International activities on thermal evaluation of insulating materials

THE introduction of many new insulating materials in recent years has led to the realization that the present national and international insulation temperature standards are not only inadequate, but difficult to interpret and often lead to ambiguities. This has resulted in relatively recent decisions to revise not only our own but also the international insulation temperature standards. In the United States, the present version of AIEE Standard No. 1, covering the “General Principals Upon Which Temperature Limits Are Based in the Rating of Electric Machines and Equipment,” dates back to the 1930's except for some revisions and additions in 1947. The definitions for the various insulation classes (Classes O, A, B, and C) originally were written around two broad chemical groups of materials, namely, “organic” and “inorganic” materials. With the development of silicone and fluorocarbon materials another insulation class (Class H) was added in 1947. Its definition was written around what might be called these “organo-inorganic” materials. The problem of developing more accurately evaluative standards for electrical insulation, especially with respect to temperature limitations, has become of international importance. These three articles deal with this situation. The activities of four groups working on the several phases of insulation evaluation in the U. S. and the International Electrotechnical Commission's actions are reviewed. The principles upon which test methods should be based also are discussed in some detail and the value of working towards temperature classification by functional test is stressed.