How to CORRECTLY Calculate Fall Clearance

How to Correctly Calculate Fall Clearance

Whether it’s working from a boom lift, walking steel, standing on an elevated order picker, or any of the myriad other reasons you may be working at heights, “tying off” will do you no good if your personal fall arrest system allows you to strike the lower level before it stops your fall.  This may sound like common sense, but if working in this field has taught me anything, it’s that “common” sense isn’t as common as we’d like to believe.

While the problem often boils down to a lack of training or solutions developed without the help of a Competent Person, the end result of a fall event in which there was a failure to calculate proper fall distance is always the same: injury or death.  Many workers do not fully understand how a Personal Fall Arrest System (PFAS) works and therefore feel that simply “tying off” is their one-size-fits-all solution to safe, compliant work at heights.  Unfortunately, this is not true.  In many instances, the length of the harness, lanyard, and anchor point, in addition to the length of the user’s body is enough to strike a lower level.  So how do you prevent this?  By making sure you understand the factors that go into calculating fall distance and ensuring you allow sufficient room for the fall to occur (clearance). 

If this clearance is not available (from anchor point to lower level), then you must come up with an alternate fall protection or fall prevention solution.

Length of Lanyard and Harness

To an untrained worker, having a 6’ lanyard may seem sufficient while working at a height of 10’ or 12’, but they often don’t know, or forget, about the deceleration device.  When a deceleration device is deployed, it adds an additional 3.5’ to the length of your lanyard.  Whether it’s a rip-stitch pack, a glue pack, or the bungee style lanyards, you must add this 3.5 feet to the initial 6’.  In addition, a harness could stretch so that the anchor point, while initially positioned properly between the shoulder blades, ends up a foot or more above the worker’s head after a fall.

Total Length to Consider: 10.5’

Length of the User’s Body

This one is simple, but perhaps the most often forgotten when calculating fall distance.  It is necessary to take into account the length of the user’s body below the D-ring.  A PFAS that stops the user at 12’ is of no use when the lower level is 14’ away, unless the user is less than 2’ tall.  I’d venture to say that this is an unlikely scenario.  On average, it is usually safe to consider approximately 5’ feet for this distance.  Remember, however, that people on more extreme sides of the height spectrum could warrant different consideration.

Total Length to Consider: 5’ (on average)

Length of Anchorage Point Connector – Including Sag

A fixed, solid anchor point is easy to calculate because the only distance to add is the length of your snap hook.  However, when dealing with an anchor like a horizontal lifeline, things get more complicated.  Horizontal lifelines sag, both as part of initial design and additionally during a fall event.  This sag needs to be taken into consideration.  Figuring out the sag in design could be as simple as pulling the lifeline tight to see what the distance is, but sag from a fall is not as easy to figure.  The forces placed on the lifeline are going to pull the line farther than you can when checking the design sag.  Estimates I’ve gathered seem to range from 3% of the total length of a high tension line to 15% of the length of a line which uses shock absorbers.  This can be significant, especially for lines that have long, uninterrupted length to allow for continuous fall protection during movement.  The longer the span, the more sag you’ll have.  Your safest bet is to have an engineer calculate the sag resulting from a fall based on the maximum number of people you intend to have tied-off to the lifeline at any one given time.  Short of that, you may want to consider shortening your spans and using double lanyards to achieve 100% fall protection.  Either way, your sag needs to be figured in.

Total Length to Consider: 3-15% of the length of line, depending on the system’s design for a horizontal lifeline.  Length of the snap hook for a fixed, solid anchor

Safety Factor

This is exactly what it states: a safety factor.  If the total length of my PFAS, including sag and body length is 16’, I certainly don’t want to use it in a situation where my clearance is 16’.  I want to build in a safety factor in case something doesn’t go exactly as calculated.  The conventional wisdom on this safety factor is an additional 3’, which brings our total length of PFAS including body length to 18.5’ (without sag). 

Now think back to your projects, your facility, or work you’ve observed.  How many times did you see a 6’ lanyard with a deceleration device used with a clearance of less than 18.5’?  Often?  Do you think those workers knew the danger they were in but chose to do it anyway?  Perhaps, but more likely than not, they were never trained in calculating fall distance and had no idea what was required.  Calculating fall distance is not only a good idea, it is critical.  Use PFAS only where you have the necessary clearance and where that clearance is not available, find an alternative.  It would be tragic to have a worker suffer an injury when both you and the worker thought you were doing the right thing.

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