Horizontal Lifeline Hazards Alert

Summary

Horizontal Lifelines (HLL; aka Static Lines) carry several risks of injury to their users because of relatively high Maximum Arrest Loads (MAL), potentially very large dynamic Total Fall Distances (TFD) and a risk of multiple falls. Due to these relatively high risks, most jurisdictions require that Horizontal Lifelines' (HLLs') design and installation are done under the supervision of a competent person.

Please note, that these hazards and this Hazard Alert apply primarily to flexible, do-it-yourself, made on-the-job Horizontal Lifelins (HLLs). Available also are off-the-shelf, engineered Horizontal Lifelines (HLLs) manufactured by the fall protection industry which are often installed by contractors who were authorized by the manufacturers. In such engineered Horizontal Lifelines (HLLs), these risks have been minimized, and could remain very low providing that the Horizontal Lifeline's (HLL’s) users follow strictly all instructions and recommendations supplied by the Horizontal Lifeline (HLL) installers and/or manufacturers.

This Alert does not apply to permanent Horizontal Lifelines (HLLs) which employ rigid rails instead of ropes.

The Hazards

What are they?

Hazard no. 1: The High Maximum Arrest Load The Maximum Arrest Load means a peak, dynamic force acting along the line, and pulling the line off its anchorages at the instant of the fall arrest (See Figure 1, page 5). When the Horizontal Lifeline's (HLL’s) user falls, and his/her Fall Arrest System (FAS), which rides along the Horizontal Lifeline (HLL), arrests the fall - the Maximum Arrest Force (MAF) generated in the Fall Arrest System (FAS), acts on the Horizontal Lifeline (HLL). The Maximum Arrest Load (MAL), which is the result of the Maximum Arrest Force (MAF) being applied to the line approximately perpendicularly, may exceed 15xMAF, if the line was made of wire rope which was installed with high initial tension. If the Maximum Arrest Force (MAF) were to be 900 lb [4 kN.], the Maximum Arrest Load (MAL) could reach 13,500 lb [60 kN]. The Maximum Arrest Load (MAL) of such magnitude could easily exceed the strength of the Horizontal Lifeline (HLL) anchorage, thus leading to the Horizontal Lifeline (HLL) failure and a non-arrested fall with all its consequences to the fall victim. The hazard of high Maximum Arrest Load (MAL) in do-it-yourself Horizontal Lifeline (HLL) comes primarily from the “designer’s” lack of information about the magnitude of the Maximum Arrest Load (MAL) and its relation to the initial tension in the line. This subsequently leads to underestimation of the required strength of the Horizontal Lifeline (HLL) anchorages.

Hazard no. 2: The Large Dynamic Total Fall Distance This hazard arises from the fact that the dynamic Total Fall Distance (a sum of a Free Fall Distance [FFD] and the Dynamic Deceleration Distance [DDD]) has to include also: the dynamic, momentary sag of the Horizontal Lifeline (HLL) line (See Fig. 1), the initial line sag; the free fall distance; the dynamic extension of the Fall Arrest System (FAS) including permanent extension of a shock absorber or a shock absorbing lanyard and other, smaller distances (e.g.: relocation of the D-ring on the harness etc.). As a result, it is not unusual to observe the Total Fall Distances (TFDs) of 25 to 30 ft [7.6 - 9.2 m], particularly in Horizontal Lifeline (HLLs) with long uninterrupted spans. When an unqualified “designer” employs special energy absorber in the Horizontal Lifeline (HLL) - the Total Fall Distance (TFD) is larger still. (The gain here is in the lower Maximum Arrest Load [MAL]).

Hazard no. 3: The Multiple Fall Hazard This hazard exists mainly in Horizontal Lifelins (HLLs) with long span and without intermediate supports. When more than one worker is attached to such Horizontal Lifeline (HLL), an accidental fall of one worker will cause subsequent falls of all others because of the downward pull on the Horizontal Lifeline (HLL) rope during the fall of the first victim. In addition to a risk of injury present in any Fall Arrest System (FAS), such multiple falls may end in all or some of the victims bumping into each other at the lowest point in mid-span and subsequently suffering injuries due to such violent, involuntary “meeting”. This risk is higher still if the fall victims are carrying some tools attached to their harnesses.

