Excessive Free Fall Distance Hazard Alert

Summary

The excessive Free Fall Distance (FFD), if it is allowed in a Fall Arrest System (FAS), may result in an injury to its user, or in a catastrophic failure of the Fall Arrest System (FAS) components (the fall protection equipment) or both. The Free Fall Distance (FFD) is in most cases controllable by the Fall Arrest System (FAS) designer, and even in cases where a large Free Fall Distance (FFD) cannot be avoided, there are methods in fall protection engineering which allow the risk of injury to the Fall Arrest System (FAS) user to be minimized.

The Hazard

What is it? Almost all falls from elevation begin with the Fall Arrest System (FAS) user falling freely until his Fall Arrest System (FAS) begins to tension. This initial tensioning commences the process of arresting the fall. In most real-life situations it is almost impossible to avoid some Free Fall Distance (FFD). What free fall distance would be considered to be excessive?

The Free Fall Distance (FFD) would be considered excessive if one or more of the following events occurred:

1. The fall victim would hit the level below or objects located in the path of the free fall before the Fall Arrest System (FAS) began to arrest the fall.
2. The product of the fall distance and the weight of its user [E=W x FFD; where W=mg] exceeded the rated capacity of the shock absorber (aka personal energy absorber) in the Fall Arrest System (FAS).
3. The Maximum Arrest Force (MAF) exceeded the legal limit of 1,800 lb [8 kN]. This may occur in Fall Arrest System (FAS) with no shock absorbers.
4. The Free Fall Distance (FFD) exceeded an arbitrary limit set by the local regulatory body, or the one which may be set in the company’s safety policy.

Why does it exist? The following are the causes of an excessive Free Fall Distance (FFD):

1. The Fall Arrest System (FAS) anchor was not located as high above the work location as reasonably achievable,
2. A lanyard (shock absorbing or not), if employed in the Fall Arrest System (FAS), was too long.
3. The Fall Arrest System (FAS) anchor was located too far to the side of the Fall Arrest System (FAS) user’s work location and resulted in a swing fall which begun with a large Free Fall Distance (FFD) (See Hazard Alert HA-005).
4. While working, the Fall Arrest System (FAS) user had to first move under an obstruction and then climb above this obstruction, thus creating a U shaped slack with his lanyard or vertical lifeline.
5. The user of a Fall Arrest System (FAS) with a rope grab and vertical lifeline (a rope) did not notice that the rope grab had inadvertently locked on the rope. Thus he/she started dragging the rope, creating a slack which will be responsible for an excessive Free Fall Distance (FFD) in case of a fall.
6. The Fall Arrest System (FAS) was attached to a Horizontal Lifeline (HLL) which had considerable initial sag.
7. The Fall Arrest System (FAS) user was involuntarily catapulted out of his Work Positioning System due to some violent, external event (e.g.: lineman thrown out of the bucket of the manlift when the booms of the latter are hit by a passing truck).

Where can you experience it? An excessive Free Fall Distance (FFD) can be present in all Fall Arrest Systems (FASs), even in those employing Self-Retracting Lifelines (SRL). The Self-Retracting Lifelin (SRL) theoretically should prevent any slack from developing in the Fall Arrest System (FAS) and then translating into an Free Fall Distance (FFD). In reality such undesired slack is quite common. In most cases the choice of the Self-Retracting Lifeline (SRL) anchorage and the Fall Arrest System (FAS) user’s inadequate training are to blame for such slack.

Even the best, state-of-the-art Fall Arrest System (FAS) can be used incorrectly and produce an excessive Free Fall Distance (FFD) as a result.

Who is affected by it and when? Fall Arrest System (FAS) users who do not have adequate training in fall protection are affected by a large Free Fall Distance (FFD) the most. The Fall Arrest System (FAS) designers, or the users who selected anchorages which were located too low in relation to the length of the Fall Arrest System (FAS) and to the working elevation of their users, are often responsible for the large Free Fall Distance (FFD).

Some Fall Arrest System (FAS) designers seem to be unaware of a variety of portable anchorages which today are an off-the-shelf product.

Particularly large Free Fall Distances (FFDs) are present in Fall Arrest Systems (FASs) with Horizontal Lifelines (HLLs). The Horizontal Lifelines' (HLLs’) initial sag adds directly to the Free Fall Distance (FFD) which would have happened without the Horizontal Lifeline (HLL).

How to Eliminate It Or Minimize Its Consequences?

1. Train the Fall Arrest System (FAS) designers, who very often are supervisors or persons who are unaware of their designer’s role, in the fundamentals of modern fall protection.
2. Train all users of Fall Arrest Systems (FASs) in the fundamentals of fall protection.
3. Minimize the Free Fall Distance (FFD( by selecting the shortest possible lanyard with which the Fall Arrest System (FAS) user can still efficiently perform his/her tasks.
4. Employ Self-Retracting Lifelines (SRL), and anchor them directly above the work place.
5. Employ Horizontal Lifelines (HLLs) if the work location is far away from the Fall Arrest System (FAS) anchorage. The length of any uninterrupted span should be limited to 30 ft [9 m]. The use of Horizontal Lifelins (HLLs) may require involvement of qualified personnel, including a PE or a P.Eng. (Canada).
6. Employ shock absorbers in Fall Arrest Systems (FASs) and energy absorbers in Horizontal Lifelines (HLLs) taking into account that their deployment increases the Total Fall Distance (TFD). This approach does not limit a Free Fall Distance (FFD) but it minimizes its consequences by lowering the shock loads (Maximum Arrest Forces [MAF] and Maximum Arrest Loads [MAL]) in the system. [For more information on Total Fall Distance see Hazard Alert no. HA-018]
7. Follow to the letter the User’s Instructions supplied with the Fall Arrest System (FAS) components by their manufacturers.
8. Seek advice from a qualified person, and if necessary, from a fall protection engineer in difficult, non-typical situations, or in those where required by law.

