MSHA - Code of Federal Regulations - 30 CFR 75.332
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MSHA - Title 30 CFR
30 CFR § 75.332
Working sections and working places.

(a)(1) Each working section and each area where mechanized mining equipment is being installed or removed, shall be ventilated by a separate split of intake air directed by overcasts, undercasts or other permanent ventilation controls.
(2) When two or more sets of mining equipment are simultaneously engaged in cutting, mining, or loading coal or rock from working places within the same working section, each set of mining equipment shall be on a separate split of intake air.
(3) For purposes of this section, a set of mining equipment includes a single loading machine, a single continuous mining machine, or a single longwall or shortwall mining machine.
(b)(1) Air that has passed through any area that is not examined under §§75.360, 75.361 or 75.364 of this subpart, or through an area where second mining has been done shall not be used to ventilate any working place. Second mining is intentional retreat mining where pillars have been wholly or partially removed, regardless of the amount of recovery obtained.
(2) Air that has passed by any opening of any unsealed area that is not examined under §§75.360, 75.361 or 75.364 of this subpart, shall not be used to ventilate any working place.