How to Correctly Calculate Your Fall Clearance
November 20, 2019
If you work at heights, whether on a cradle or on a scaffolding, you know that calculating fall clearance and swing fall is paramount to safety. Miscalculating this distance can result in serious injuries and even death.
There are a lot of confusing definitions in the world of fall protection, so we thought we’d help you get a better understanding of the key concepts involved in determining fall clearance.
What is fall clearance?
Fall clearance is the distance required to prevent workers from hitting the ground or lower level, in case of a fall. Here is how to calculate fall clearance:
Free fall or free fall distance - the distance before the fall arrest system begins to apply force and slow the worker down or arrest the fall. According to OSHA standards, this distance shouldn’t be more than 6 feet (1.8 meters) and it depends on lanyard length and where the attachment point is located.
“Personal fall arrest systems, when stopping a fall, shall:
- limit maximum arresting force on an employee to 900 pounds (4 kN) when used with a body belt;
- limit maximum arresting force on an employee to 1,800 pounds (8 kN) when used with a body harness;
- be rigged such that an employee can neither free fall more than 6 feet (1.8 m), nor contact any lower level;
- bring an employee to a complete stop and limit maximum deceleration distance an employee travels to 3.5 feet (1.07 m); and, have sufficient strength to withstand twice the potential impact energy of an employee free falling a distance of 6 feet (1.8 m), or the free fall distance permitted by the system, whichever is less.”
A qualified person is responsible of selecting the components, materials, anchorage and connectors to fit the application, the hazards and the work environment. This is called a competent person, which requires professional training.
When it comes to anchorage and anchorage systems, it’s important to select stable anchorages. Location is also key in reducing the possible free fall distance, to prevent swing fall hazards and provide clear space in the potential fall path. Anchorage locations above the harness attachment point are preferred in this case.
The trigger heights are dictated by specific industry standards. In 2017, OSHA changed the trigger height for general fall protection from 10 feet to 6 feet, for instance. Check out this OSHA fact sheet for more information.
The shock-absorbing lanyard or self-retracting lanyard - both play a critical role in a fall arrest system, as they reduce fall arrest forces in the event of a fall.
The lanyard is a flexible line securing a full-body harness to an anchorage point. Energy-absorbing lanyards reduce the energy the user’s body receives in the event of a fall. Additionally, shock-absorbing lanyards also provide deceleration distance, reducing the fall-arresting forces by 60-90%. Shock-absorbing packs can also be included in non-shock absorbing lanyards.
Self-retracting lifelines (SRLs) are known to reduce the free-fall distance, while enabling greater horizontal and vertical mobility than standard lanyards. Other benefits include shorter activation and arresting distances, which reduce the risk of impact with the ground or lower level. Standard shock-absorbing lanyards allow for up to 6 feet or free-fall distance before activating, while SRLs require less than 2 feet.
Regardless of the PPE option, connecting devices need to be in line with the type of work performed, the environmental conditions, the system component compatibility and of course, product quality and regulations.
Length of the lanyard – the measured length of the lanyard or retractable device the workers uses to connect the harness to the anchorage point. Avoid tying a knot in lanyard to make it shorter, as it also impacts strength. Instead, go for an adjustable lanyard.
Deceleration distance – This is the additional vertical distance between a worker’s body harness attachment point when activated at the location of the attachment point when the person stops falling. OSHA limits this distance to 3.5 feet or less.
Manufacturer instructions on the fall arrest system need to be reviewed to determine the deceleration distance, as well as the elongation. Vertical elongation is the temporary elastic stretch incurred by the lifeline and helps reduce jolting when stopping a fall.
Height and weight of suspended worker should also be considered when calculating fall clearance.
Safety Factor – it’s advisable to allow for an additional safety distance (3 feet or 1 meter) below the feet of the fallen worker.
Because calculating fall clearance is not always easy, here is a useful video on how to user the Miler Fall Clearance App to calculate your fall clearance.