The interpretation of thermal endurance tests on electrical insulation

The thermal classification of insulating materials has a long history with the first significant consideration of the subject by Steinmentz and Lamme now fifty years old. Fifty years ago, insulation systems were relatively simple. Arbitrary separation into thermal classes defined by description and supported by operating experience was relatively simple. About 15 years ago it became very apparent that the old thermal classification of insulating materials by broad definition was no longer adequate for the extremely wide variety of materials which were rapidly becoming available.