Fall Arrest Distance Calculation
Fall arrest distance calculation determines the minimum vertical clearance needed for a personal fall arrest system (PFAS) to stop a worker before striking a lower level. It accounts for free fall distance (before deceleration) and total fall distance (to full stop), including lanyard length, energy absorber deployment, harness shift, worker height, anchor location, and a safety factor.
On a shop floor, fall arrest distance calculation is performed before selecting or rigging a harness, lanyard, lifeline, or anchor to ensure sufficient clearance below the worker. OSHA and WorkSafeBC require accounting for lanyard length, energy absorber/deceleration distance, worker height, harness/D-ring shift, anchor location, and a safety factor, as the worker can drop significantly after system engagement. If clearance is insufficient, fall restraint, different anchor positions, self-retracting lifelines, or task rearrangement are used instead of accepting short clearance.
- Ignoring actual clearance below the work point: Crews measure only lanyard length and forget deceleration distance, D-ring shift, and worker height, causing bottoming out.
- Using an energy-absorbing lanyard where clearance is too short: Shock absorbers increase total fall distance, and WorkSafeBC warns to account for this; if insufficient, fall restraint or other configurations may be needed.
- Assuming one standard formula fits all anchors: Free fall changes with anchor geometry (above, level, or below D-ring); OSHA notes free-fall distance varies with anchor position.
What is the difference between free fall distance and total fall distance?
Free fall distance is the distance traveled before the PFAS begins deceleration; total fall distance is the full distance from the start of the fall to complete stop.
Why is deceleration distance critical?
Because an energy absorber or shock-absorbing lanyard stretches during arrest, adding distance after system engagement; OSHA and WorkSafeBC require this in clearance planning.
When should fall restraint be preferred over fall arrest?
When available vertical clearance is insufficient for a safe arrest distance, OSHA training materials note that a fall restraint system might be used instead.