MSHA - Code of Federal Regulations - 30 CFR 75.343
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MSHA - Title 30 CFR
30 CFR § 75.343
Underground shops.

(a) Underground shops shall be equipped with an automatic fire suppression system meeting the requirements of §75.1107-3 through §75.1107-16, or be enclosed in a noncombustible structure or area.
(b) Underground shops shall be ventilated with intake air that is coursed directly into a return air course.