The intent of this standard is to establish a uniform method by which the thermal endurance of electrical insulation systems for dry-type specialty and general-purpose transformers can be compared. These insulation systems are intended for use in the types of transformers described in NEMA ST 1-1988 and NEMA ST 20-1992. In general, these dry-type transformers are used with the primary windings connected to secondary distribution circuits of 600 V nominal and below. Thermal degradation generally is one of the principal factors affecting the function of most insulating materials. It is the principal environmental factor in this standard. Other environmental factors, such as vibration, thermal shock, and moisture, are included to simulate operating conditions. These factors have been chosen in such a way as to develop and promptly disclose any significant weakness during the temperature aging of an insulation system. This standard provides a statistical method for establishing a life-temperature relationship for an insulation system. This life-temperature relationship is relative. To have any significance, it must be supported with adequate field service data or it must be compared to similar life test data for insulation systems with known field service reliability.