. . . F1 A F2worker shall not be guilty of an offence under this Act in the case of any failure to return to the surface within the time limited by this Act if he proves that without default on his part he was prevented from returning to the surface owing to means not being available for the purpose.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F3

If a F2worker is below ground for a longer period during any consecutive twenty-four hours than the time fixed by this Act he shall be deemed to have been below ground in contravention of this Act unless the contrary is proved.