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 and a review of the elements to be considered in applying the principles to specific cases. Guiding principles are included for the development of test procedures for 1) Thermal evaluation of electrical insulating materials; 2) Thermal evaluation of insulation systems; 3) Thermal classification of insulation systems for use in rating electric equipment. For many types of equipment proper evaluation of their electrical insulation for service conditions requires controlled exposure to a range of aging stresses which may or may not include thermal aging. The aging factors of influence may include thermal, electric, ambient (environmental), and mechanical stresses. Thermal aging has been widely used as a single factor test method. The other factors may also be evaluated by single factor tests or they may be combined with or without thermal aging into a multifactor test. Guiding principles for developing insulation systems test procedures for single and multifactor aging are included in this standard. Insulating materials, as referred to herein, are substances in which the electric conductivity is very small (approaching zero) and provide electric isolation. Insulation systems, as referred to herein, include an insulating material or a suitable combination of insulating materials specifically designed to perform the functions needed in electric and electronic equipment. Other definitions necessary for the use of these principles are listed in Section 1. The Principles are presented in the following order: Part I. General; Part II. Evaluation of Thermal Capability of Insulating Materials; Part III. Limiting Temperatures and Their Measurement for Electrical Insulation Systems; Part IV. Evaluation of Electrical Insulation Systems In the application of these principles, variations will be necessary to suit the widely different types of equipment and service conditions that are considered in equipment standards. The temperature limits and other, provisions given herein are not intended to be used for rating or testing equipment for which specific IEEE, or other recognized standards based on these principles, are available.