This report makes recommendations for new approval and testing criteria for high-voltage permissible loadcenters (transformer and switchgear, greater than 4,160 v) for underground U.S. mines and describes the development and testing of a prototype loadcenter to meet the criteria. The recommended criteria included new findings in several areas: minimum clearances between live conductors, special precautions for using organic insulators, tests for dielectric strength and electrical stress, and explosion testing. A research test program was conducted to evaluate the pressure rise caused by a high-energy arcing fault (10,000 a) with and without a simultaneous methane-air explosion inside the enclosure. The prototype loadcenter included a 1,250-k V.A, 7,200-v primary transformer inside a center compartment with separate, attached enclosures for high-voltage switchgear on one end and low-voltage distribution and protection on the other end. A comprehensive literature search was conducted, and a state-of-the-art review of research and industrial experience with high-voltage explosion-proof transformers is included.