Determination of Insulation System Thermal Class: History and State of Standards

Interest in the development of an insulation system for a device will lead one to the determination of the thermal class of said insulation system. There are a series of standards that can be utilized to determine the thermal class of an electrical insulation system (EIS). These can range from IEEE standards to UL standards and IEC standards, and others, depending on the region of the world. But the basis for most of these standards is IEEE 117: IEEE Standard Test Procedure for Thermal Evaluation of Systems of Insulating Materials for Random-Wound AC Electric Machinery. This standard was developed many decades ago and is based on actual testing data from a series of motors, not simply theoretical considerations.Also developed shortly after IEEE 117 is IEEE 259 and IEEE 304. These standards use the same method for testing, but each utilizes a different model that is specific to their application area. For IEEE 117 (random wound), IEEE 259 (transformers) and IEEE 304 (dc machines). UL 1446 as well as the related IEC 61857 documents are to be considered as commonalities will be reviewed. The premise for the testing, as well as the future application of these standards for emerging markets such as wind, automotive, or transportation electrification. This paper strives to educate the reader through a review of the history of IEEE 117 as well as exploring the development of other standards that are utilized for thermal class determination of insulation systems.