Experience and continuing rapid development in electrical insulation have demonstrated the need for periodic review and revision of this document. During the time since the 1962 revision of this document, there has been a need for a more distinct separation of insulation systems and insulating materials. Confusion in nomenclature be tween insulating materials and insulation systems has continued to exist. This present document attempt to overcome this conflict through the complete separation of insulating materials from insulation systems and by the introduction of the temperature index concept for insulating materials. To clarify this separation a distinct section, Part IV, is now provided for insulating materials.

These Principles are intended to serve as a guide in the preparation of IEEE and other Standards that deal with the selection of temperature limits and the measurement of temperature for specific types of electric equipment. They include an outline of the fundamental considerations that determine the selection of temperature limits, appropriate methods of temperature measurement, and a discussion of the factors to be considered in applying the principles to specific cases. Guiding principles are also included for the development of Test Procedures for the thermal evaluation and classification of electrical insulating materials and of insulation systems for use in electric Equipment.