Abstract: A safety unit (11) in the form of a network or mesh for location across an opening to receive and restrain an object falling into the opening. The network or mesh is a trellis (12) comprising: a first array (13) of parallel rigid members (13A); a second array (14) of parallel rigid members (14A); and pivoting mechanism whereby each member (13A) of the first array (13) is pivotably attached to at least some of the members (14A) of the second array (14) so that the first and second array can be pivoted relative to one another between a first housed, configuration wherein the difference between the width (W′) of the trellis (12) and the length (L′) of the trellis (12) tends to a maximum; and a second, opened, configuration wherein the difference between the width (W) of the trellis (12) and the length (L) of the trellis (12) tends to a minimum.