Why does it exist? In general, all three Hazards exist because of the phenomena taking place in Horizontal Lifelines (HLLs) which, from the physics view point, represents a flexible line suspended freely between two, same elevation, anchor points. The curve in which such free hanging line will hang is called a catenary. A mathematical model of a catenary can be found in any engineering handbook. When a Fall Arrest System (FAS) slides along a Horizontal Lifeline (HLL) and a fall happens, the Horizontal Lifeline (HLL) is suddenly impacted by the shock loading generated in the Fall Arrest System (FAS). This shock loading is perpendicular to the theoretical horizontal line between the Horizontal Lifeline's (HLL’s) anchorages. In other words, the Maximum Arrest Force (MAF) from the Fall Arrest System (FAS) causes loading of the Horizontal Lifeline (HLL) which as a result sags, while being pulled out of its anchorages. The strength of these anchorages must comply with the appropriate fall protection regulations and standards. This Maximum Arrest Load (MAL) is a reaction in the Horizontal Lifeline (HLL) to the input of the Maximum Arrest Force (MAF) from the Fall Arrest System (FAS). Due to the Maximum Arrest Loads (MALs) the Horizontal Lifeline (HLL) sags, and the Fall Arrest System (FAS) user is exposed to a Total Fall Distance which includes his/her free fall, the initial sag of the Horizontal Lifeline (HLL), its dynamic, momentary sag, and the Dynamic Deceleration Distance of the Fall Arrest System (FAS) itself. All attempts to calculate magnitudes of Maximum Arrest Loads (MALs) acting on anchorages and the distances travelled by the fall victim(s) lead to computerized calculations at a level of difficulty normally associated with a degree in structural, civil or mechanical engineering. No accurate assessment of the Maximum Arrest Load (MAL) and the Total Fall Distance (TFD) in Horizontal Lifeline (HLL) can be made without either running a computer program or testing. The latter gives better accuracy but can be prohibitively expensive. The author’s experience from training of the Horizontal Lifeline's (HLL’s) users is that, on average, the magnitudes of both Maximum Arrest Load (MAL) and Total Fall Distance (TFD) expected by the audience were several times lower than those which were calculated using a computer program. This fact of underestimating the loads and the distances in Horizontal Lifelines (HLLs) makes them particularly hazardous for those who attempt to procure do-it-yourself Horizontal Lifelines (HLLs).

When more than one person is attached to an Horizontal Lifeline (HLL), his/her fall may trigger falls of all others because the sagging Horizontal Lifeline (HLL) will pull them down.

Where can you experience it? The Horizontal Lifeline (HLL) hazards exist when unqualified persons simply string the rope between two structures, consider this a safe Horizontal Lifeline (HLL) and let others attach their Fall Arrest System (FAS) to such lines.

Who is affected by it and when? The most frequent underestimating of the loads and distances in Horizontal Lifelines (HLLs) can be found in the construction industry (particularly steel erection) and in maintenance operations on railway bridges. However, anybody, in any industry, who undertakes to use do-it-yourself, on-the-job-made Horizontal Lifelines (HLLs) without consulting with a P.E. exposes the users of such lines to very high risk of fatality.

How to Eliminate It Or Minimize Its Consequences?

1. Do not attempt to install a length of rope between two points and expect it to be a safe Horizontal Lifeline (HLL) without consulting a PE. or a P.Eng. with regard to the loads acting on anchorages and the total fall distances and subsequently the necessary clearances required for such Horizontal Lifeline (HLL). A Professional Engineer’s stamp of approval is mandatory in many jurisdictions.
2. Use temporary, off-the-shelf Horizontal Lifelines (HLLs), which were pre-engineered by their manufacturers, and follow strictly the User’s Instructions.
3. Ask for Energy Absorber(s) in your Horizontal Lifeline (HLL) in order to lower the Maximum Arrest Loads (MALs), but remember that their use increases the Total Fall Distance (TFD).
4. Ask for Self-retracting Lifelines in your Fall Arrest System (FAS) in order to minimize the free fall distance, but remember about the possible cascading effect in some of those Self-Retracting Lifelins (SRLs) [See Hazard Alert HA-009].
5. Ask for a Shock Absorber in your Fall Arrest System (FAS) but remember that lowering of the Maximum Arrest Force (MAF) will come at a price of greater deceleration distance.
6. Ask for intermediate supports along the span of your Horizontal Lifeline (HLL) spaced maximum every 30 ft [9 m.].
7. Beware of over-tensioning the wire rope Horizontal Lifeline (HLLs) even if the presence of a turnbuckle is tempting to do just that. Strictly follow recommendations of a competent person regarding the initial tension of the line.
8. Beware of large dynamic Total Fall Distance (TFD) in temporary Horizontal Lifelines (HLLs) made of three strand, twisted synthetic fibre rope.
9. Do not attach the Horizontal Lifeline (HLL) rope directly to any sharp edge anchor. Employ anchorage connectors.
10. Terminate wire rope with a thimble eye which prevents too sharp a bend in the rope.
11. For temporary wire rope Horizontal Lifeline (HLL) consider ½ inch [12 mm] nominal diameter wire rope. The 3/8" [9.5 mm] diameter rope, while acceptable in pre-engineered permanent Horizontal Lifelines (HLLs), is too risky in a do-it-yourself Horizontal Lifeline (HLL) when the number of simultaneous users may be difficult to control. Ropes of diameter larger than ½ inch [12 mm] are not user friendly when handling them. Do not use any do-it-yourself Horizontal Lifeline (HLL) without it being first approved by a P.E. (P.Eng in Canada).
12. For temporary synthetic fibre rope consider minimum 5/8 inch [16 mm] static kernmantel of polyester/polyester with minimum breaking strength of 10,000 lb (approx. 45 kN) or special ropes (e.g.: Kevlar) of higher strength. Attach your Fall Arrest System (FAS) to such Horizontal Lifeline (HLL) using travelling rings. Avoid riding of the Fall Arrest System (FAS) snap hook directly on the rope. Do not use any do-it-yourself Horizontal Lifeline (HLL) without it being first approved by a P.E.
13. Beware of UV enhanced deterioration of synthetic fibres.
14. Never allow more users on the Horizontal Lifeline (HLL) than the line was designed for.
15. Perform the Before-use and Periodic Inspections of your Horizontal Lifeline (HLL) and your Fall Arrest System (FAS).
16. Provide proper training in fall protection to all users of Fall Arrest System (FAS).
17. When using off-the-shelf Horizontal Lifelines (HLLs) strictly follow the supplied User’s Instructions.

