Conductivity measurements on aging of synthetic beams for catenary support structures

For railways systems the overhead contact wire is normally electrically insulated from the portals or poles by means of insulators. The size of these insulators adds to the required distance between the contact wire and the grounded portals. In tunnels and under viaducts this distance is kept to a minimum for economical reasons. A possible option to decrease the required space is to abandon the concept of insulators, but to use electrically insulating beams as catenary supports. A major point of concern in the application of fibreglass-reinforced synthetics is the electrical behaviour with respect to aging. In this paper DC conductivity measurements are presented on a synthetic fibre glass-reinforced beam, which is in service for about twenty years. The voltage was varied up to 25 kV, and the current could be measured with sensitivity better than 1 nA during the field measurements, corresponding to over 10¹³ Ω. Reference measurement on a small sample of the same material showed a conductivity of five orders of magnitude lower. The surface conductivity can be attributed to deposited material, probably carbon and metal from the catenary, in conjunction with moisture.