[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 649]
 
                       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.  77.901  Protection of low- and medium-voltage three-phase circuits.

    (a) Low- and medium-voltage circuits supplying power to portable or 
mobile three-phase alternating equipment shall contain:
    (1) Either a direct or derived neutral grounded through a suitable 
resistor at the power source;
    (2) A grounding circuit originating at the grounded side of the 
grounding resistor which extends along with the power conductors and 
serves as a grounding conductor for the frames of all the electric 
equipment supplied power from the circuit.
    (b) Grounding resistors, where required, shall be of an ohmic value 
which limits the ground fault current to no more than 25 amperes. Such 
grounding resistors shall be rated for maximum fault current 
continuously and provide insulation from ground for a voltage equal to 
the phase-to-phase voltage of the system.
    (c) Low- and medium-voltage circuits supplying power to three-phase 
alternating current stationary electric equipment shall comply with the 
National Electric Code.