Abstract: A system to prevent a crane turret and the boom carried thereby from rotating about the axis of the turret beyond a predetermined angular disposition, thereby to prevent the crane boom from being brought too close to high-tension wires or some other structure in the vicinity of which the crane is being operated. Means are provided for locked-in disposition at at least one, and preferably, two points about the 360.degree. arc circumscribing the turret with such means including projections adapted to trip limit switches extending radially outwardly from the rotatable turret. Each limit switch is connected into an electromagnetic circuit with the first switch to be tripped serving to actuate an audio and/or visual warning system, and the second switch to be tripped, an electromagnetic control of the fluid flow in a hydraulic or compressed air driving system.