Cal. Code Regs. tit. 8, § 7060 – Fire Prevention--Underground.

(a) (4-9) All electrical equipment which might communicate fire to adjacent flammable material shall be of a type which will confine the heat and flames within the equipment, or it shall be so located and installed that flammable material will not be exposed.
(b) If the Division considers that a serious fire hazard exists at mine openings or underground stations, it may require that underground employees be protected by one or more of the following methods:
1. (1) By making fire resistant the mine openings for a distance of 50 feet from the surface.
2. (2) By making fire resistant underground stations and adjoining portions of the shaft.
3. (3) By making fire resistant underground buildings or enclosures used to house machinery, or as lunchrooms or places of assembly for men.
  It is recommended that the main shafts and haulageways be provided with fireproof sections at regular intervals.
(c) In mines where serious fire hazards exist, the division may require fire inspection of timbered areas after the crews have left the working places at the end of the shift.