The two most important aspects of fall arrest systems are force and clearance – in that order.
These are the basis for every decision made when designing a fall arrest system, and it specifies the equipment to be used with that system. That's because the force must be high enough to arrest a fall in as short of a distance as possible, yet still low enough not to injure the user as the fall is arrested.
In this article, we'll look at both of these factors and go through the calculations you can use to determine how much clearance you'll need when selecting fall protection equipment.
The Impact of Force on the Body
So, how do we go about determining what the maximum amount of force is that the human body can sustain without causing serious injury to the body?
Fortunately, we have a long history of scientific research and data to help us with these calculations. We can trace it back to the 1950s and 60s when researchers used pigs to study the effects of ejection seat acceleration on the body. This allowed them to determine the maximum force the body can withstand before serious trauma occurs, approximately 2,700 lbs. They also calculated the duration the force could exerted on the body, finding that high forces could be tolerated but only for extremely short periods of time (40 milliseconds or less).
These measurements were adopted by the fall protection industry to establish the upper limit of force that can be exerted on the human body when stopping or arresting a fall. To err on the side of caution, they decided to reduce the force by a third, to 1,800 lbs (8 kN), which has been universally accepted as the Maximum Arrest Force (MAF).
Personal energy absorbers (PEA) and other fall arrest devices were first designed to deploy at half the maximum allowable arrest force, which was a good place to start. The downside of using a PEA that deploys at a lower force, however, is that more deployment distance will be required to arrest the fall of a worker of the same weight and free fall as a worker using a PEA with a higher deployment force. This added deployment distance can make a difference to the outcome, since high forces can be exerted on the body for a short duration without causing serious trauma.
(Learn more about Suspension Trauma: The Danger of Fall Arrest Systems)
Getting the Clearance Right
The second most important aspect of a fall arrest system is clearance.
A fall arrest happens very quickly, but the user can travel quite a distance in that short time frame. There needs to be enough space for the worker to free fall far enough to generate sufficient energy to deploy their fall arrest system. Deploying the energy absorbing device requires some additional distance as well to slow the fall before stopping it.
Without sufficient clearance, the fall arrest system won't deploy in time to stop the user from hitting the ground or striking against an obstruction, thereby negating all the protection.
Required Clearance vs. Available Clearance
Understanding clearance is as important as understanding the dynamics of fall arrest and the forces involved to arrest a fall.
There are really two types of clearance: required clearance and available clearance.
The designer of the fall arrest system must first determine the required clearance – the clearance the system will need to arrest the fall of the worker. Then compare this value to the clearance available below the working surface to ensure that the worker will not hit, strike, or land on a lower level or obstruction.
The available clearance must be equal to or greater than the required clearance. This gives the fall arrest system some additional leeway in case the equipment does not deploy exactly as planned.
The Two Reference Points for Evaluating Clearance
There are also two distinct reference points that must be referred to when evaluating clearance. And there is a mathematical relationship between the two that the designer, competent person supervising the worker, and the worker using the fall arrest system all need to understand in order to be sure that the fall arrest system being used is actually going to do what it is intended to do.
These two reference points are the working platform (the surface the worker is standing on) and the location of the anchor (preferably above the working platform). The clearance below these two reference points are known as the clearance below the platform (Cp) and the clearance below the anchor (Ca). The relationship between these two reference points is the height of the anchor above the working platform (Ha).
The mathematical relationship between the two distinct reference points is represented by the formula:
Ca = Cp + Ha or by rearranging Cp = Ca – Ha
Which can also be rearranged to:
Cp = Ca – Ha
The most useful value here is the clearance below the platform (Cp) as this is the height of the working surface that the worker may fall from.
(Learn more in How to Choose Your Fall Protection Anchorage)
Fixed Lanyards, SRDs, and SRLs
There is a fundamental difference between a fall arrest system that uses a fixed length lanyard versus one that uses an automatic length lanyard (either a Self-Retracting Device or Self-Retracting Lanyard). The clearance required will depend on which of these is being used.
Calculating Clearance for Fixed Length Lanyard Systems
For a fixed length lanyard system, the clearance is calculated from the reference point of the anchor and the clearance below the anchor (Ca). The formula to calculate Ca for a fixed length lanyard is:
Ca = Ly + Xpea + 2.4m = Ly + Xpea + 8 ft.
Where:
Ly = fixed lanyard length (recommended maximum 1.8m = 6 ft)
Xpea = full deployment distance (elongation) of the personal energy absorber (PEA)
2.4m / 8 ft = a fixed value combining the height of the D-ring on the worker's back (typically 1.5.m [5 ft]) + stretch out of the worker (typically 0.3m [1 ft]) + 0.6m (2 ft) safety buffer
Once the Ca is determined, and knowing the height of the anchor above the working platform, Ha and Cp can be calculated from the formula:
Cp = Ca – Ha
Calculating Clearance for Automatic Length Lanyards
For SRLs and SRDs, clearance is calculated from the reference point of the platform and the clearance below the platform (Cp). The formula used to calculate Cp for an automatic length lanyard system depends on the position of the worker:
For a standing worker: Cp = 1.8m = 6ft
For a kneeling or crouching worker: Cp = 2.6m = 8.5ft
For a leading edge application: Cp = 4.5m = 15ft
The values are determined by the average free fall and lock-off of a typical clutching SRL as follows:
- Standing worker: 0.45m (1.5 ft) free fall + 0.45m (1.5 ft) lock-off + 0.9m (3 ft) safety buffer
- Kneeling or crouching working: 0.45m (1.5 ft) free fall + 0.45m (1.5 ft) lock-off + 0.8m (2.5 ft) worker stretch-out + 0.9m (3 ft) safety buffer
- Leading edge: 1.5m (5 ft) free fall + 2.1m (7 ft) lock-off + 0.9m (3 ft) safety buffer
Leave Clearance Calculations to Those Most Qualified
Clearance calculations are essential. Misjudging the amount needed can result in serious injury or a tragic fatality.
For that reason, this and all other aspects of fall arrest system design must be undertaken by a trained and qualified person in fall protection. Preferably, a professional engineer who is competent, qualified, trained, and experienced in the complexities of published standards such as CAN/CSA Z259.16-04 – Design of Active Fall Protection Systems, ANSI/ASSE Z359.6-2009 – Specifications and Design Requirements for Active Fall Protection Systems, or their equivalents.
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