Railway signal wire insulation investigations and monitoring of degradation undertaken by London Underground

Signalling systems on London Underground date from the 1940s onwards and consequently a wide range of polymeric insulated wire and cable is present. The investigations reported centre on the chlorinated diene rubber insulation surrounded by a lacquered glass braid used on earlier vital signalling equipment. The main degradation processes affecting the insulation are described and it is concluded that temperature is a critical factor; chloride salts formed on the surface of the conductor by reaction with chlorine released from the insulation also have a deleterious effect. The results of accelerated aging tests at 100°C are presented. These have been combined with observations of degradation in service in an Arrhenius type analysis that permits the lifetime of insulation in given time-temperature conditions to be estimated.