[Code of Federal Regulations]
[Title 30, Volume 1]
[Revised as of July 1, 2002]
From the U.S. Government Printing Office via GPO Access
[CITE: 30CFR18.51]

[Page 104-105]
 
                       TITLE 30--MINERAL RESOURCES
 
  CHAPTER I--MINE SAFETY AND HEALTH ADMINISTRATION, DEPARTMENT OF LABOR
 
PART 18--ELECTRIC MOTOR-DRIVEN MINE EQUIPMENT AND ACCESSORIES--Table of Contents
 
             Subpart B--Construction and Design Requirements
 
Sec. 18.51  Electrical protection of circuits and equipment.

    (a) An automatic circuit-interrupting device(s) shall be used to 
protect each ungrounded conductor of a branch circuit at the junction 
with the main circuit when the branch-circuit conductor(s) has a current 
carrying capacity less than 50 percent of the main circuit conductor(s), 
unless the protective device(s) in the main circuit will also provide 
adequate protection for the branch circuit. The setting of each device 
shall be specified. For headlight and control circuits, each conductor 
shall be protected by a fuse or equivalent. Any circuit that is entirely 
contained in an explosion-proof enclosure shall be exempt from these 
requirements.
    (b) Each motor shall be protected by an automatic overcurrent 
device. One protective device will be acceptable when two motors of the 
same rating operate simultaneously and perform virtually the same duty.
    (1) If the overcurrent-protective device in a direct-current circuit 
does not open both lines, particular attention shall be given to marking 
the polarity at the terminals or otherwise preventing the possibility of 
reversing connections which would result in changing the circuit 
interrupter to the grounded line.
    (2) Three-phase alternating-current motors shall have an 
overcurrent-protective device in at least two phases such that actuation 
of a device in one phase will cause the opening of all three phases.

[[Page 105]]

    (c) Circuit-interrupting devices shall be so designed that they can 
be reset without opening the compartment in which they are enclosed.
    (d) All magnetic circuit-interrupting devices shall be mounted in a 
manner to preclude the possibility of their closing by gravity.