This standard gives principles for the development of test procedures to evaluate the thermal endurance of solid electrical insulating materials and simple combinations of such materials. The results of accelerated thermal endurance tests, which are conducted according to prescribed procedures, may be used to establish temperature indexes for insulating materials. Thermal endurance tests and temperature indexes can be used to provide relative comparisons between the thermal capability of new materials and older, more accepted materials. Temperature indexes, while giving guidance on materials, should not in themselves be taken to predict the relationship between temperature and life of an insulation material or system in service. Thermal endurance tests provide a means for determining the rate at which important properties of insulating materials deteriorate irreversibly as a function of temperature and time. Such tests and the temperature indexes derived from them can aid in selecting insulating materials for use in electrical equipment operating under a wide variety of service conditions. Tests for specific insulating materials are not within the scope of this standard. Procedures should be developed for each important kind of insulating material, such as wire enamel, varnish, sheet, and tape, etc. The test procedures covered by this standard apply to solid insulating materials, including processed compositions of raw materials and simple combinations thereof, before they are fabricated into insulating structures identified with specific parts of electric equipment. These procedures may or may not apply to the aging characteristics of dielectric fluids or of porous materials impregnated with dielectric fluids. A distinction between such material tests and tests to evaluate the performance of insulation systems is necessary because the details of the particular use of a material may not be known to its manufacturer. General principles for designating the thermal capability of insulating systems are given in IEEE Std 1-1969 [11]. See also IEEE Std 99-1980 [12] and IEC 611-1978 [10].