Abstract: A safety apparatus for a derrick on an oil drilling rig includes a cable having a first end for connection to one side of the derrick at a predetermined location spaced below the crown. A series of pulleys is secured to the oil derrick to define a predetermined path for the cable extending from the fixed first end across the derrick at the predetermined location and then down the derrick to a location adjacent the lower end. The portion of the cable extending across the derrick is positioned in the path of a traveling block as it is pulled up the derrick. An alarm device is linked to the cable to produce an output alarm signal if the cable is pulled up by the traveling block as it approaches the top of the derrick, giving the operator a warning that the traveling block should be stopped to avoid impact with the crown.