Cal. Code Regs. tit. 8, § 7105 – Mechanical and Electrical Equipment for Gassy Mines.

(a) Before installing mechanical equipment in a gassy mine, consideration shall be given to the use of power which will not be a source of ignition in a gaseous atmosphere.
Where practical, locomotives, material hoists, scraper hoists, auxiliary fans, and other equipment in the mine should be driven by compressed air.
(b) Before installing or using any electrical equipment in a gassy mine, the Division of Industrial Safety shall be advised of actual conditions. Such installations or operations shall be made according to specific orders to be issued by the Division for the particular location.