Guiding Principles in the Thermal Evaluation of Electrical Insulation [includes discussion]

The rating of electrical insulating materials for use at various temperatures for an adequate service life requires a knowledge of the manner in which the insulation deteriorates in service, and the rate of deterioration. This paper reviews some of the basic principles of the aging process in various classes of the newer materials, which should serve as a guide in developing the test information necessary in the new method of insulation classification. Specifically the paper summarizes the various mechanisms of insulation deterioration and describes the various chemical and physical changes which take place in insulating materials as a result of the application of heat. Rates at which these changes take place and the manner in which they are influenced by temperature, environment, and other factors are discussed. Test methods for evaluating materials and simple combinations of materials as well as systems of materials as they are used in various types of apparatus, are described. The manner in which these test methods may be used to develop new insulation standards and the analysis of the data derived from them are given special attention. A number of examples of aging behavior are given. Finally the paper outlines a specific proposal for the revision of AIEE and corresponding international insulation temperature standards. This includes the development and adoption of standard tests by which temperature ratings can be obtained on materials and systems of materials as they are used in various apparatus.