What a Personal Fall Arrest System Has to Do and In What Order

Why this matters

A harness is what you reach for when everything better has already failed. That sentence is the whole card, and most crews have the order backwards: they arrive with a harness, look for somewhere to clip it, and never ask the two questions that come before it. If you are planning a job right now and the first thing you wrote down was "tie off," stop and go back two steps.

The order that actually governs is eliminate the exposure, prevent the fall from being possible, restrain the worker so the fall cannot start, and only then arrest a fall that has already begun. Arrest is last because it is the only option in the list where the worker leaves the walking surface, takes an impact load through their body, and then hangs there needing somebody to come get them. Everything above it in the order ends with a person still standing on something.

What a personal fall arrest system consists of is covered in this library's Fall Protection Systems card. This one is about what the system has to accomplish, in what sequence, and how to tell when it should not have been the answer at all.

Restraint is the step nearly everybody skips

Between the guardrail and the harness sits a system almost nobody plans for: fall restraint, sometimes called travel restraint. Same harness, different rigging philosophy. The tether is fixed-length and short enough that the worker physically cannot reach the edge. No fall occurs, so no arrest occurs.

Restraint changes four things at once, and this is why it is worth the extra ten minutes at the tailboard:

• No clearance problem. There is no free fall, no deceleration distance and no stack to compute, because nothing falls.
• No rescue clock. Nobody ends up suspended, so the hardest part of any arrest plan disappears.
• No arrest force on the body. The worker does not take an impact load through their pelvis and thighs.
• A far smaller load path. The anchorage is loaded by a body leaning against a tether, not by an arrest event.

That last point comes with a hedge you must keep attached: rig a restraint anchorage to the arrest anchorage criteria anyway. OSHA's construction anchorage rule at 29 CFR 1926.502(d)(15) is written for arrest, and a restraint system that is mis-measured by a few feet becomes an arrest system in the instant it matters. Every restraint rig this trade gets wrong gets wrong in the direction of too much tether.

Two hardware rules that follow directly and that people violate without noticing. A restraint tether is fixed-length with no energy absorber, because an absorber's whole function is to lengthen under load, which converts your restraint standoff into an overrun. A self-retracting lifeline is not a restraint device, because it pays out on demand and its length is whatever the worker walks to.

What the arrest system has to do once it is genuinely the answer

Five duties, ordered by when in the event each one is discharged. Four of them come from the criteria at 29 CFR 1926.502(d) for construction, and 29 CFR 1910.140 carries closely matching criteria for general industry.

1. Begin arresting before the free fall exceeds the limit. Construction caps free fall at 6 feet under 29 CFR 1926.502(d), or at less if the system's own rating is lower. Free fall is the distance the attachment point travels before the system starts to do anything, and it is set almost entirely by where the anchorage sits relative to the dorsal D-ring. This is the term that doubles when the anchor moves from overhead to foot level.

2. Hold the peak force on the body under the ceiling. 1,800 pounds maximum arresting force when used with a body harness, per 29 CFR 1926.502(d). That ceiling is a limit on what a human tolerates, not on what the hardware survives, and the reasoning is in What an Arresting Force Limit Is Protecting.

3. Complete the stop within the allowed deceleration distance. 3.5 feet maximum in construction under 29 CFR 1926.502(d), measured as the additional travel from the point where arrest begins to the point where the body stops.

4. Have done all of the above above the lower level. This one is not a separate physical event, it is the accounting of duties 1 through 3 against the space you actually have, plus harness and D-ring effects and a margin. It is checked last and it fails first. The arithmetic is in How to Compute Fall Clearance and Why People Get It Short.

5. Hold the body upright and permit prompt rescue. 29 CFR 1926.502(d) requires the employer to provide for prompt rescue or assure that employees can rescue themselves. A worker hanging in a harness is a live emergency, not a stable state.

The criteria at 1926.502(d) are written for a combined employee-and-tools weight of under 310 pounds. Above that, the system has to be modified accordingly and the answer comes from the manufacturer's instructions for a system rated at that weight, not from scaling anything yourself.

The gate, and two jobs that resolve opposite ways

The gate: if a passive system or a restraint system can be rigged with the anchorage and the clearance actually present on this roof, arrest is not the answer. Run it against two tickets on the same building.

Job A: rooftop unit service, 8 feet in from the edge

Measured facts. Roof deck 14 feet above grade. Parapet 30 inches tall, against the 39-inch wall height 29 CFR 1926.500(b) requires before an edge stops being unprotected, so it is a trip-over wall and not protection. A structural roof anchor is present, installed by the building owner and documented. The rooftop unit sits 8 feet from the parapet. Anchor measures 22 feet 0 inches from the nearest edge, perpendicular. Tech's standing height 6 feet.

