the work of Cenelec TC 11. The U/sub 50/ values and standard deviations of a number of key configurations have been measured.