30 CFR § 57.22603
Blasting from the surface (II-A mines).
(a) All development, production, and bench rounds shall be initiated
from the surface after all persons are out of the mine. Persons shall not
enter the mine until the mine has been ventilated for at least 15 minutes and
the ventilating air has passed over the blast area and through at least one
atmospheric monitoring sensor.
(b) If the monitoring system indicates that methane in the mine is
less than 0.5 percent, competent persons may enter the mine to test for
methane in all blast areas.
(c) If the monitoring system indicates that methane in the mine is
0.5 percent or more, the mine shall be ventilated and persons shall not enter
the mine until the monitoring system indicates that methane in the mine is
less than 0.5 percent.
(d) If the monitoring system is inoperable or malfunctions, the mine
shall be ventilated for at least 45 minutes and the mine power shall be
deenergized before persons enter the mine. Only competent persons necessary
to test for methane may enter the mine until the methane in the mine is less
than 0.5 percent.
(e) Vehicles used for transportation when examining the mine shall be
approved by MSHA under the applicable requirements of 30 CFR parts
18 through 36. Vehicles shall not be used to examine the
mine if the monitoring system is inoperable or has malfunctioned.