30 CFR § 57.4761 - Underground shops.
To confine or prevent the spread of toxic gases from a fire originating in an underground shop where maintenance work is routinely done on mobile equipment, one of the following measures shall be taken: use of control doors or bulkheads, routing of the mine shop air directly to an exhaust system, reversal of mechanical ventilation, or use of an automatic fire suppression system in conjunction with an alternate escape route. The alternative used shall at all times provide at least the same degree of safety as control doors or bulkheads.
(a) Control doors or bulkheads. If used as an alternative, control doors or bulkheads shall meet the following requirements:
(1) Each control door or bulkhead shall be constructed to serve as a barrier to fire, the effects of fire, and air leakage at each opening to the shop.
(2) Each control door shall be—
(i) Constructed so that, once closed, it will not reopen as a result of a differential in air pressure;
(ii) Constructed so that it can be opened from either side by one person or be provided with a personnel door that can be opened from either side;
(iii) Clear of obstructions; and
(iv) Provided with a means of remote or automatic closure unless a person specifically designated to close the door in the event of a fire can reach the door within three minutes.
(3) If located 20 feet or more from exposed timber or other combustible material, the control doors or bulkheads shall provide protection at least equivalent to a door constructed of no less than one-quarter inch of plate steel with channel or angle-iron reinforcement to minimize warpage. The framework assembly of the door and the surrounding bulkhead, if any, shall be at least equivalent to the door in fire and air-leakage resistance, and in physical strength.
(4) If located less than 20 feet from exposed timber or other combustibles, the control door or bulkhead shall provide protection at least equivalent to a door constructed of two layers of wood, each a minimum of three-quarters of an inch in thickness. The wood-grain of one layer shall be perpendicular to the wood-grain of the other layer. The wood construction shall be covered on all sides and edges with no less than 24-gauge sheet steel. The framework assembly of the door and the surrounding bulkhead, if any, shall be at least equivalent to the door in fire and air-leakage resistance, and in physical strength. Roll-down steel doors with a fire-resistance rating of 1 1/2 hours or greater, but without an insulation core, are acceptable provided that an automatic sprinkler or deluge system is installed that provides even coverage of the door on both sides.
(b) Routing air to exhaust system. If used as an alternative, routing the mine shop exhaust air directly to an exhaust system shall be done so that no person would be exposed to toxic gases in the event of a shop fire.
(c) Mechanical ventilation reversal. If used as an alternative, reversal of mechanical ventilation shall—
(1) Be accomplished by a main fan. If the main fan is located underground:
(i) The cable or conductors supplying power to the fan shall be routed through areas free of fire hazards; or
(ii) The main fan shall be equipped with a second, independent power cable or set of conductors from the surface. The power cable or conductors shall be located so that an underground fire disrupting power in one cable or set of conductors will not affect the other; or
(iii) A second fan capable of accomplishing ventilation reversal shall be available for use in the event of failure of the main fan;
(2) Provide rapid air reversal that allows persons underground time to exit in fresh air by the second escapeway or find a place of refuge; and
(3) Be done according to predetermined conditions and procedures.
(d) Automatic fire suppression system and escape route. If used as an alternative, the automatic fire suppression system and alternate escape route shall meet the following requirements:
(1) The suppression system shall be—
(i) Located in the shop area;
(ii) The appropriate size and type for the particular fire hazards involved; and
(iii) Inspected at weekly intervals and properly maintained.
(2) The escape route shall bypass the shop area so that the route will not be affected by a fire in the shop area.