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A method is described for evaluating the life of cable insulation subjected to the varying temperature and time electrical service conditions of a locomotive. This type of evaluation becomes more important as the current ratings of higher fuel efficiency locomotives are increased, subjecting cable insulation materials to higher operating temperatures. Insulations evaluated by this proposed technique are first tested at several elevated constant temperatures to determine their thermal aging characteristics. Based on the material performance data, the insulation life consumed by the thermal stresses imposed by the locomotive duty cycle is calculated. This method can be readily applied to measure the effects of temperature cycles on cable life, objectively compare the performance of different insulations, and assess the design margin of locomotive cables. It may also be used to establish Association of American Railroads (AAR) specification requirements for thermal aging tests based on the conditions of actual field application. A method for evaluating cable life through testing and duty cycle analysis has been developed and verified by examination of cables in service. A new high performance insulation developed by the General Electric Wire and Cable Business Department is shown to offer reliable service life under the varied and demanding operating conditions of today's higher performance locomotives.