The gate says check restraint before arrest, so here is the restraint rig with every qualifier from the section above printed as its own line.

Anchor to nearest edge (perpendicular)        22 ft 0 in
Required standoff (worker standing height)   -  6 ft 0 in
                                              ----------
Maximum permissible tether length             16 ft 0 in
Rounded DOWN to stocked length                15 ft 0 in
Absorber fitted?                              none - fixed
                                              length tether
Closest approach to edge (22 - 15)             7 ft 0 in
  against 6 ft standoff passes, 1 ft
Anchor to rooftop unit (22 - 8)               14 ft 0 in
  against 15 ft tether reaches, 1 ft
Anchor to every OTHER edge, measured all over 22 ft
Anchorage rated to arrest criteria yes

Read the two rounding calls, because both run one way. The tether rounds down from 16 to 15 feet, because rounding a tether up is the direction that puts a body over a 30-inch parapet. The standoff is set at the worker's full standing height rather than an arm's length, because a person who trips projects their whole body past their D-ring, and I would rather lose a foot of working radius than model a falling human as a point.

Outcome: restraint. No clearance calculation, no rescue plan for a suspended worker, no arrest force on anyone. The gate resolved before the harness was ever an arrest device.

Job B: replacing the parapet coping and the edge roof drain

Same building, same anchor, same 14-foot deck. The work is at the edge, and the surface a guardrail would clamp to is the surface being removed.

Restraint fails immediately: the job requires reaching the edge, so no tether short enough to keep the worker off the edge lets them do the work. A guardrail fails because its mounting surface is the scope. So arrest is now genuinely the answer, and the first thing arrest does is hand back a problem.

The existing anchor is at deck level. With a foot-level anchorage and a 6-foot shock-absorbing lanyard, the D-ring falls 5 feet just reaching the anchor's level before the lanyard pays out, so free fall is 11 feet, not 6. That breaks the free fall cap at 29 CFR 1926.502(d)(16)(iii) before any clearance question is asked. Add up to 3.5 feet of deceleration and duties 1 and 3 alone consume 14.5 feet, before a single harness term, against 14 feet of deck height with the worker's D-ring starting about 5 feet up. That stack does not close, and it is worth being blunt about what "does not close" means: the system would be paying out at the moment the worker reached the ground.

Outcome: the job changes, not the harness. Three real options, and the crew picks by what the building actually offers. Raise the attachment point with a rated overhead anchorage or a rigid rail so free fall drops toward zero. Substitute a self-retracting lifeline specifically rated for anchorage at or below the dorsal D-ring, and use the clearance figure the manufacturer publishes for that configuration rather than the one for overhead use. Or take the exposure away entirely by working the edge from a scaffold or a mast-climbing platform on the ground side, which is the elimination step the gate started with and which is often faster than it sounds once someone prices the alternative honestly in crew hours.

The failure mode here is the crew that reads "arrest is the answer," clips a 6-foot lanyard to the deck-level anchor, and works the edge all afternoon feeling protected. That system is legally attached and physically incapable of doing duty 4. Nothing on the roof looks different.

How to verify you got the order right

Work backwards through the four steps and require a written reason for each one you skipped, not a shrug.

• Elimination. What would it take to do this from the ground or from a platform? If the answer is "more time," that is a scheduling answer, not a safety answer, and it belongs to the person who owns the schedule.
• Passive prevention. Is there a guardrail, a cover, or a hole plug that solves this for everyone who ever walks here, not just today's crew? Permanent fixes on fixed workplaces are what 29 CFR 1910 Subpart D is built around.
• Restraint. Measure anchor-to-edge for every edge and run the four lines from Job A. If the tether that reaches the work is shorter than the tether that reaches the edge, restraint is on the table.
• Arrest. Only now. And the first thing you do in this step is compute clearance, before anyone buys hardware, because clearance is the constraint that most often sends you back up the list.

One tell that the order was skipped rather than worked: the plan names a harness before it names an anchorage. Anchorage availability is what actually decides whether restraint or arrest is possible on a given roof, so a plan that starts with the harness has assumed the hard part.

References

• 29 CFR 1926.502(d), criteria and practices for personal fall arrest systems in construction, including free fall, arresting force, deceleration distance, anchorage and prompt rescue.
• 29 CFR 1910.140, personal fall protection system criteria for general industry.
• 29 CFR 1910 Subpart D, walking-working surfaces, for the passive-protection duties on fixed workplaces.
• See related: Fall Protection Systems, How to Compute Fall Clearance and Why People Get It Short, What an Anchorage Has to Be Rated For and Who Decides, What an Arresting Force Limit Is Protecting.