How much clearance you need below a fall arrest anchor

More than you have, on most of the jobs where people clip on. A harness does not stop a fall — it stops it after a distance, and that distance is a stack of five things rather than one. A two-metre energy-absorbing lanyard clipped level with your D-ring needs 6.65 metres of clear space below you. A scaffold lift is two. The person clipped on there is not protected; they are attached to the thing they will hit the ground beside. FALL adds the stack up against the space actually beneath, shows every part separately so any figure can be argued with, and runs in the browser.

The five things, added up

A two-metre lanyard with an energy absorber, a person of average height, anchor level with the D-ring, to EN 355:

The deceleration line is the one that surprises people. A European energy absorber may tear out to 1.75 metres — longer than the person wearing it is tall. It is not a fault; it is how the force on your body is kept survivable. But it is a metre and three quarters of height that has to exist underneath you, and it is not printed on the lanyard in a way anybody reads.

The fourth line is the one people forget entirely. The system catches your harness, at the D-ring between your shoulder blades. Everything below that point keeps going.

Clipping low is worse than it sounds

Anchoring below your D-ring does not add a little; it adds the drop twice, because you fall past the anchor before the lanyard begins to take any load. The same two-metre lanyard, clipped at foot level:

And the free fall factor goes to 1.75. Above a factor of one, a standard lanyard is outside what it was tested to: the absorber was rated on the assumption you would not fall further than the length of the lanyard. Clipping to the scaffold tube by your boots is the commonest way people leave that assumption behind, and it is done because it is the tube within reach.

A retracting lifeline is shorter, and still not short enough

A self-retracting lifeline locks in a foot or two rather than letting you fall the length of a lanyard, and the arithmetic improves accordingly — free fall 0.60 m, deceleration 1.20 m, 4.70 m needed instead of 6.65.

That is a large gain and it does not rescue a two-metre lift. It is still short by 2.70 metres. The honest conclusion for low-level work is not a different lanyard: it is that fall arrest is the wrong system, and what is needed is restraint — a line short enough that you cannot reach the edge — or a net, or a platform, or not being there.

Two more that get left out of every calculation

• Swing. An anchor off to one side means falling in an arc rather than straight down. That costs extra height before the system loads at all, and it ends with a horizontal arrival into whatever is over there — a column, a stack, the scaffold you fell off. Thirty degrees from vertical is the usual limit, and it is worth knowing what your angle actually is rather than assuming it is fine.
• What is underneath. The margin at the bottom of the stack assumes clear air. Arriving one centimetre above a stillage, a rebar cage or a handrail is not being caught. Measure to the first thing you would hit, not to the deck.

What to do with a figure that does not fit

1. Raise the anchor. Every metre of anchor height is a metre off the free fall and a metre off the requirement, and it is usually the cheapest change available.
2. Shorten the connector, or change it. A retracting lifeline instead of a lanyard is worth roughly two metres of clearance.
3. Change the system. Restraint prevents the fall instead of arresting it and needs no clearance at all, which is why it is the right answer far more often than it is chosen.
4. Do not clip on to something that will not catch you. This is the one the arithmetic exists for. A harness clipped to an anchor without the room below it gives everybody on site — including the person wearing it — the impression that the hazard has been dealt with. It has not. It has been made invisible.

What this is and is not

The published figures are EN 355 and EN 363, with the ANSI Z359 equivalents offered for work in the United States, and every part of the stack is shown separately so a figure can be disagreed with rather than swallowed. It runs in the tab with no network call at all, which matters for a method statement written on somebody else's site.

It is not a rescue plan, and it is not a competent person. It cannot see your anchor, it does not know what is below you better than you do, and suspension trauma after a successful arrest is a separate problem with its own clock. This is the arithmetic, done before somebody clips on rather than in the investigation afterwards.

Alongside: RAMP does the equivalent for gradients and landings, and EVACUATE for getting people out of a building rather than off the side of one.