We recently received a question concerning the difference between Fall Arrest Distance and Free Fall Distance. A safe fall protection program depends on knowing what both of these are. If both of these aren't known, a fall protection lanyard can be in use but the user can still hit the ground or another obstacle during a fall.
Fall protection lanyards contain shock absorbing packs. These packs can be an actual pack on the lanyard or they can be built into the lanyard itself. A stretchable lanyard, for example, has the absorbing pack built in. See the below picture.
If the shock absorber is deployed during a fall, the lanyard will extend out further than its initial length. This needs to be accounted for an any fall protection program. The only way to properly do so is to understand both Free Fall Distance and Fall Arrest Distance. If these are not properly calculated, an individual is at risk of crashing into the ground, a beam, or another obstacle in the fall path.
Free Fall Distance
The first thing that needs to be understood to avoid this problem is what is called Free Fall Distance. In order to calculate Free Fall distance, two points need to be established.
First, Point 1 is the point at which the lanyard attaches to the harness while in use by the worker on the working surface. Second, Point 2 is the point at which the lanyard end opposite the end attached to the anchor point would freely dangle. These points are pictured in the illustration below.
The distance between Point 1 and Point 2 equals the Free Fall Distance. This distance can't exceed six feet. If this distance is longer than six feet, there is a problem.
Free Fall distance is why you should always attach the lanyard above your head. If the lanyard is attached at your feet, the maximum Free Fall distance will almost always be exceeded. In such cases, a more expensive leading edge SRL will be required.
Fall Arrest Distance
Fall Arrest Distance is a second crucial feature of fall protection that needs to be properly understood. Fall Arrest Distance is the Free Fall Distance plus the greatest distance the lanyard's shock absorber extends when arresting a fall. It is pictured in the illustration below.
Most shock absorbers can extend the distance a worker will fall by as much as 42 inches (some more). In such a case, the Fall Arrest Distance equals the Free Fall Distance plus 42 inches. If your Free Fall Distance is 6 feet, then your Fall Arrest Distance equals 6 feet plus 42 inches. This means a worker will fall almost 10 feet before he or she is fully stopped!
It should be obvious at this point that the distance to the nearest obstruction also needs to be established. If the Fall Arrest Distance is long enough that the worker's feet can still hit the nearest obstruction, then an alternative solution must be found.
Generally, the easiest solution is to raise the anchor point. If that is not possible, a certain kind of SRL (self retracting lifeline) might be required.
Always factor in more distance than you think. As is evident in the illustrations, the worker's lower body extends well below the lanyard's point of attachment on the harness. This distance also needs to be accounted for.
It is also the case that, because of the force exerted in a fall, the d-ring on a harness will slide up a number of inches and that harness fabric will stretch. Each of these factors will add distance to the length of the fall.
There is no room for error when making these calculations. Be careful and diligent. Read your lanyard's instruction manual.