Stay Clear with the Right Gear

Here's How to Calculate Accurate Fall Distances and Clearances for Fall Arrest Systems

By Craig Firl, Project Manager, DBI-SALA and PROTECTA -- Industrial Distribution, 4/1/2006

Anytime an employee works on a structure where he or she could fall more than six feet, it is necessary to have a fall arrest system in place. Unfortunately, if the proper fall distance or clearance has not been calculated, the system's effectiveness is lessened, and can cause greater injury if the worker falls. Knowing the required clearance for the particular type of fall arrest system in use is crucial, and can minimize the potential for injury in the event of a fall.

Calculating fall distance and allowing for proper clearance is a time-worthy task that is relatively easy once the basic factors involved are understood; to do so requires knowledge of the terms used in the calculations.

Free fall distance is the distance or height that the attached person would fall before the lanyard or connecting system becomes taut and starts to absorb energy and arrest the fall. A free fall is typically unobstructed.

Deceleration distance is the distance after the free fall necessary for the connecting system (for example, a lanyard or self-retracting device) to stop the attached person at the onset of fall arrest. For an energy-absorbing lanyard, some refer to this as the tear-out distance.

Safety factor distance is the amount of distance that is added into the clearance calculation to allow for unknowns and provide an added margin of safety.

Required fall clearance distance is the total distance needed below a worker in order to safely stop him without the chance of hitting an obstruction. This typically involves the free fall distance plus the deceleration distance plus the safety factor distance.

An anchor point or anchorage is the fixed or moveable point to which the fall arrest system connecting component is attached. An example of this would be an overhead beam.

Fall arrest systems with energy-absorbing lanyards are the easiest systems for calculating required clearance. First, by using the surface that the employee is standing or working on as a base point, determine the free fall distance by measuring how far his feet can move or travel downward before the lanyard becomes taut if he falls.

If the fixed anchorage point for the lanyard is parallel with the harness' dorsal d-ring when standing, and the lanyard is six feet, the free fall distance will be six feet. If the anchorage point for the lanyard is two feet above the dorsal d-ring when standing with a six foot lanyard, the free fall distance will be four feet.

Next, add in the deceleration distance. This is typically three-and-a-half feet for an energy-absorbing lanyard. Finally, add in a safety factor (normally two feet) as an overall safety distance and to allow different possibilities, including the harness/dorsal d-ring sliding during fall arrest and/or the harness tightening around the body.

Using all of the above information, a standard equation for calculating the required clearance for a fall arrest system using an energy-absorbing lanyard is: free fall distance plus deceleration distance plus safety factor distance equals required clearance.

Here is an example: using a six-foot, energy-absorbing lanyard with a fixed anchorage point two feet above the dorsal d-ring on the harness when standing, the required clearance is nine-and-a-half feet (four-foot free fall, three-and-a-half foot deceleration distance and a two-foot safety factor). This is all measured from the initial surface on which the worker is standing.

If the anchorage point for the energy-absorbing lanyard is not fixed, which would be the case if it was attached to a horizontal lifeline, an additional amount needs to be added for the deflection of the horizontal line. This amount is typically provided by the supplier or designer of the horizontal lifeline system and will vary based on the type of material the line is made of, the amount of tension (or sag) in the line and the amount of energy absorption or type of energy absorber in the horizontal system.

Calculating the required clearance distance of a fall arrest system that uses a self-retracting lifeline is slightly more difficult. This is because the SRL compensates while the worker moves about below the device, making the initial location of a fall a movable point.

While standing with the SRL attached overhead to a fixed anchorage, the required clearance calculation would be as follows: fall arrest distance (three-and-a-half feet maximum) plus safety factor (normally two-and-a-half feet when using an SRL).

If an SRL is attached overhead from a standing position, six feet of clearance is needed (arrest distance of three-and-a-half feet maximum plus two-and-a-half feet of safety factor). Again, this is measured from the initial surface on which the worker is standing. When a worker is seated or crouching down, modifications need to be made to the clearance calculation to account for additional fall distance. If the overhead SRL is attached to a horizontal lifeline (HLL), it is necessary to figure in the HLL deflection distance in the same manner as when using an energy-absorbing lanyard.

One additional factor can come into play and should be considered when calculating the required clearance distance for a fall arrest system that uses an SRL: the added vertical drop from a swing fall. Because of the mobility an SRL provides, it is possible to be working out to the side of the SRL and not directly below it. If that is the case, the pendulum swing upon completion of a fall must be added to the vertical free fall distance and, therefore, factored into the required clearance. In some cases, the added vertical drop distance in a swing fall can be substantial. For example, if a worker is 15 feet to the side and the SRL is 30 feet overhead, the added drop distance when swinging is four feet.

The most difficult fall arrest systems to calculate are those with a vertical lifeline/energy-absorbing lanyard/rope grab system. With this type of system, three components must be accounted for: the lifeline, the rope grab and the lanyard.

To calculate clearance for this type of system, you can base the clearance distance from the location of the rope grab prior to the fall. From this initial starting point, add in the lanyard length, deceleration distance of the lanyard/rope grab, the workers body height, rope lifeline stretch and a safety factor. Please note that the amount of stretch in the lifeline can be large given the type of lifeline and the amount of line above the worker.

For example, a worker is 50 feet below the anchorage point of his vertical lifeline, using a three foot energy-absorbing lanyard attached to a rope grab. The required clearance can be computed to approximately 18 feet based on lanyard length (three feet) plus deceleration distance of lanyard/rope grab (three-and-a-half feet) plus body height (six feet) plus lifeline stretch (three feet) plus safety factor (two feet). Again, measure from the initial starting position of the rope grab.

Calculating the required fall distance for a fall arrest system may initially appear to be unimportant and not worth the time investment. But if the required clearance distance is unknown or miscalculated, the results of a fall can be much worse. Thankfully, most fall arrest system component manufacturers provide data that makes the calculation easier and painless. The benefits of taking the time are limitless, and give those valued employees confidence in the equipment that may one day save their lives.

For more information on DBI-SALA and PROTECTA and its line of products, call Craig Firl at (800) 328-6146, email him at cfirl@capitalsafety.com or visit www.dbisala.com.