How good is the 8kN Maximum Arrest Force limit in industrial Fall Arrest Systems?
Andrew C. Sulowski, P.Eng.
Sulowski Fall Protection Inc.
Toronto, Ontario, Canada
2006
A PDF version of this article,
"How good is the 8kN Maximum Arrest Force limit in industrial Fall Arrest Systems?"
is available for download.
This paper was presented by the author at the International Symposium on Fall Protection - IFPS’06 - held in Seattle, WA on June 14-15,2006
Abbreviations used in the paper
- MAF — Maximum Arrest Force
- FAS — Fall Arrest System
- PFAS — Personal Fall Arrest System
- FPS — Fall Protection System
- OSHA — Occupational Safety and Health Administration, US Department of Labor
- OHSAct — Occupational Health and Safety Act, Province of Ontario, Canada
- O.Reg. — Ontario Regulation
- EA — Energy Absorber
- G — Gravitational acceleration
- ANSI — American National Standards Institute
- CSA — Canadian Standards Association
- CEN — Committee European de Normalization
- ISO — International Standards Organization
- ASTM — American Society for Testing and Materials
The existing legal limit on MAF in USA and Canada
- OSHA 29 CFR Parts 1910 and 1926 Safety Standards for Fall Protection in the Construction Industry; 1926.502 (d) (16)(ii) "Personal FAS when stopping a fall, shall limit maximum arresting force on an employee to 1,800 pounds (8 kN) when used with a body harness."
- OHSAct and Regulations for Construction Projects; O.Reg.213/91 as amended by O.Reg.145/00 Section 26.6 (5) "The FAS shall not subject a worker who falls to a peak fall arrest force greater than 8 kilonewtons."
Fall arrest — full body harness; (+Z)
System of Coordinates
The question at hand:
- Does the 8 kN (1,800 lbs) limit on the Maximum Arrest Force (the shock load) in industrial Fall Arrest Systems depend on the direction of the force and the point of application to the human body?
Our hypothesis
- The 8 kN limit for the Maximum Arrest Force transmitted to the fall victim via full body harness applies ONLY to the force acting along the spine and applied via the sub-pelvic bone.
Note: the connecting subsystem of the FAS may be attached to the D-ring located at the back of the harness in the middle of the shoulder blades, or alternatively it may be attached to a D-ring located frontally in the sternum area, as long as the harness design allows the straps to transfer most of the MAF to the sub-pelvic region.
When is the MAF acting not along the spine?
- The MAF experienced in fall protection systems (FPS) for ladders in which the attachment to the slider is frontal and at the waist level. (+X direction)
- The MAF transmitted to the hip area via a side D-ring which may be experienced when misusing the Y-style (double-leg) lanyards. (Y direction)
Rail FPS for ladders; (+X)
Seat Harness; (+X)
Side D-ring; (Y)
Searching for the answer
- What are the human body’s endurance limits and/or injury thresholds for the shock load (MAF) acting along the +X and Y axis and applied from the back into the spine and through the hip respectively?
- Note: the origins of the 8 kN (USA, Canada) or the 6 kN (Lower MAF and larger safety margin, European Union) limits on the MAF are well documented and are outside the scope of this presentation.
The existing +Z MAF limits
Searching for the +X shock load into the spine
Doctoral Thesis by Helmut Magdefrau: "Die Belastung des menschlichen Körpers beim Sturz ins Seil und deren Folgen". Ludwig-Maximilians University, Munich, Germany, December 1989.
"The Force And Its Consequences To the Human Body when Falling Into a Rope"
Field of safety: mountaineering, recreational activity.
Magdefrau’s results
The tests with the manikin in a seat-harness have shown that the shock load of 4 kN [900 lbs] directed into from the back and into the spine at the waist level (due to the frontal, waist level attachment) via the waist strap of the seat harness resulted in a broken spine. This type of injury normally leaves its victim either paraplegic or dead.