Additional Information and Comments

Fall Protection is the most complicated of all Personal Protective Equipment (PPE). Fall Arrest Systems (FASs) are the most complex of all Fall Protection Systems (FPSs), and Horizontal Lifelines (HLLs) are in turn the most difficult to model mathematically and then calculate, of all Fall Arrest System (FAS) components. High Maximum Arrest Loads (MALs) and Total Fall Distances (TFDs) require very careful assessment, and professional engineers are the best equipped to do so. Testing of Horizontal Lifelins (HLLs) produces more accurate results than any modelling could, but it is extremely expensive.

The Total Fall Distance (TFD) shown in Figure 1 represents the lowest position of the victim during the fall arrest action. This position lasts fraction of a second and can be observed only if one uses a high speed video to record the fall. After the arrest, the post-fall position of the victim is at a higher elevation. This is because the high Maximum Arrest Load (MAL) is a short lasting, dynamic force equal approximately 4 times the victim’s weight, if a shock absorber was employed in his/her Fall Arrest System (FAS). The post-fall position is accompanied by a force equal to just the weight of the victim.

It is our recommendation not to resort to do-it-yourself Horizontal Lifelines (HLLs). There are several, excellent off-the-shelf, temporary-use Horizontal Lifelines (HLLs). There are tens of manufactured permanent Horizontal Lifelins (HLLs). If this is not enough - an advice of a P.E. or a P.Eng. should be obtained, as it is mandatory in most of the North American jurisdictions.

References

1. Miura, N., Sulowski A.C. “Introduction to Horizontal Lifelines” in the “Fundamentals of Fall Protection”, Sulowski et al, ISFP, Toronto 1991, ISBN 0-921952-01-5.
2. Arteau, J., Lan, A. “Protection contre les chutes de hauteur - conception de cable de secours horizontaux” Guide technique, IRSST, Montreal, 1991.
3. Sulowski, A.C., “Fall Arrest Systems - Practical Essentials”, the CSA International, January 2000, Toronto, Ontario, Canada; ISBN 1-894416-92-9.
4. Ellis, J.N., “Introduction to Fall Protection”, Second Edition, ASSE, Des Plaines, IL, 1993.
5. Riches, D., Feathers, L.J., “Research, Development and Testing of Multiple Span - Multiple Use Horizontal Lifelines from the Designer’s Perspective”, IFPS’98, International Fall Protection Symposium, 1998, Wuppertal, Germany.
6. Baszczynski, K., Zrobek Z., “Dynamic Performance of Horizontal, Flexible Anchor Lines During Fall Arresting - the Numerical Method of Simulation”, ditto.
7. Timmermans, M.F. “Ancrage mobile sur cable horizontal (AMC)”, ditto.
8. 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.
9. “Fundamentals of Fall Protection”, Sulowski A.C. et al, International Society for Fall Protection, Toronto, ON, 1991.
10. CSA Z259.16-04 Design of active fall-protection systems. Canadian Standards Association, 5060 Spectrum Way, Mississauga, ON, L4W 5N6.
11. CSA Z259.13-04 Flexible horizontal lifeline systems.

Figures

Figure 1 - Horizontal Lifeline