California Code, Labor Code - LAB § 7975

A “kill” button capable of cutting off all electrical equipment shall be maintained in any gassy tunnel or underground mine. The safety representative or his designated representative shall cut off power at any time gas or vapor levels reach 20 percent of the lower explosive limit or more. Before work is restarted every employee underground shall be informed of the level of gas or vapor recorded, and a permanent record shall be called to the surface and retained in a special log.