Fall Clearance Calculations for Lanyards, SRLs and Lifelines

Rules of thumb get you started. They stop working when the anchor is low, the rope is long, the lifeline sags, or a stack of pallets sits under the edge. A fall arrest system that bottoms out on the level below has failed, even if every part of it is rated. Both OSHA rule sets say a system must be rigged so that you cannot contact a lower level. This is the math a lead or competent person does before anyone ties off. All figures here are examples, so use the numbers on your own equipment label and in the maker's manual. Those, and your competent or qualified person, have the final word.

What You Will Be Able to Do

• Build a clearance total from a label and a tape measure.
• Convert between figures measured from the surface, the D-ring and the anchor.
• Work out free fall with a low anchor, and say when it breaks the limit.
• Add rope stretch, lifeline sag and swing to the total.
• Choose what to change when the math says the system will not work.

Why the Limits Exist

A 6-foot free fall takes about 0.6 seconds. You are moving at about 13 mph at the end of it. The energy that the system has to absorb grows with the distance you fall. A 220-pound worker falling 6 feet carries 220 x 6 = 1,320 foot-pounds of energy. Falling 11 feet from a foot-level anchor, the same worker carries 2,420 foot-pounds, almost twice as much. OSHA's guidance warns that a few extra feet of free fall can raise the force on your body enough to injure you, and can exceed what the system was built to take.

The Clearance Sum

Measure from the walking surface, and add four things:

Clearance needed = free fall + deceleration + D-ring shift + safety margin

Your height drops out of this sum, because your feet start at the walking surface and fall the same distance your D-ring does.

Check What a Chart Measures From

OSHA's Technical Manual adds in the 5-foot D-ring height and uses a 2-foot margin, so the same lanyard comes to 17.5 feet. None of these conflict. The fall is the same, and only the starting point and margin differ. Before you compare two figures, find out what each was measured from. Workers over 6 feet tall need a larger D-ring height.

Free Fall With a High or Low Anchor

Free fall = connector length - distance the anchor sits above your D-ring.

If the anchor sits below your D-ring, add that distance instead.

Worked example: A mechanic works on a warehouse mezzanine with a deck 14 feet above the floor. A strap on a steel beam sits 7 feet above the deck, which is 2 feet above his D-ring. He has a 6-foot shock-absorbing lanyard.

1. Free fall = 6 - 2 = 4 feet.
2. Total = 4 + 3.5 + 1 + 3 = 11.5 feet.
3. The floor is 14 feet down, so the system fits with 2.5 feet to spare.
4. Then a forklift parks a 10-foot stack of pallets below him. The clearance is now 14 - 10 = 4 feet, and the system fails.

Clearance belongs to a location and a moment. If what sits below can change, someone has to own that check.

Worked example: A concrete worker ties off to an anchor 1 foot below her D-ring, using a 6-foot lanyard.

1. Free fall = 6 + 1 = 7 feet.
2. That is over the 6-foot limit, so stop there. Do not add up the rest.
3. With a 4-foot lanyard, free fall = 4 + 1 = 5 feet, and the total is 5 + 3.5 + 1 + 3 = 12.5 feet.

When Free Fall Can Go Past 6 Feet

Construction has a firm limit of 6 feet under 1926.502(d)(16)(iii). In general industry, 1910.140(d)(2)(ii) allows more only if the employer can show the maker designed the system for it and tested it to keep the arresting force at or under 1,800 pounds. Lanyards sold for foot-level tie-off exist, but owning one does not settle which rule applies to your job. Ask your competent person before you use one.

Self-Retracting Lifelines

An SRL anchored overhead cuts free fall to a short distance. Labels differ in how they state it. Some list free fall and deceleration separately. Others give one arrest distance. Find out which one yours gives, and do not assume. If you cannot tell, add both, which gives the bigger and safer number.

Worked example: An overhead SRL's label lists 2 feet of free fall and up to 3.5 feet of deceleration. Total = 2 + 3.5 + 1 + 3 = 9.5 feet. A rule of thumb of about 8 feet would have been short for this unit.

Three cases need the manual open in your hand:

• Anchored at or below your feet. Standard SRLs are built to be anchored overhead. The ANSI/ASSP Z359.14 standard uses Class 1 for anchors at or above the D-ring, and Class 2 for units also tested for anchors below it. Class 2 units can need far more clearance.
• Line running over an edge. Only use a unit rated for that edge. The label states the edge type, the setback and the extra clearance.
• Working far to the side. Many manuals limit how far you may go from under the anchor.

Vertical Lifelines and Rope Stretch

A rope grab on a long vertical line adds a term to the sum: stretch. The more rope above you, the more it stretches. That means your worst case is near the bottom, which is also where you have the least room.

Worked example: A painter works on a bridge pier with a rope grab, a 3-foot lanyard and a line anchored 50 feet above the ground. The rope grab stays at D-ring height. In this example, the maker's data shows up to 10% stretch.