Similar falls, arrested also with the frontal attachment, but this time in the sternum area through a full body harness with MAF acting on the sub-pelvic bones structure, produced no injury.
Tests with the humanoid manikin were supplemented by tests with volunteers. The test were fully instrumented and the shock loads were measured. One of the volunteers was Magdefrau himself. These tests were performed in the Alps with the volunteers jumping while attached to a dynamic kernmantel rope, which constituted their primary FAS component. All test-jumps by the volunteers were done with full body harnesses only, and none resulted in any injury.
Conclusions from Magdefrau’s research
- The mountaineers should wear full body harnesses.
- The seat harness should be banned.
- The injury threshold for the shock load directed into the spine (+x direction) from the rear, at the waist level, causes the so called "reversed jackknife" body reaction, and can be fatal at 4 kN level.
- The injury threshold of human body subjected to transitory deceleration depends on the direction of the force and the part of the body it is applied to. Note: Amphoux, Brinkley and others proved that when the shock load is applied to the body via the pelvic bone and the force is directed upwards along the spine — our body can tolerate 6 or 8 kN without an injury.
MAF limits for +X, spine
Searching for the limit of the shock load (MAF) into the hip area (Y)
Research by A. Zaborowski, USAF
- A series of 50 controlled deceleration experiments was performed on 37 male volunteers to establish, if possible, human body’s injury threshold to lateral (Y axis, waist level application) shock loads. The subjects were restrained in a seat with a lap belt. The D-ring of the lap belt was in the hip area.
- The volunteers were exposed to shock loads from 3.25G to 9.02G for durations of 0.3 to 0.1 second.
- No permanent injuries occurred.
- Minor physical complaints were reported by 50% of volunteers when exposed to 6.25G or more.
- Tests were stopped at 9.02G due to the danger from lateral flexion of the torso of up to 30 degrees from the vertical.
MAF limits for the Y, hip
MAF limits for +Z;+X and Y
We can now present the documented results for the effects of the shock loading of the human body along the three major axis and with the specific points of application.
MAF limits for +Z;+X and Y
MAF in FAS with peronal energy (shock) absorbers
Almost all fall arrest systems employ personal energy absorbers in order to control the maximum arrest force. The current North American and European standards allow for the upper limit of the shock load to vary from 8 kN (Canada) to 6 kN (EU). The commercially available energy absorbers are capable of generating MAFs well below those limits.
MAF limits vs. MAF from available personal energy (shock) absorbers
Conclusions
- The existing legal limit of 8 kN for the Maximum Arrest Force in the industrial , personal fall arrest systems applies exclusively to the shock load (MAF) directed into the sub pelvic area and acting upwards along the spine. Our initial hypothesis was correct.
- The MAF of 8 kN (the existing limit) may lead to a serious injury or a fatality when applied perpendicular to the spine, from the back towards the front in the waist area (+X). Such loading may occur in some ladder fall protection systems, unless the length of the link between the user and the system’s rail (or a wire rope) is severely restricted.
- The MAF of 8 kN (the existing limit) may lead to a serious injury when applied laterally, perpendicular to the spine, from the side in the waist area (Y). Such loading may occur when the FAS is attached to the side D-ring on the harness.
- Not all of the currently available North American and European personal energy (shock) absorbers can be considered safe for the MAF acting along the +X or the Y axis.
- Note: Majority of the North American and European personal energy absorbers generate the MAF below 4 kN. This limit seems to be safe for the +Z (sub-pelvic) and for Y (lateral) impact loads. However for the +X (the reversed jack-knife body reaction) direction, the 4 kN force is unacceptably high. Also, this below 4 kN performance is not required for compliance with the current ANSI, CSA, CEN and ISO fall protection standards.
We Recommend
- That results of research by Dr. Helmut Magdefrau and by Albert Zaborowski are made known to the users of fall protection equipment thus allowing the users to make appropriate decisions when selecting the equipment. This is needed because changes to regulations and standards take years to materialize.
