Abstract: A load arrestor for use in conjunction with a primary lift, the load arrestor comprising a spool about which winds a safety line attached to a load supported at least in part by the primary lift, a lock mechanism comprising a lock plate connected to and rotatable with the spool, the plate defining a plurality of slots; and at least one cylinder having a locking pin for insertion into at least one of the plurality of slots to inhibit rotation of the lock plate and the spool; a sensor for sensing motion of the spool; and a controller which sends a signal to the at least one cylinder to activate the locking pin when the motion of the hoist drum exceeds a predetermined limit. Alternate aspects of the load arrestor are also provided.