Abstract: An embodiment of the invention discloses a restraint mechanism (12) of a braking device (10) for location in tubing (16) such as the casing of an oil or gas well. The braking device (10) forms part of a tool string which is run into the tubing (16) on a wireline and, in the event that the wireline breaks or is severed, the braking device (10) and the associated tool string falls through the tubing. The restraint mechanism (12) includes a sensor assembly (18), and a brake mechanism (22). The sensor assembly (18) senses the uncontrolled movement and provides an appropriate output causing activation of the brake mechanism (22), bringing the braking device (10) and tool string to a controlled stop.