Maximum Arresting Force

Maximum Arresting Force is one of those epic fall protection terms. I say it’s epic because it plays an important role in the safety of anyone who uses a fall protection system or device. Those words also appear on the label of all Rigid Lifelines fall protection systems (and if it makes the label, it must be important). But, it also has a really snazzy abbreviation (M.A.F.) that we will be using periodically throughout this blog.

So, why do you need to care about maximum arresting force?

Maximum arresting force is the largest amount of force that the fall protection system and the person attached to the system will experience as generated by the deceleration device.

OSHA 1910.140(d)(1) provides the system performance criteria for a personal fall arrest system. This section states that “the employer must ensure that personal fall arrest systems must: (i) limit the maximum arresting force on the employee to 1,800 pounds (8 kN).

This figure is based on the use of a full body harness. OSHA 1910.140(b) requires the use of a harness that will “distribute the fall arrest forces over at least the thighs, pelvis, waist, chest, and shoulders.” This requirement means that climbing harnesses that secure only the waist and thighs, like those used in recreational climbing, are not acceptable for use with a personal fall arrest system.

ANSI Z359 (2009 version) defines the maximum arresting force as:

2.106–Maximum Arresting Force: The peak force measured by the test instrumentation during arrest of the test weight in the dynamic tests set forth in these standards.

(In the 2007 version of ANSI Z359 Fall Protection Code, this definition is listed as number 2.93)

The deceleration device in a fall protection system is the part of the system that has the most control over the amount of arresting force that a person will experience. The amount of arresting force that a deceleration device exerts is typically established during the manufacturing process.

The arresting force of an energy absorbing lanyard (also known as a Rip Stitch Lanyard) is established during the stitching process. The more stitches there are in the fabric, the higher the force that the lanyard will exert during deceleration. Meanwhile, the arresting force for a self-retracting lifeline is determined by the amount of torque that is placed on the main bolt in the center of the ratchet pawl device. A higher amount of torque on the bolt will result in a higher arresting force being exerted, whereas a lower amount of torque will lower the arresting force.

The flip-side of the arresting force of individual fall arrest lifelines is that lifelines with a lower arresting force require greater distance to come to a complete stop. A longer stopping distance spreads the deceleration force over a greater span, which can reduce both the average arresting force and the maximum arresting force.

SRLs with a maximum arresting distance of 24 inches are classified as Class A, while SRLs with a maximum arresting distance of 54 inches are classified as Class B. Both Class A and Class B have an M.A.F. of 1,800 pounds, but a Class B SRL may produce a lower overall arresting force. Additionally, the M.A.F. may not be reached in an actual fall event, depending on the circumstances, so a Class B SRL can further reduce the actual force felt by a worker during a fall event. Class B SRLs should only be used when the distance to a lower level allows a sufficient factor of safety for a 54-inch maximum arresting distance.

The physical science for fall protection systems is important because it ultimately determines how much bodily damage people will experience on a fall protection system. The numbers get more complicated if the fall protection system uses a wire as an anchorage point instead of a rigid track. Wire systems get more complicated because you need to factor in deflection rates and distances in addition to the force and deceleration speed of the deceleration device. Consult a safety professional to help you determine the right fall arrest solution for your system to minimize the risk of injury due to arresting forces.