Fall clearance is the total amount of unobstructed vertical space needed below a worker to ensure that, in the event of a fall, the fall protection system has enough room to arrest the fall before the worker strikes the ground or another obstruction.
Measure fall clearance from the top down to determine freefall distance. "Freefall" is t
Steve Lee
Warren Faber, Engineer Manager, Pure Safety Group
he portion of a fall in which there is no force acting to arrest the fall. It's the distance from the beginning of a fall until the lanyard becomes taut or, for a self-retracting lifeline (SRL), the braking system engages and begins to arrest the fall.
Permitted freefall can vary from 0 to 12 feet. Fall protection manufacturers must indicate the maximum amount of freefall permitted (if any) for a connector, so it is important the competent persons who assemble fall protection solutions know how to properly calculate freefall to maintain compliance.
"Deceleration distance" is the portion of a fall in which the connector is actively working to arrest the fall. The amount of freefall, worker weight and location of the anchor relative to the location of the worker at the onset of the fall affect how much vertical distance the connector requires to fully arrest the fall. Per ANSI Z359, a shock-absorbing lanyard that permits 6 feet of freefall must arrest a fall within 48 inches.
A full-body harness will exhibit some degree of stretch as it is pulled from one direction by the connector and the opposite direction by the momentum of the worker, which needs to be added to the calculation. As specified by ANSI Z359.11, during dynamic testing, full-body harness stretch should not exceed 18 inches. Given that the ANSI Z359 test is designed to generate a minimum of 3,600 pounds of force on the harness and that complete fall protection systems must limit maximum arrest forces to 1,800 pounds, it is logical to use 18 inches as the maximum potential amount of stretch that could occur under even the most extreme circumstances.
Fall clearance doesn't have a mathematical safety margin calculation; however, like many other factors in fall protection, ANSI provides guidance in the Z359 Fall Protection Code. ANSI Z359 suggests a 2-foot safety margin be added to fall clearance calculations, which gives the worker confidence should the initial fall clearance calculation be slightly off.
Finally, take into consideration the swing of a potential fall. According to ANSI Z359, a "swing fall" is "a pendulum- like motion that occurs during and/ or after a vertical fall. A swing fall results when an authorized person begins a fall from a position that is located horizontally away from a fixed anchorage." Swing falls occur most frequently with the use of an SRL or vertical lifeline, which can allow the worker to walk 30 feet or more away from the anchor point. When the amount of extended lifeline exceeds the amount of fall clearance under a worker, the worker is at serious risk of a ground impact in the event of a fall.
Even if the worker does not impact the ground during a fall, they are still at risk of striking structures, equipment or other workers as they swing unrestricted underneath the anchor. This post-fall swinging is not constrained merely to a straight line, back and forth. Rather, it occurs in three dimensions, forming a cone underneath the anchor, which greatly expands the potential danger area. It takes great vigilance to ensure the area underneath a worker is free from hazards, since the dynamic nature of a jobsite means people, equipment, and materials are continually moving in and out of the danger zone.
Properly calculating fall clearance is one of the most important factors of configuring an effective personal fall arrest system. Given the number of variables to consider, it can also be one of the most challenging. Knowing the components of total fall clearance and how they work together can help you more accurately calculate this critical measurement to ensure workers remain safe in the unlikely event of a fall.
For more information, visit www.puresafetygroup.com or call (800) 466-6385.