At 12 feet up, she needs 14.3 feet but has 12, so the line will not protect her there. In real work, that means a different method for the last stretch of the descent. Each worker needs a separate vertical line, with rare exceptions.

Horizontal Lifelines and Sag

A horizontal lifeline sags when it catches a fall. That drop adds directly to your clearance. Sag depends on the span, line tension, line material, and the number of people on it. You cannot estimate it in the field. A qualified person must design, install and supervise these systems, and that design states the clearance.

Worked example: A 60-foot cable lifeline runs along a hangar dock. The design drawing gives 4 feet of deflection with one user, and 5 feet with two. A worker uses an SRL on it, with 2 feet of free fall and 2 feet of deceleration.

1. One user: 2 + 2 + 1 + 3 + 4 = 12 feet.
2. Two users: 2 + 2 + 1 + 3 + 5 = 13 feet.
3. If the dock is 12.5 feet above the hangar floor, only one person can use that span at a time.

Follow the design for the number of users and the connector type. Adding a third worker, or swapping the SRL for a lanyard, means the numbers no longer apply.

Adding Swing to the Total

If you are not directly under the anchor, you drop more and you swing.

Swing drop = line length - vertical distance from the anchor to your D-ring.

Worked example: An SRL is anchored 12 feet above a roof deck. A worker falls over an edge 9 feet to the side of the point under the anchor. His D-ring is 5 feet above the deck.

1. Vertical distance from anchor to D-ring = 12 - 5 = 7 feet.
2. Line length = square root of (7 x 7 + 9 x 9) = square root of 130 = 11.4 feet.
3. Swing drop = 11.4 - 7 = 4.4 feet.
4. Total = 4.4 + 2 (lock-up) + 2 (deceleration) + 1 + 3 = about 12.4 feet, compared with 8 feet directly under the anchor.

A 4.4-foot swing also means that he reaches the low point at about 11 mph. Anything in that arc gets hit at that speed, whether it is a wall, a column or a parapet. Being far enough off to the side can also drag the line over an edge.

Weight

OSHA's general industry rule treats a system that meets its test criteria as compliant for a combined weight under 310 pounds. That is body, clothes, tools and anything else you carry. Many products are labeled for a capacity of up to 310 pounds, and some are rated higher.

Worked example: A 245-pound electrician wears 15 pounds of boots and clothes and a 35-pound tool belt, and his harness and SRL add 15 more. His total is 245 + 15 + 35 + 15 = 310 pounds. That is at the limit. Before he adds a drill and a bag of fittings, he needs gear rated for more, and the label's clearance for that rating. Go by what the maker's manual says counts toward capacity.

When the Math Fails

Do not shave the margin to make it fit. Change the system.

OSHA's Technical Manual makes the same point: if the space below is not more than the total clearance, a fall arrest system is the wrong choice, and restraint or a different system should be used. Low work is the hard case. Between about 6 and 14 feet, a shock-absorbing lanyard often cannot do the job, even though fall protection is required.

A Clearance Worksheet

Use these steps at each location.

1. Measure the distance to the first thing you would hit. Check the whole path, not just the floor.
2. Note what could change, such as stacks, vehicles or scaffolding.
3. Measure the anchor height above the deck, and your D-ring height.
4. Work out free fall. Stop if it is over the limit.
5. Read deceleration and shift from the label.
6. Add stretch, sag and swing if they apply.
7. Add your program's safety margin.
8. Compare the total with what you measured.
9. Record it, and recheck if anything moves.

Common Mistakes

Check Your Understanding

1. A crew plans to run an SRL line over a steel roof edge. What do you check before they start?

Only use a unit rated for that edge. Its label states the edge type, the setback and the extra room it needs.

2. A chart says 17.5 feet for a lanyard. Your own math says 12.5 feet below the surface. Which is wrong?

Maybe neither. If the chart measures from D-ring height, subtract the 5-foot D-ring height: 17.5 - 5 = 12.5 feet. Confirm the start point.

3. You use a rope grab on a 60-foot line, with 8% stretch in the maker's data. How much stretch do you add if 45 feet of rope is above you?

45 x 0.08 = 3.6 feet.

4. A worker's overhead anchor is well off to one side. He says the label total fits, so he is fine. What has he left out?

Swing drop. The line length minus the vertical distance to his D-ring adds to the total, and he could hit something in the arc.

5. A crew has 10 feet of clearance, an anchor at D-ring height and 6-foot lanyards. What do you tell them?

The lanyard needs about 13.5 feet, so it will not work. Look at an overhead SRL if the label total fits, or use travel restraint or guardrails.

Key Takeaways

• Below the walking surface, add free fall, deceleration, D-ring shift and a margin.
• Before you compare figures, check what they are measured from.
• Stop at free fall if it is over the limit, because no clearance fixes that.
• Rope stretch is worst near the bottom of the line, and sag comes from the qualified person's design.
• Add swing drop when you are not under the anchor, and look for what you would hit.
• If the math fails, change the system. Do not cut the margin.