A method and device for rendering safe the opening or removal of a machine door which is normally kept locked to guard machinery or an area which is potentially hazardous. The device includes a lock unit having a lock body for connection to the frame panel of machinery to be guarded and supports an electrical switch. A key unit includes a key body which supports a rotatable control knob and a key adapted to cooperate with the lock body when the door is closed on the frame panel. The key body includes a radially extending peripheral flange formed with a hole towards its outer edge which rotatably receives the head of a screw. The threaded end of the screw cooperates with a threaded hole in the frame panel when the door is closed and must be screwed fully into the hole to fully close the door onto the panel. The control knob which can be depressed and rotated to OFF and ON positions includes a mask in the form of an arcuate flange which renders the head of the screw accessible for operation only when the control knob assumes a particular angular OPEN position. The screw must be screwed into the threaded hole in the frame unit and the control knob depressed and rotated to the ON position before power is supplied to the machinery. Unlocking cannot be effected until the control knob is turned to the OFF position and further turned to the OPEN position to unmask the screw head whereupon the screw can be unscrewed from the frame panel to allow the door to be opened. The delay produced by the time taken to unscrew the screw allows rotating machinery to come to rest before the door can be opened.