International Activities in the Temperature Classification of Electrical Insulating Materials

Until recently, the definitions of the various insulation temperature classes in both the United States national and the corresponding international temperature standards were based upon chemical composition as largely determined by the words "organic" and "inorganic." With the advent of many new synthetic materials having thermal stabilities extending over a wide temperature range a new approach to insulation classification by "accepted test and experience" has been developed both in the United States and internationally. This has resulted in the revision in 1955 by the International Electrotechnical Commission (IEC) of IEC Publication 34 and issuance of the revised standard as IEC Publication 85 in 1957. It also resulted in a similar revision of AIEE No. 1 in 1956 and the issuance of the revised AIEE No. 1 and supplements 1D and 1E in June 1957. The evolution of this new approach to insulation temperature classification is reviewed with emphasis on its international aspects.