This standard provides principles for the development of test procedures to evaluate the thermal endurance of solid electrical insulating materials (EIMs) thermally aged in air. The test procedures use accelerated thermal endurance test(s), conducted in accordance with prescribed procedures herein. The thermal endurance value for a measured property can be assigned based on one of two following methodologies: -- Temperature index (TI), which establishes the temperature index value by means of a preselected time coordinate.; -- Relative temperature index (RTI), which establishes the temperature index value by comparison of the performance of a new material to the performance of the same property of a known material. For RTI, there is no preselected time coordinate; the correlation time is determined based on the results of the known material under laboratory conditions. Both TI and RTI are long-term thermal aging (LTTA) methodologies.