9. The manager shall ensure that, apart from those persons who are going to or leaving their place of work at the beginning or end of a shift, not more than 50 persons are employed below ground in any part of the mine unless—
(a)there are two separate intake airways into that part of the mine which are connected only in such a way that in the event of a fire, transmission of the products of combustion from one airway to the other is prevented so far as is reasonably practicable; or
(b)there is one intake airway which is constructed of suitable fire resistant materials and is free, so far as is reasonably practicable, from the risk of fire.