An Experimental Investigation of Electrical Clearance in Air for Overhead Traction System

While converting the existing rail network to electric traction, provision of specified minimum requirement of air gap clearance under old bridges and tunnels has always posed problems. Demand in specified electrical clearance of air-gap often forced the Indian Railways in construction of new bridges for the traction overhead lines to pass when the existing bridges still had their life left, which resulted in very high cost of electrification schemes. Extensive experimental investigation work is presented in this article for the performance of air-gap on actual catenary of contact wire assembly. Measurements were made under natural conditions of rain, fog, fair weather and under artificial rain with ac power frequency, lightning and switching impulse voltages. Investigations were also performed with the insertion of a solid insulating barrier in the gap. Considering the severest condition that of stream of water falling directly on the live conductor from the grounded object, the requirement for minimum air-gap clearance could be recommended to reduce to 170 mm for short time and 220 mm for long time applications, which is 37% lower than the air-gap clearance practiced hitherto. Development of twin contact wire assembly forming horizontal bundle is recommended for reducing the air-gap clearance further.