The invention relates to a safety device for a crosshead of a decoking plant arranged by means of a carrying device to be liftable and lowerable on profiles of a frame above a drum adapted to receive material, such as coke, and guided on guide rails of the profiles by means of rolls, and carried by a cable of a winch slung around return pulleys at the top of the frame and around a carrying pulley rotatably supported at the top end of the crosshead; and equipped with brakes for stopping the crosshead in an emergency, when the carrying device fails, and for intercepting the crosshead and fixing it on the guide rails or on the profiles. For a safety device, a test apparatus is provided for testing the brakes on site, comprising a carrying device, such as a brace, attachable on the frame, on which the crosshead is mountable for simulating a free fall of the same by means of a release device, and an arrestor device for the crosshead releasably attachable on the frame at a distance below the brace.