Additional Information and Comments

At the instant when the fall victim ends his/her free fall and the Fall Arrest System (FAS) begins arresting the fall, the victim has an energy which is equal to: E=WxFFD, where W=mg is simply the weight of the falling person. This energy has to be absorbed in the process of arresting the fall. All modern Fall Arrest Systems (FASs) employ personal Shock Absorbers (SAs) attached to the back D-ring of the harness and, if a Fall Arrest System (FAS) employs an Horizontal Lifeline (HLL), the Horizontal Lifeline (HLL) usually has its Energy Absorber (EA). These Shock Absorbers (SAs) may come either as shock absorbing lanyards or as pouch style shock absorbers. However, both the personal Shoch Absorber (SA) and the Energy Absorber (EA) (in the Horizontal Lifeline [HLL]) have certain rated energy absorbing capacities which should not be exceeded. It is therefore of paramount importance that the Fall Arrest System (FAS)/Horizontal Lifeline (HLL) designer and the person who designates an anchorage for the Fall Arrest System (FAS) both minimize the Free Fall Distance (FFD) to the largest possible extent. Who is this Fall Arrest System (FAS) designer and anchorage selector? In 90% of cases it is a supervisor, a foreman, a safety officer or another company employee whose expertise lies outside fall protection engineering. The above may be unexpected and not necessarily the most desired “news”, but who else selects, purchases and issues various components made by different manufacturers to the plant’s work crews?

The overall rated capacity of a shock absorber or a shock absorbing lanyard is defined as the amount of energy which these devices can absorb without exceeding the standardized Maximum Arrest Force (MAF) and Standardized Extension (SAE). These limits in North America are governed by the appropriate standards [7], [9]. The nominal rated capacity represents this portion of the overall rated capacity which is of interest to the users of these devices (considered here are only the free fall and the weight of the Fall Arrest System [FAS] user). It is recommended that the evaluation of the shock (energy) absorbers is performed by an engineer, or by a qualified person.

In Fall Arrest Systems (FASs) with lanyards only, the shock load (aka the Maximum Arrest Force [MAF]) acting on the Fall Arrest System (FAS) user (as well as on the Fall Arrest System [FAS] anchor) is proportional (but not directly)[1] to the Free Fall Distance (FFD). In Fall Arrest System (FAS) with rope grabs on vertical lifelines, the Maximum Arrest Force (MAF) depends on the fall factor, and not directly on the Free Fall Distance (FFD). The fall factor is a ratio of Free Fall Distance (FFD) to the arresting length of the Fall Arrest System (FAS). In Fall Arrest Systems (FASs) with Horizontal Lifelines (HLLs), both the Maximum Arrest Force (MAF) and the MAL (the Maximum Arrest Load acting on the HLL anchorages) depend on the Free Fall Distance (FFD). For more information about the hazards of high Maximum Arrest Force (MAF) go to Hazard Alert no. HA-019.

The Total Fall Distance (TFD) always increases with the rise of the Free Fall Distance (FFD) as the TFD=FFD+DDD, where DDD means the Dynamic Deceleration Distance [3]. An excessive Total Fall Distance (TFD) is dealt with in the Hazard Alert no. HA-018. An example of a Total Fall Distance (TFD) with a large dynamic component which is typical to all Fall Arrest Systems (FASs) with Horizontal Lifelines (HLLs) is shown in Figure 1 on page 4.

In the process of arresting a fall the initial energy carried by the fall victim is absorbed by all components of the Fall Arrest System (FAS) and by its user. To minimize the part which goes “into” the user and which may cause an injury, we strive to absorb as much as possible in the Fall Arrest System (FAS) itself. However there is a limit to what the Fall Arrest System (FAS) can take without failure, and therefore the Free Fall Distance (FFD) should always be analysed, particularly when an Fall Arrest System (FAS) is to be used by a person weighing more than 310 lb [100 kg] [See Hazard Alert no. HA-016], or if an Horizontal Lifeline (HLL) is to be used by several workers at a time.

References

1. “Fundamentals of Fall Protection” Sulowski, A.C. et al., ISFP, Toronto, ON, 1991
2. “Introduction to Fall Protection” Second Edition, Ellis, J.N., ASSE, Des Plaines, IL, 1993
3. “Fall Arrest Systems - Practical Essentials”, Sulowski, A.C., CSA international, Toronto, ON, 2000
4. “A Study of Personal Fall-Safety Equipment”, NBSIR 76-1146, Product Systems Analysis Division, Institute for Applied Technology, National Bureau of Standards, Washington, DC, USA
5. “Selecting Fall Arresting Systems”, Sulowski, A.C., National Safety News,Vol.20, No.4, October 1979, NSC, Chicago, IL, USA
6. “Horizontal Lifelines with Fall Arrest Systems Equipped with Class A Shock Absorbers and Modified Computer Program” Miura, N., Sulowski A.,C. Research Report No. 89-43-H, Ontario Hydro, Research Division, Toronto, ON, March 1989
7. ANSI Z359.1-1992 (R1999) American National Standard safety requirements for personal fall arrest systems, subsystems and components. American National Standards Institute Inc., New York, NY, and American Society of Safety Engineers, Des Plaines, IL.
8. “The Fundamentals of Fall Protection” Seminar, CSA International, Toronto, ON, 1999.
9. CSA Z259.11-05 Energy absorbers and lanyards.

Figures

Figure 1 - An example of a FAS in which the Total Fall Distance has a large dynamic component