When a worker falls to another level, they should come to rest suspended by their personal fall arrest system. For for that to happen, however, some informed forethought and calculation has to occur.
Clearance Requirements
A fall arrest system has three basic components:
Depending on the type used, each of these parts add some length to the overall arrest mechanism, and thus the clearance requirement. From top to bottom, the elements go like this:
- Anchorage/connection length
- Lanyard length
- Shock absorber length
- D-ring slide
- Height of the worker (D-ring to heels)
- A margin of safety
Some specification for each of these is given in OSHA 29 CFR 1926.500 and the Canadian standard CSA Z259.16-04.
According to fall protection standards, the lanyard connecting the harness to the anchorage has to include a shock absorbing device. Either the rip-stitch or stretching shock absorber lanyards are designed to partially give way in the process of a fall. They lengthen under high tension to dissipate some of the energy of the falling mass so that less force is placed on the worker as they come to rest. This is important to note, because the length of the shock absorber mechanism after the fall will contribute to the overall fall clearance calculation.
Calculating Total Fall Clearance
The total fall clearance is the sum of the free fall distance, deceleration distance, D-ring shift, D-ring height and safety factor. Some of these exact v
alues are not known beforehand, but for practical purposes three of these can be safely estimated: the D-ring shift, D-ring height, and safety factor.
The D-ring shift is the difference in D-ring position when the harness is simply being worn versus its position when the worker's full weight is suspended. Usually this is factored in at one foot – probably more than is realistically needed for a properly donned harness.
The D-ring height is generally assumed to be 5 feet for workers up to 6 feet tall, as the D-ring should sit between the shoulder blades. For workers taller than 6 feet, the D-ring height inclusion should be increased accordingly.
Finally, the fall clearance calculation includes a safety factor that may vary from one application to the next. It is typically factored in at about 2 feet, but can be increased to make sure to account for any unforeseen or slightly miscalculated clearance requirements. This is ultimately the distance that remains between the soles of the worker's boots and the ground after the fall. Why cut that closer than it needs to be?
(Learn more in Four Steps to Calculating Fall Arrest Distance.)
Self-Retracting Devices and Fall Clearance
Self-retracting lanyard systems use a braking mechanism that decelerates the fall, which, according to OSHA 29 CFR 1926.502, still must occur in under 3.5 feet. Theoretically, the self-retracting lanyard does offer the advantage of potentially lowering the free fall distance to under 2 feet. This means that the fall clearance can be somewhat less when these devices are used.
However, when using self-retracting devices, it is possible to play out a long length of cable and walk a distance away from the anchorage point. This creates the potential for a swing fall. If the played out line distance is greater than the distance to the ground (minus the worker's height), the worker will come in contact with the ground. In a case like this, total fall clearance calculations are only part of the picture and have to be combined with appropriate safe work practices.