The Mines (Control of Ground Movement) Regulations 1999

3.  In the case of face workings where powered supports are used–

(a)such supports should be advanced as soon as is practicable after extraction and set so that the maximum distance between the end of the beam closest to the face and the face shall be 0.5 metres;

(b)during normal production at any place where a machine is used which shears mineral to a depth exceeding 0.4 metres, the said machine must not be permitted to approach within 27 metres of any powered support which has not been advanced from the previous cut; and

(c)where an immediate forward support system is used, the supports shall be advanced–

(i)as close as is practicable behind the coal getting machine, and

(ii)in any event, no more than 10 metres behind the coal getting machine.