MSHA - Code of Federal Regulations - 30 CFR 57.22205
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MSHA - Title 30 CFR
30 CFR § 57.22205
Doors on main fans (I-A, II-A, III, and V-A mines).


In mines ventilated by multiple main fans, each main fan installation shall be equipped with noncombustible doors. Such doors shall automatically close to prevent air reversal through the fan. The doors shall be located so that they are not in direct line with explosive forces which could come out of the mine.