Insulation for high-voltage a.c. railway electrification in Great Britain

The paper traces the development of ceramic and synthetic insulators used on the high-voltage a.c. electrification schemes of British Railways (BR). Details are given of operating experience and the factors which led to each successive development. Most of the earlier difficulties were due to severe pollution from steam locomotives, and the paper describes the measures taken to overcome these difficulties, and also the innovations which are now possible owing to the cessation of steam traffic. Another important aspect of insulation is the air clearance required, since this has a great effect on the extent and expense of civil-engineering work. The paper details the original requirements for air clearances and arrangement of the overhead equipment at bridges and tunnels, and also the present reduced requirements and the tests which led to their adoption.