Fall arrest equipment does not stop a fall instantly. It stops it over a distance. If that distance is greater than the space between the worker and the next surface down, the equipment will work exactly as designed and the worker will still hit the ground.
This is the calculation that gets skipped most often, and it is the one that decides whether the rest of the system means anything.
What has to fit in the space below
Required clearance is the sum of several things, measured from the anchor point down:
1. The length of the connecting subsystem. For a lanyard, its full length. A 2 m lanyard contributes 2 m before anything else happens.
2. Energy absorber deployment. The absorber tears open under load to limit force. That tearing has a length — commonly up to around 1.75 m for a webbing absorber, though it varies by product and by the mass being arrested. Take this figure from your absorber’s own instructions, not from a rule of thumb.
3. The height of the worker below the attachment point. The dorsal D-ring sits high on the back, so most of the body hangs below it. Around 1.5 m is a common planning figure for the distance from the D-ring to the feet.
4. A safety margin. Space between the lowest point the feet reach and the surface below. 1 m is a widely used minimum. This absorbs harness stretch, D-ring slide, variation between workers, and the plain fact that estimates are estimates.
Add them together and, with a 2 m lanyard, you are already in the region of 6 m of clearance required below the anchor before any site-specific factors. That is why lanyard-based fall arrest so often does not suit work at low or intermediate heights.
The anchor position changes the answer completely
Two workers with identical equipment can need very different clearance depending on where the anchor sits relative to the D-ring.
- Anchor overhead — the best case, and what the numbers above assume. The worker falls the slack, the absorber deploys, and the fall is arrested.
- Anchor at D-ring height — the worker now falls the full length of the lanyard plus everything else. Add the lanyard length again.
- Anchor below the D-ring — the worst case. The free fall is longer, the forces are higher, and there is a real risk of a swing fall. Many systems are simply not rated for it.
Keeping the anchor high is not a nicety. It is the single most effective way to reduce both required clearance and arrest force.
Swing falls
If the worker is not directly below the anchor, a fall becomes a pendulum. Two things follow. The worker can strike structure to the side during the swing, which the equipment does nothing to prevent. And the arc means the lowest point of travel is below the point directly under the anchor, so more clearance is needed, not less.
Reduce the horizontal offset, or use a horizontal lifeline or a trolley so the attachment point tracks with the worker.
Retractable blocks change the arithmetic
A self-retracting lifeline keeps the line taut, so free fall is short and arrest distance is much lower — often around 1 m or less of arrest distance, depending on the device. This is why an SRL is frequently the right answer where clearance is tight and a lanyard would not fit.
But this only holds when the device is used as intended. Most SRLs are designed for overhead anchorage. Anchoring at or below foot level, unless the device is specifically rated for it, produces a longer free fall and higher forces than the device was tested for. Check the rating — some are approved for leading-edge or foot-level use, and many are not.
Do the sum before the work, not after
Write the calculation into the method statement with real numbers taken from the instructions for the actual equipment being used, and check it against the real distance to the next surface — not to the ground, but to the first thing a falling worker would hit: scaffolding, a slab edge, plant, a stack of material.
If the number does not fit, the answer is not a longer lanyard. It is a different system: an SRL, a higher anchor, a restraint system that prevents reaching the edge at all, or a change to how the work is done.
The figures here are common planning values used to illustrate the method. Deployment distances, arrest distances and clearance requirements vary by product and by standard. Always calculate using the values in the manufacturer’s instructions for the specific equipment in use, and have the result checked by a competent person.
