by Toby Hayward Dip1OSH, TechSP, AIIRSM

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It is well documented and acknowledged within the HS industry that falls account for the majority of HS related fatalities and injuries in the workplace and foremost in the construction sector.

However, certainly in the Middle East, we all see instances where proper fall arrest measures are either ignored or just not in place, nearly on a daily basis. But saying this we must remember that fall arrest systems used poorly can be as dangerous as no protection at all.

In this article I shall be discussing the ways to calculate the Total Fall Distance (TFD) that is often a factor that is ignored. How many times have you seen a person working at 8’ high with a 6’ lanyard on his harness? Or personnel attaching to anchorage points at waist height? Of course any fall arrest device should only be used when other hierarchical measures have been exhausted but if fall arrest equipment is to be effective then we need to understand its limitations.

Knowing how to calculate TFD is just as important as selecting the proper harness, lanyard, anchorage connector and anchorage point for the specific task to be performed. TFD is defined as the sum of Freefall Distance (FFD), Deceleration Distance (DD), Harness Effects (HEFF) and Vertical Elongation (VEL). It is also wise to include Safety Factor (SF) of at least 1’ in the formula. TFD can be calculated using the following formula: –

Before we begin to calculate TFD, we first need to define the variables in the formula above.

The vertical distance a person travels between the onset of a fall until just prior to the point where the Fall Arrest System begins to arrest the fall. To keep FFD to a minimum, you should always try to keep the anchor point as far above the back d-ring of the harness as possible.

The vertical distance a person travels between the activation of the Fall Arrest System and final fall arrest. The DD that each shock-absorbing fall arrest device will permit is typically stated on the product label.

The stretch of the harness during fall arrest. This is typically 1’ or less for a properly fitted harness. However, some harnesses use elastic-type webbing that can increase harness effects to 2’ or more.

The stretch in the lifeline of the Personal Arrest System. Vertical Elongation is measured on the part of the lifeline that is under tension during deceleration and final fall arrest. This variable will change drastically depending upon the type of Fall Arrest System you are using. For example, most shock absorbing lanyards are designed to have a maximum deceleration distance of 3.5’, which includes the vertical elongation of the lanyard. However, if you are using a rope grab system of horizontal lifeline, vertical elongation must be calculated based on the stretch of the vertical or horizontal lifelines in those systems. You will need to check the specific manufacturers product for exact stretch percentages.

An additional factor of safety to ensure that you have the required clearance below your working surface. This variable should be at least 1’, but can reflect any number with which is both practical and with which you feel comfortable.

For illustration purposes, we will use the following equipment:

- Full body harness (non-elastic)
- 6’ shock-absorbing lanyard
- Fixed, rigid anchorage connector (such as a D-plate bolted to a structural I-beam)

In figure 1, we see a person with a 6’ shock-absorbing lanyard on an elevated platform. In this example, let’s assume that his attachment point is 2’ above the back D-ring of the harness. For every 1’ the lanyard attachment point is above the harness back D-ring, 1’ is deducted from the freefall distance. For every 1’ that the lanyard attachment point is below the harness back D-ring, 1’ is added to the freefall distance. In this scenario, if the person falls, the freefall distance (FFD) will equal 4’ since the lanyard attachment point is 2’ above the back d-ring of the harness. So, our formula looks like this:

The next variable to consider is declaration distance (DD). Many regulations including OSHA, limit this to a particular figure. In OSHA’s case this distance should not exceed 3.5’. Since all manufacturers product is slightly different, you should read the label on the product you intend to use to determine the maximum deceleration distance of that product. When calculating the TFD, the maximum that a product will permit should always be used. In our example, the maximum deceleration distance would be 3.5’.

The harness effects variable is relatively constant at less than 1’. This will vary slightly due to the adjustment of the harness, so we generally use 1’ to account for these slight differences. However, elastic-type harnesses can have more than 1’ of stretch, possibly 2’ or more, and that additional distance must be accounted for in your calculation. In our example, we are using a non-elastic harness to keep TFD to a minimum.

Most manufacturers design their shock absorbing lanyards so that the vertical elongation is included in statutory mandated deceleration distances. However, if we were using a rope grab or horizontal lifeline, or if you were attaching to a non-rigid anchorage connector, VEL would need to be calculated based on the specifications of those components in your fall arrest system. Since we are using a 6’ shock absorbing lanyard in our example and the VEL is already considered in the lanyard design we will enter a “0” for the VEL variable.

The final variable of the formula is the safety factor. It is always a good idea to include at least a 1’ safety factor; however, the safety factor could reflect any number that makes you feel comfortable with your calculation.

Now we can solve our TFD formula:

TFD=4’+3.5’+1’+0’+1’

TFD = 9.5’

Now we know that if the worker in figure 1 would happen to fall, his total fall distance will be 9.5’. But what does this number really mean? It means that the clearance between the working surface and the next closest object in the fall path must be at least 9.5’.

It is important to remember the TFD is not always measured form the working surface to the ground because sometimes the ground is not the closest object beneath the working platform. If there is any type of obstruction in the fall path of the worker (see figure 2), your available clearance is measured from the working platform to the top of that obstruction. Sometimes these distances can be very short, and a fall protection means other than a 6’ shock absorbing lanyard is necessary.

Total fall distance calculations can become more complex than those demonstrated here. The numbers and variables will change depending on the type of personal fall arrest system used. For example, when calculating TFD’s for horizontal lifeline systems you have additional variables to consider, such as cable deflection and the number of people on the system.

The important thing to remember is that calculating TFS is just as important as selecting the right product for the job.

Forgetting to calculate total fall distance is just as dangerous as forgetting to put your harness on before you begin to work at any height.

Published: 10th May 2009 in Health and Safety Middle East