20.—(1) In each shaft and outlet which is connected by a drift to a fan on the surface of the mine, which was so connected after the thirty-first day of July, nineteen hundred and forty-seven, and which is ordinarily used for winding or haulage there shall be provided and maintained an efficient air-lock.
(2) An inspector may serve on the manager of a mine a notice requiring the provision and maintenance of such an air-lock at any shaft or outlet specified in the notice being a shaft or outlet which is connected and used as aforesaid but which was not so connected after the said date. The provisions of Part XV of the Act with respect to references upon notices served by inspectors shall apply to a notice served under this paragraph and any of the following shall be a relevant ground of objection to such a notice, namely—
(a)that there is insufficient space for an efficient air-lock;
(b)that by reason of the shortness of the period during which the mine is expected to be worked the requirement is unreasonable;
(c)having regard to the provision made to ensure the proper ventilation of all parts of the mine, the requirement is unnecessary.
(3) If the Minister is satisfied that the requirements of this regulation are inappropriate to the circumstances of any mine, he may by notice served on the manager exempt that mine from those requirements.
(4) The provisions of this regulation shall not apply to a mine at which not more than thirty persons are employed below ground.