Self-Retracting Lifeline Hazard Alert No. 2

Summary

Some Self-Retracting Lifelines (aka Retractable Lifelines or Safety Blocks) may fail to arrest a fall of their user when, after an initial arrest and a brief stop, the Self-Retracting Lifeline (SRL) enters a cycle of lock-unlock actions. This cycle may continue until the user hits the lower level. This phenomenon of the lock-unlock actions is also referred to as the cascading effect.

The Hazard

What is it? Some Self-Retracting Lifelines (SRLs), after an initial arrest of the user’s fall, return to additional paying-out of the line, and the further descent of its user. This in turn leads to a brief arrest, which is followed by another free paying-out of the line (and another fast descent). This in turn leads to another brief stop (arrest) followed by another paying-out/descent, and so on and so forth in a cycle which ends with the Self-Retracting Lifeline's (SRL’s) user hitting the level or an object below.

This phenomenon is equivalent to a non-arrest, and depending on the nature of the level/object below, it may carry a very high risk, such as electrical contact, landing in a container filled with chemicals, or deep water. The velocity of the Self-Retracting Lifeline's (SRL’s) user at the time of contact with the lower level is not as high as if his/her fall were not arrested at all. Nonetheless, a risk of injury does exist, even if the nature of the level below is not hazardous in itself. This risk has to be minimized. Preferably, the Self-Retracting Lifeline (SRL) is tested under “real life” conditions for cascading before use and the hazard is eliminated.

Why does it exist? This hazard exists because the internal locking mechanism in a Self-Retracting Lifeline (SRL) contains two (or more) small ratchet pawls held by the pawl springs. This mass-spring assembly may, under certain conditions, enter a cyclical mode of operation. When the reel with the Self-Retracting Lifeline (SRL) line rotates faster than the design limit (and such fast rotation occurs during a fall), the ratchet pawls engage the locking ring (or equivalent) and the pawl springs are under tension. The locking action takes place, and is accompanied by the dynamic force (the Maximum Arrest Force [MAF]) which acts on the Self-Retracting Lifeline (SRL) user at one end, and on the Self-Retracting Lifeline (SRL) anchorage at the other. If this anchorage deflects momentarily and then rebounds, the rebound is accompanied by unloading of the main line. Now the pawl springs bring the ratchet pawls back to the unlock position and the line is paid out. This is equivalent to re-starting a fall. This in turn leads to momentary locking, and to a repeat of the above cycle. If the natural frequency of the Self-Retracting Lifeline (SRL) anchorage, the pawls’ mass-spring constant combination, and the victim’s weight all match the conditions necessary for cascading, we have to deal with this Hazard. The cascading effect is not very well researched, and the above mentioned conditions for its occurrence cannot be at this time expressed quantitatively. We know with certainty that it happens when a rebound of either the Self-Retracting Lifeline (SRL) anchorage, or of another Fall Arrest System (FAS) component (the Self-Retracting Lifeline [SRL] line, or a horizontal lifeline) causes momentary unloading of the Self-Retracting Lifeline (SRL) line.

Where can you experience it? This hazard exists in Fall Arrest Systems (FASs) with some (but not all) Self-Retracting Lifelines (SRLs) which are anchored to either a Horizontal Lifeline (HLL) or an extended arm of the davit style anchorage. In both instances, the vibration of such anchor point is quite normal and is to be expected. If the frequency and the amplitude of this vibration are within the sensitive range of these parameters on the Self-Retracting Lifeline (SRL), the cascading effect will occur. This Hazard may also be present in Self-Retracting Lifelins (SRLs) with a relatively elastic line (not a wire rope) in which the line’s elasticity results in a re-bound after an initial arrest. During this rebound the unlocking of the Self-Retracting Lifeline (SRL) mechanism occurs leading to paying-out of the line which causes another locking. This locking may not be a final one, and the Self-Retracting Lifeline (SRL) may go into the cascading mode.

Who is affected by it and when? Users of Fall Arrest Systems (FASs) with some Self-Retracting Lifelins (SRLs) which are attached to flexible Horizontal Lifelines (HLLs) or to other anchorages with a potential of developing vibrations (e.g: an end of a davit arm) may experience the cascading effect.

Unfortunately there is no analytical method of predicting which Self-Retracting Lifelins (SRLs) are prone to cascading without testing them while attached to a specific anchorage. This hazard may carry a very high risk to workers using an Self-Retracting Lifeline (SRL) over energized equipment, water or an open chemical tank or container, as well as confined spaces.

How to Eliminate It Or Minimize Its Consequences?

The only way to assure that cascading will not occur is to perform the before-use testing of the Self-Retracting Lifeline (SRL) attached to the “real life” anchorage, be it a Horizontal Lifeline (HLL) or a davit type. It is recommended that such testing is conducted by the supplier of equipment, either on- or off-site. In the latter case the “real life” anchorage should be simulated with reproduced mechanical characteristics at the point of attachment of the Self-Retracting Lifeline (SRL).

Additional Information and Comments

This little known, but potentially fatal hazard, was documented in the early ‘80s by the US Air Force during tests of Fall Arrest Systems (FASs) with Self-Retracting Lifelins (SRLs) employed on transporters of the Minuteman missiles. Since that time it was also observed during tests of Self-Retracting Lifelins (SRLs) attached to horizontal lifelines.

The cascading effect is not a common occurrence, but the Self-Retracting Lifeline (SRL) users should be very much concerned about this hazard in cases when the final landing of the fall victim would involve another serious hazard e.g: electrical contact, drowning or suffocation (bulk powdered goods).

References

1. “Minuteman Transporter Erector Fall Test” video, USAF
2. Laboratory evaluation of horizontal lifelines with various fall arrest systems. Proprietary information.
3. Harris, C.M., “Shock and Vibration Handbook”, Fourth Edition, McGraw Hill, New York, NY, 1996.
4. “Fall Arrest Systems - Practical Essentials”, Sulowski, A.C., CSA International, Toronto, ON, 2000.
5. “Introduction to Fall Protection”, Second Edition, Ellis, J.N., ASSE, Des Plaines, IL, 1993.
6. 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.
7. “The Fundamentals of Fall Protection” Seminar, CSA International, Toronto, ON, 1999.