MSHA - Code of Federal Regulations - 30 CFR 57.22215
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MSHA - Title 30 CFR
30 CFR § 57.22215
Separation of intake and return air (I-A, II-A, III, and V-A mines).

Main intake and return air currents shall be coursed through separate mine openings and shall be separated throughout the mine, except--

(a) Where multiple shafts are used for ventilation and a single shaft contains a curtain wall or partition for separation of air currents. Such wall or partition shall be constructed of reinforced concrete or other noncombustible equivalent, and provided with pressure-relief devices.

(b) During development of openings to the surface--

(1) Ventilation tubing approved by MSHA in accordance with 30 CFR part 7 or previously issued a BC or VT acceptance number by the MSHA Approval and Certification Center may be used for separation of main air currents in the same opening. Flexible ventilation tubing shall not exceed 250 feet in length.

(2) Only development related to making a primary ventilation connection may be performed beyond 250 feet of the shaft.

[52 FR 24941, July 1, 1987, as amended at 54 FR 30508, July 20, 1989]