Principles are given 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. The test procedures apply to materials before they are fabricated into insulating structures identified with specific parts of electric equipment. Tests for specific insulating materials are not covered. The procedures may or may not apply to the aging characteristics of dielectric fluids or of porous materials impregnated with dielectric fluids.