Abstract: A safety device for cable climbers, hoists and winches and like apparatus which prevents the insertion of wrong size of cable is described. The device comprises a housing having a first hole mounted along the axis of the cable, a guide block having a second hole and mounted in the housing in alignment with the first hole, and a gate block having a third hole moveably housed along transverse axis of the guide block. Also described are means for aligning all three holes when a cable of right size is inserted.