A safety block (101, 201) for use in a fall arrest system, the safety block comprising: an attachment device (143, 243) for attaching the safety block to a support structure; a safety line drum mounted for rotation within a housing and upon which a safety line is wound; a bob sheath (180, 280) provided proximate to the end of the safety line; a speed responsive engagement mechanism (10, 11) responsive to the speed of rotation of the drum, which is activated above a predetermined rotational speed of the drum to engage the drum or a component connected to the drum in order to inhibit rotation of the drum; a rewinding or re-spooling mechanism arranged to act to rotate the drum to rewind the safety line onto the drum in the absence of sufficient tension in the safety line to pay out the line; and an exit arrangement (144, 244) provided in the housing; wherein the exit arrangement comprises an elongate slot (170, 270) extending from a lower end to an upper end of the safety block and arranged to permit the safety line to exit the housing over a range of different exit positions to the side of a line between the attachment device (143, 243) and the axis of the safety line drum, wherein as the rewinding mechanism rewinds the safety line into the housing toward its maximum extent the position at which the safety line exits the slot (170, 270) moves along the slot, such that the slot serves as a guide to raise the exit position of the safety line as an end of the safety line is retracted into the block (101, 201), wherein a docking collar is provided at the top of the elongate slot and under the influence of the rewinding or re-spooling mechanism the bob sheath (180, 280) is guided upwardly along the slot until it locates in the docking collar. an attachment device (143, 243) for attaching the safety block to a support structure; a safety line drum mounted for rotation within a housing and upon which a safety line is wound; a bob sheath (180, 280) provided proximate to the end of the safety line; a speed responsive engagement mechanism (10, 11) responsive to the speed of rotation of the drum, which is activated above a predetermined rotational speed of the drum to engage the drum or a component connected to the drum in order to inhibit rotation of the drum; a rewinding or re-spooling mechanism arranged to act to rotate the drum to rewind the safety line onto the drum in the absence of sufficient tension in the safety line to pay out the line; and an exit arrangement (144, 244) provided in the housing; wherein the exit arrangement comprises an elongate slot (170, 270) extending from a lower end to an upper end of the safety block and arranged to permit the safety line to exit the housing over a range of different exit positions to the side of a line between the attachment device (143, 243) and the axis of the safety line drum, wherein as the rewinding mechanism rewinds the safety line into the housing toward its maximum extent the position at which the safety line exits the slot (170, 270) moves along the slot, such that the slot serves as a guide to raise the exit position of the safety line as an end of the safety line is retracted into the block (101, 201), wherein a docking collar is provided at the top of the elongate slot and under the influence of the rewinding or re-spooling mechanism the bob sheath (180, 280) is guided upwardly along the slot until it locates in the docking collar.