30 CFR § 75.327
Air courses and trolley haulage systems.
(a) In any mine opened on or after March 30, 1970, or in any new working
section of a mine opened before that date, where trolley haulage systems are
maintained and where trolley wires or trolley feeder wires are installed, an
authorized representative of the Secretary shall require enough entries or
rooms as intake air courses to limit the velocity of air currents in the
haulageways to minimize the hazards of fires and dust explosions in the
(b) Unless the district manager approves a higher velocity, the velocity
of the air current in the trolley haulage entries shall be limited to not
more than 250 feet per minute. A higher air velocity may be required to limit
the methane content in these haulage entries or elsewhere in the mine to
less than 1.0 percent and provide an adequate supply of oxygen.