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]