Fall Arrest Clearance Calculator: Clear Space Needed Below the Worker
The distance below the worker that must be clear for a fall arrest system to stop a fall in time.
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How this calculator works
When a worker on a lanyard falls, they fall freely until the lanyard goes taut, then the energy absorber opens to slow them, and the harness stretches. All of that distance, plus a safety margin, must be clear below the working position, or the worker may hit the ground or a structure.
The free fall depends on the lanyard length and on where the anchor is. An anchor above the harness D-ring cuts the free fall; an anchor at foot level adds to it. The calculator adds the free fall, the absorber deployment, the harness stretch and the margin to get the clear space needed, and compares it with the space you have.
Fall arrest is the last line of defence. Preventing the fall, by a guard rail, a work platform or a restraint system, is better, and a fall arrest system needs a rescue plan and an anchor rated for the load. The equipment figures must come from the maker instructions.
Formulas used
- F = \max(0,\; L - h_a)
- C = F + D_a + S + M
Worked example
With the default numbers in the calculator above:
- Free fall before the absorber acts: 2.00 m
- Clear space needed below the working position: 4.50 m
- Spare clear space: 0.50 m
Change the numbers in the calculator to see the full working, every step, for your own case.
A second worked example
A different case, with these numbers: Height of the anchor above the harness D-ring (negative if the anchor is lower) -1.5 m.
- Free fall before the absorber acts: 3.50 m
- Clear space needed below the working position: 6.00 m
- Spare clear space: -1.00 m
Common questions
Why does an anchor at foot level need more clearance?
The worker falls the whole lanyard length plus the height of the D-ring above the anchor before the lanyard goes taut, so the free fall is longer.
Where do I find the absorber deployment distance?
On the marking and instructions of the lanyard or energy absorber. It is a rated maximum.
Does this cover a swing fall?
No. If the anchor is not directly overhead, the worker can swing into an obstruction. That needs a separate check.
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Got a question about this? Ask an engineering question and get a plain-language answer with references.
Further reading: AS/NZS 1891 series (industrial fall-arrest systems and devices, licensed); Managing the risk of falls at workplaces Code of Practice (Queensland); the maker instructions for the equipment.
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