When choosing a fall arrest system, one of the most common mistakes is failing to consider, or incorrectly calculating, the minimum clearance we have to stop the fall safely.
This variable, the fall clearance distance, can make the difference in work at height between a safe fall arrest and an impact with the ground or a nearby structure.
In this post we explain how to calculate this distance and what factors we must consider to choose one type of system or another.
What Is Fall Clearance Distance?
Fall clearance distance, as the name suggests, is the vertical space from the anchor point where the worker is attached to the next obstacle (the ground) that is needed for the fall arrest system to work correctly.
What Factors Influence Clearance Distance?
When calculating fall clearance distance, we must take into account various factors or variables, such as:
The length of the connecting element we are going to use
One part of the fall arrest system is the connecting element, which connects the worker’s harness to the anchor point. Depending on the connecting element we use, a different length must be taken into account:
- Lanyards: Whether single or double, lanyards have a predefined length when we buy them. For example, we can find them in 1 m, 1.5 m, or up to 2 m. This length must be taken into account when calculating the fall clearance distance.
- Self-retracting lifeline: Retractable fall arrest devices lock in the event of a fall, and the length is whatever the worker has extended at that moment.
Length of the energy absorber
When working at high fall factors (1 and 2), a fall can generate enough force to activate the energy absorbers that the connecting elements have. These come in different lengths, and they must also be taken into account when calculating the fall clearance distance.
The fall factor
When working at height we can work at three different fall factors: factor 0, factor 1, or factor 2. To simplify a bit:
- Fall factor 0: the anchor is above the worker’s head; in the event of a fall, the worker falls 0 times the distance
- Fall factor 1: the anchor is at the same level as the anchor ring on the worker’s harness; in the event of a fall, the worker falls 1 times the distance.
- Fall factor 2: the anchor is below the worker, usually at foot level; in the event of a fall, the worker falls 2 times the distance.
This will help us know whether or not the energy absorber is activated in the event of a fall.
System elongation
Depending on the anchor device we work with, we must know and take into account the elongation or sag that will occur in the event of a fall, since for example:
- On a temporary lifeline it is usually around 10% of the line length (for example: for a length of 5 meters, we will have a sag of 50 cm)
- On a flexible cable lifeline, this sag is given by the manufacturer and the calculations and tests performed
- On a 795A anchor point there is no sag or elongation that we need to take into account
Safety distance
For safety, a minimum distance of 1 meter must always be allowed to ensure that the worker does not reach the ground in the event of a fall.
How Do You Calculate Fall Clearance Distance?
Fall clearance distance is the sum of the factors mentioned above, plus the height of the worker:
Fall clearance distance = Length of connecting element + Length of energy absorber + System elongation + Safety distance + Height of the person
Let’s look at it with different examples:
Fall clearance distance when working at fall factor 2
Suppose we are working on a roof and are anchored to a rail lifeline at floor level (fall factor 2), and our connecting element is a 1.5-meter lanyard with an absorber that reaches 1.8 m when fully deployed.
- Lanyard: 1.5 meters
- Absorber: 1.8 meters
- System elongation: 0 (negligible, since it is a rail line)
- Safety distance: 1 meter
- Height of the person: 2 meters (worst-case scenario)
Clearance distance = 1.5 + 1.8 + 0 + 1 + 2 = 6.3 meters
In other words, in this scenario we will need a working height of at least 6.3 meters so that the worker does not reach the ground in the event of a fall.
If, instead of a rail lifeline, the worker were working on a 5-meter temporary lifeline, they would need almost 7 meters of fall clearance to avoid reaching the ground:
- Lanyard: 1.5 meters
- Absorber: 1.8 meters
- System elongation: 0.5 meters
- Safety distance: 1 meter
- Height of the person: 2 meters (worst-case scenario)
Clearance distance = 1.5 + 1.8 + 0.5 + 1 + 2 = 6.8 meters
Fall clearance distance when working at fall factor 0
Suppose we are working on a roof and are anchored to a rail lifeline that is 2.2 meters above the ground (fall factor 0), and our connecting element is a 1.5-meter lanyard with an absorber that reaches 1.8 m when fully deployed.
In this case, since we are working at fall factor 0, with the anchor above the head, the worker would not reach the ground and would only travel the length of the connecting element, in this case 1.5 meters.
Key Tips for Choosing the Right System
For all these reasons, when we have to choose the fall arrest system, we must consider where we are going to work and what PPE we will use, since, as we have seen, it will influence whether or not we reach the ground in the event of a fall:
- Assess the work environment: The clear space we have in the event of a fall, that is, the distance to the ground
- Low fall factors: Try to prioritize working at fall factor 0, since it is the most favorable
- With short fall clearance distances, use self-retracting lifelines, since they lock in the event of a fall
- Consult the technical data sheets: These will tell us the elongation of the fall arrest system, the length of the absorber in the event of a fall, etc.
- Train and qualify personnel: Ongoing, up-to-date training lets teams know how to work safely and choose the most suitable PPE in each case
Conclusion
Correctly calculating fall clearance distance is essential to ensure the effectiveness of a fall arrest system.
Failing to take this value into account can put the life of the worker at height at risk. Therefore, as a manufacturer or supplier, it is essential to educate the end customer about these factors and offer solutions adapted to each real work-at-height situation.
Do you have questions about which system best suits your project? Contact us and our technical team will help you do the specific calculations and choose the safest solution.