The persons to whom this Convention applies shall be employed under a system providing for at least four shifts.

The hours of work of such persons shall not exceed an average of forty-two per week.

This average shall be calculated over a period not exceeding four weeks.

The length of a spell of work shall not exceed eight hours.

The interval between two spells of work by the same shift shall not be less than sixteen hours: Provided that this interval may where necessary be reduced on the occasion of the periodical change-over of shifts.