[Code of Federal Regulations] [Title 30, Volume 1] [Revised as of July 1, 2006] From the U.S. Government Printing Office via GPO Access [CITE: 30CFR77] [Page 648] TITLE 30--MINERAL RESOURCES CHAPTER I--MINE SAFETY AND HEALTH ADMINISTRATION, DEPARTMENT OF LABOR PART 77_MANDATORY SAFETY STANDARDS, SURFACE COAL MINES AND SURFACE WORK AREAS OF UNDERGROUND COAL MINES--Table of Contents Subpart J_Low- and Medium-Voltage Alternating Current Circuits Sec. 77.900 Low- and medium-voltage circuits serving portable or mobile three-phase alternating current equipment; circuit breakers. Low- and medium-voltage circuits supplying power to portable or mobile three-phase alternating current equipment shall be protected by suitable circuit breakers of adequate interrupting capacity which are properly tested and maintained and equipped with devices to provide protection against undervoltage, grounded phase, short circuit, and over-current.