Procedures for demonstrating the adequacy of new Class 1E transformers, located in a mild environment of a nuclear power generating station, to perform their required safety functions under postulated service conditions are presented. Single- and three-phase transformers rated 601 V to 15 000 V for the highest voltage winding and up to 2500 kVA (self-cooled rating) are covered. Because of the conservative approach used in the development of this new standard for new transformers, the end-point criteria cannot be used for in-service transformers.