30 CFR §57.22207

1. (a)Booster fans shall be approved by MSHA under the applicable requirements of 30 CFR part 18, and be—
   1. (1)Provided with an automatic signal device located so that it can be seen or heard by a person designated by the mine operator to give an alarm when the fan stops or when methane reaches the following levels:
      1. (i)1.0 percent at the fan in Subcategory I-A, Category III, and Subcategory V-A mines; and
      2. (ii)0.5 percent at the fan in Subcategory II-A mines.
   2. (2)Equipped with a device that automatically deenergizes power in affected workings should the fan stop; and
   3. (3)Equipped with starting and stopping controls located at the fan and at another accessible remote location.
2. (b)Booster fan installations, except for booster fans installed in ducts, shall be—
   1. (1)Provided with doors which open automatically when all fans in the installation stop; and
   2. (2)Provided with an air lock when passage through the fan bulkhead is necessary.