A fall arrest block comprises of a retractable lifeline made from synthetic fibre rope, webbing or wire rope that is stored on a reel within a protective housing. The reel is spring-biased so as to wind the retractable lifeline in. This ensures that the lifeline is always under light tension to give the shortest possible length between the housing and the user. The lifeline tension allows the user to move freely around the work area.
The working length of fall arrest blocks is usually in the range of 2m to 50m and is measured as the distance from the bearing point of the housing connector to the bearing point of the hook when the lifeline is fully extended. In the event of a fall, the lifeline is rapidly pulled out of the housing until it reaches a critical velocity (the “lock-on speed”) at which point the brake mechanism engages and the clutch mechanism decelerates the user over a short distance (maximum 2.0m total – EN360).