How it adds up
Picture the worker standing at the edge, then follow the D-ring down. It drops the length of the lanyard past the anchor, then the shock absorber opens and stretches, then the harness stretches and the D-ring slides up the back. The worker's feet hang below the D-ring by about the D-ring's standing height. Add a margin so the feet stop short of the ground, not at it.
- Free fall = lanyard length + D-ring height − anchor height
- Clearance needed below the working surface = lanyard length + deceleration distance + harness stretch + D-ring height − anchor height + safety margin
OSHA limits free fall to 6 feet and deceleration distance to 3.5 feet for a personal fall arrest system (1926.502(d)(16)). The calculator flags either one.
Use the manual's numbers
Deceleration distance and harness stretch vary by product. With the manual's values in the box, the result is built on them, and the line under the result says which numbers it used. If the manual gives its own total clearance, the calculator uses whichever is larger.
What it doesn't cover
- Swing fall: an anchor off to the side adds distance and a swing into whatever's nearby. Keep the anchor overhead.
- Self-retracting lifelines and leading-edge equipment, which have their own clearance charts
- Horizontal lifelines, whose sag needs a qualified person's design
- Rescue: a worker hanging in a harness needs to come down quickly, so plan the rescue before the work starts