(a) General rule.--Underground use of inby and outby diesel-powered equipment, including mobile equipment, stationary equipment and equipment of all horsepower ratings, shall only be approved, operated and maintained as provided under this chapter, except for emergency fire-fighting equipment to be used specifically for that purpose.

(b) Required attendant.--All diesel-powered equipment shall be attended while in operation with the engine running in underground mines. For purposes of this subsection, “attended” shall mean an equipment operator is within sight or sound of the diesel-powered equipment.

(c) Required certifications or approvals.--Inby and outby diesel-powered equipment may be used in underground mines if the inby or outby diesel-powered equipment uses an engine approved or certified by MSHA, as applicable, for inby or outby use that, when tested at the maximum fuel-air ratio, does not require a MSHA Part 7 approval plate ventilation rate exceeding 75 c.f.m. per rated horsepower. If MSHA promulgates new regulations that change the MSHA Part 7 approval plate ventilation rate, the c.f.m. requirement per rated horsepower shall be revised either up or down on a direct ratio basis upon recommendation of the technical advisory committee in accordance with section 424.¹