- That the authorities around the world should be advised that the existing MAF limits are not safe under some conditions, and that the research work which was reviewed here can make certain organizations liable for injuries to the workers. The authorities to be advised are:
- the governmental Health&Safety groups
- the technical standard writing organizations (CSA; ANSI; ASTM, ISO, CEN), and
- the fall protection equipment manufacturers.
- That the following legal limits are adopted world-wide for the maximum arrest force (MAF) according to its direction and the point of application to the human body:
- 6 kN for the +Z; subpelvic,
- 2.75 kN for the +X; spinal (waist),
- 4 kN for the Y; hip area.
Note: notwithstanding the above limits, FAS designers should always strive to keep the MAF as low as reasonably achievable.
References and selected bibliography
- Magdefrau H. "Die Belastung des meschlichen Korpers beim Sturz ins Seil und deren Folgen", Ludwig-Maximilians-Universitat, Munich, 1989
- Zaborowski A.B. "Human Tolerance to Lateral Impact With Lap Belt Only" Proceedings of the 8tStapp Car Crash and Field Demonstration Conference, Detroit, October 21, 1966. Detroit, Wayne University Press, 1966, p.34-71
- Crawford H. "Survivable Impact Forces on Human Body Constrained by Full Body Harness" Report HSL/2003/09 for the Health and Safety Executive, U.K.
- Brinkley J.W., Raddin J.H. "Biodynamics: Transitory Acceleration" in Fundamentals of Aerospace Medicine, R.L.DeHart Editor, Lea&Febiger, Philadelphia, 1985
- Amphoux M. "Psychopathological Aspects of Personal Equipment for Protection Against Falls" in the Fundamentals of Fall Protection, A.C.Sulowski Editor, IFPS, Toronto, 1991
- Snyder R.G. "Human Tolerances to extreme Impacts in Free Fall" Aerospace Medicine, Vol.34, No.8, August 1963, p.695-709.
- Snyder R.G. "Human Impact Tolerance", International Auto Safety Conference,p.712-755, SAE, 1970)
- Noel, G. et al "Some Aspects of Fall Protection Equipment Employed in Construction and Public Works Industries", in the Fundamentals of Fall Protection, A.C.Sulowski Editor, IFPS, Toronto, 1991.
- Eiband A.M., "Human Tolerance to Rapidly Applied Accelerations. A Summary of Literature", NASA, Memo No.5-19-59E, Washington, D.C., 1959
- Snyder R.G. et al "Study of Impact Tolerance Through Free-Fall Investigations", The University of Michigan, UM-HSRI-77-8
- Ruff S. "Brief Acceleration: Less than One Second" German Aviation Medicine WWII, Vol.1, p.584-597, USAF, Washington, D.C.,1950
- Sulowski A.C. "Performance Requirements for Class C Fall Protection Equipment for Wood Pole Climbing and Laboratory Evaluation of Commercially Available Equipment", the OHT Fall Protection Centre, Toronto, Dec.1996. CEA Report No. 096 D 1007
- Arteau J., Giguere D. "Proposed Method to Test Harnesses for Strength and Human Factors Criteria", ISFP’88, Orlando, FL
- Ellis J.N. "Introduction to Fall Protection", ASSE, Des Plaines, IL, 1993
- "Fundamentals of Fall Protection", Sulowski A.C. (Editor) et al; ISFP, Toronto, 1991.
- Sulowski A.C. "Fall Arrest Systems — Practical Essentials", Canadian Standards Association, Toronto, 2000.
- OSHA 29 CFR Parts 1910 and 1926 Safety Standards for Fall Protection in the Construction Industry; 1926.502 "Fall protection systems criteria and practices." - section (d) "Personal fall arrest systems."
- OHSAct & Regulations for Construction Projects; Ontario Regulation 145/00 Subsection 26.6 — Fall arrest systems