Fundamental aspects of temperature classification

For almost half a century temperature classification of electrical insulation has been a primary basis for the rating of electrical equipment and apparatus. In 1913, Lamme of Westinghouse and Steinmetz(1) of General Electric, recognized in a joint AIEE paper that electrical insulation is expendable at a rate which is variable with operating temperature. Their work laid the basis for the establishment of a system of temperature classification which was later officially recognized in AIEE No. 1. The original concept for the selection of these limiting temperatures was that the electrical and mechanical properties of the insulation must not be impaired by the prolonged application of the limiting insulation temperature. However, as time went on it became apparent that changes in characteristics of insulation take place slowly at these limiting temperatures. Thus, the concept of thermal endurance developed gradually and became coupled with temperature limitation.