Cal. Code Regs. tit. 8, § 7058 – Fire Doors and Fire Bulkheads.

(a) If the Division considers that a serious fire hazard exists at a mine, fire doors or fire bulkheads shall be installed so as to prevent smoke and gases from endangering the men. Such doors may also be used for mine ventilation.
(b) Fire doors shall be constructed of steel and shall be set in steel or concrete walls so constructed that fire on one side cannot pass to the other side when the door is closed.
(c) Fire doors shall fit closely so they can be readily made gas tight when closed. They shall be provided with suitable latches or devices so they can be readily opened from either side without the use of tools, but cannot be opened by a reversal of the air current.
(d) Fire doors shall be tested each month and maintained in good working order.
(e) Where practical, fire bulkheads shall be installed in solid rock away from timbers and stoped-out areas. Fire bulkheads shall be of noncombustible construction if located within 100 feet of mine timbers or other combustible materials.
(f) Fire bulkheads shall be installed in such manner that fire, smoke, and gases on one side of the bulkhead cannot pass to the other side.
Note: In order to provide for sampling air or gas behind the bulkhead, in case fire or other emergency makes such sampling desirable, it is recommended that a metal pipe be installed through the bulkhead. Such pipe should be not less than 1-inch inside diameter and provided with a cap or gate-type valve on the outer end.