How to Properly Calculate Fall Clearance

How to Correctly Calculate Fall Clearance

Have you ever seen someone tied off to an anchor and thought that they would hit the ground if they fell?

Sure, fall protection PPE was being used. But the connector used was a coiled rope that lay between the user and the anchor. And the edge was only 15 feet off the ground.

This oversight should not happen. Fall clearance needs to be calculated every time a personal fall arrest system is used.

What is fall clearance?

Fall clearance is the distance a worker can safely fall without contacting a lower level. With some simple calculations, you could prevent a life-changing mistake.

The factors you need to calculate fall clearance can happen in any order. But, for the sake of clarity, we will list them as five steps:

  1. Free Fall Distance
  2. Deceleration Device
  3. Back D-Ring Height
  4. D-Ring Shift
  5. Safety Factor

Some of these factors may seem insignificant, but I assure you that they are not. Two feet could mean the difference between a broken ankle and a safely arrested fall. Make sure that you take the time to include each of these in your calculations.

Step 1: Calculate Free Fall Distance

Free fall, as it relates to fall safety, is the distance you drop before your PPE is engaged. This is going to change based on your application. You could be working on a bus with a self-retracting lifeline (SRL) attached to an overhead lifeline. Or you could be tied off to a non-penetrating roof anchor while working on a rooftop unit.

The calculation for each application is going to be a little different. Here are the 3 different ways that you can figure this out.

  • If your anchor point is level with your D-ring, the lanyard length is your free fall. This is typically 6 feet, but lanyards come in many lengths.
  • If your anchor point is below your D-ring, you need to include the distance from the D-ring to the anchor point. Then, add the length of your lanyard to determine your free fall.
  • If your anchor point is overhead, your free fall distance is the difference between the distance from D-ring to the anchor point from the lanyard length. However, if you use a Class A SRL, then the free fall will be limited to 2 feet per OSHA regulation.

fall clearance D-ring location

Step 2: Include the Deceleration Device

A deceleration device is an additional mechanism added to your connector to reduce the force applied to the user. For example, if you’re using a lanyard as your connector, the shock pack will slow your descent by unraveling once it is engaged.

Per OSHA 1910.140 regulation, once engaged, the max a deceleration device can extend before it must bring you to a full stop is 3-1/2 feet. It’s easiest to use 3-½ feet as the addition to the formula regardless of the OSHA-compliant device that you use.

Step 3: Determine the Back D-Ring Height

The back D-ring height measurement is the distance from the sole of your shoes to the D-ring between your shoulder blades. This is especially important if the anchor is below the D-ring of the harness.

Let’s assume your standard worker is about 6 feet tall. In that case, this measurement would be about 5 feet. The standard addition to fall clearance is 5 feet.

Step 4: Add the D-Ring Shift

To fit a harness correctly, your back D-ring should fall between your shoulder blades. However, during a fall, the D-ring shifts upwards, stretching closer to the back of your head.

To make this simple, just add 1 foot to the fall clearance calculation.

Step 5: The Safety Factor

There’s always room for human error, and since we’re working with human lives, it’s important to get it right. The safety factor is a reasonable distance between the user and the lower level when the fall stops. This is typically 2 feet.

Fall Clearance Formula

Free Fall Distance – X (see above)

+ Deceleration Device – 3.5 feet

+ Back D-Ring Height – 5 feet

+ D-Ring Shift – 1 foot

+ Safety Factor – 2 feet

= Total Fall Clearance - X + 11.5'

Free Fall Distance – X (see above) + Deceleration Device – 3.5 feet + Back D-Ring Height – 5 feet + D-Ring Shift – 1 foot + Safety Factor – 2 feet = Total Fall Clearance - X + 11.5'.

Horizontal Lifeline Addendum

If you are using a horizontal lifeline system, then you will need to add the sag of the cable lifeline when it is engaged. The manufacturer will typically have these numbers based on the length of the lifeline that you use.

Your fall clearance needs to be less than the height between you and any object you could come in contact with. That could be a lower roof level, a tree, or any other obstruction you could hit during a fall.

Safety should not be a guessing game. There will always be room for errors, so let’s minimize the risk. Do your part to ensure your safety and the safety of anyone else working with you.

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