(a) Rock dust to be used to render coal dust inert shall come within the following specifications:

Combustible matter not more than five percent by volume, quartz or free silica particles not more than five percent by volume, and not unduly absorbent of moisture and preferably light in color. Rock dust shall be pulverized so that 100 percent will pass through a 20 mesh screen and 70 percent or more will pass through a 200 mesh screen.

(b) Rock dust shall be applied and maintained upon the top, floor, and sides of all open places, passages, and haulageways in such quantities that the incombustible contents of mine dust that could initiate or propagate an explosion will not be less than 65 percent, but the incombustible content in the return air courses shall be no less than 80 percent. Rock dust shall be so applied and maintained to include the last open breakthrough of rooms and entries and to within 40 feet of the faces or closer if necessary.

(c) In back entries and air courses, rock dust barrier protection in lieu of generalized rock dusting may be authorized by the chief of the division upon request by the operator. The chief of the division will prescribe the methods of protection that may be substituted.

(d) Where methane is present in any ventilating current, the percentage of incombustible content shall be increased one percent and four-tenths percent for each one-tenth percent of methane where 65 and 80 percent, respectively, of incombustibles are required.

(e) Mines or locations in mines that are too wet or too high in incombustible content for a coal dust explosion to initiate or propagate are not required to be rock dusted during the time such conditions prevail.