Ageing of insulating materials and equipment insulation in service and in tests

After a discussion of the concepts of ageing of insulations and of the endurance of insulating materials, the importance of verifying the relevance of ageing to insulation failure in equipment is pointed out. Information regarding frequency distributions of service stresses and of the stress dependence of insulation strength is also required. After a discussion of ageing versus changes of property, problems of evaluating the endurance of insulating materials, functional testing of insulation systems, and of the acceptance testing of equipment insulation are briefly reviewed. The paper concludes by recommending certain areas which should be studied in order to improve rational, consistent and economic evaluation methods.