Evaluation of shedded polymeric insulators for use on BR 25 kV overhead lines

Whilst the mechanical and tracking/weathering properties of polymeric insulators are not in doubt, there remains the problem of shed punctures reported both from BR service and in the literature whereby after a period of outdoor exposure small punctures appear at the root of the sheds. These punctures effectively short out the sheds thereby reducing the creepage distance, and the flashover performance should be impaired. Polymeric insulators from world wide sources have therefore been examined on long-term tracking tests, shed puncture tests and coating adhesion tests to assess their suitability for use on BR. Sixteen insulator designs were acquired for tests. Materials included polycarbonate, epoxy resin, epdm, silicone elastomers and PTFE. All but two had glass fibre cores, one being of a porcelain core and one without a core but using the shed resin in a series of compressive shells. Several insulators of each design were subjected to a series of tests to assess bonding integrity, tracking resistance and shed puncture resistance