Published Date: February 23, 2012
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Section 151 Clearance, maximum arresting force and swing
Subsection 151(1) Clearance distance
To ensure the safety of a fallen worker, two conditions must be met. The first condition is that the worker’s personal fall arrest system is arranged so that the worker cannot hit the ground, an object which poses an unusual possibility of injury, or a level below the work area. The second condition is there must be sufficient clearance distance including a safety factor. Figure 9.14 shows that using a 1.8 metres long (6 feet) lanyard, a worker needs approximately 5.7 metres (18.5 feet) to 6.8 metres (22.1 feet) of clear space below the level of the anchor point.
Clearance distance using a vertical life safety rope
The most important consideration when using vertical life safety ropes to arrest falls is knowing how much clearance is required. In general, vertical life safety ropes require more clearance than self retracting devices and should therefore only be used when large clearances are available.
The lock-off distance of the fall arrester, lanyard length, stretch of the vertical life safety rope, swing drop, deployment of the shock absorber and the type of harness that the worker is wearing all contribute to the required clearance distance. The following example illustrates how to calculate the required clearance distance below the working platform in accordance with the methods described in CSA Standard Z259.16-04, Design of Active Fall-Protection Systems.
Figure 9.14 Clearance distance
The worker is 1.8 m (6 ft.) tall using a 1.8 m (6 ft.) long lanyard. The combined weight of the worker, clothing, and tool belt is at least 100 kg (220 lbs).
A Length of lanyard – 1.8 m (6 ft.)
B Shock absorber pulling apart – 1.1 m (3.6 ft) CSA E4 or ANSI-compliant shock absorber; 1.75 m (5.7 ft) CSA E6 or European EN-compliant shock absorber.
C Harness stretch plus D-ring sliding – 0.3 m (1 ft.) for normal harness and 0.75 m (2.5 ft) for stretch webbing harness
D Height of worker – 1.8 m (6 ft)
E Safety factor – clearance below feet of 0.9 m (3 ft)
F A+B+C+D+E Minimum clearance distance varies between 5.7 m (18.5 ft) and 6.8 m (22.1 ft) depending on the components used in the system
Clearance distance example:
A worker uses a Class E4 energy absorbing lanyard that is 1.8 metres long and can deploy up to 1.07 metres at a force of 4 kN. The lanyard connects the dorsal D-ring on the worker’s harness to an automatic fall arrester that is known to lock onto the vertical lifeline within 0.3 metres. The automatic fall arrester will initially hang the lanyard length (1.8 m) below the D-ring on the harness. The rigid anchorage of the vertical lifeline is 29.7 metres above the location of the fall arrester at the onset of the fall. The lifeline is known to stretch 22 percent at 8 kN and 15 percent at 4 kN. The worker is 8.4 metres laterally from the anchor and therefore subject to a swing drop distance of 1.2 metres. The worker is wearing a “comfort” harness that will stretch 0.75 metres at peak fall arrest forces, and may fall from a kneeling position.
The length of lifeline above the fall arrester after it has locked onto the lifeline = 29.7 metres + 0.3 metre lock-off distance for the fall arrester = 30 m
|Free fall = 2 x lanyard length + lock off of the fall arrester = 2 x 1.8 m + 0.3 m||3.90 m|
|Stretch of the vertical life safety rope = 15% of the rope length = 15% of 30 m||4.50 m|
|Maximum deployment of the shock absorber||1.07 m|
|Swing Drop||1.20 m|
|Stretch of the harness||0.75 m|
|Stretch-out of the worker (falling from a kneeling position)||0.75 m|
|Mandatory Safety Buffer||0.60 m|
|Total Required Clearance below the working platform||12.77 m|
While the above example is an extreme case, it illustrates how choices of equipment and equipment configuration affect the required clearance distance. It also provides a template for calculating clearances for other configurations and choices of equipment.
Strategies for reducing the required clearance distance include using short lanyards, low stretch life safety rope and conventional harnesses (0.3 metre stretch). Workers can also be trained to manually “park” the fall arrester as high up the rope as possible once they get to their working position. This will help reduce their free fall distance in the event of a fall.
The above methodology will show that approximately 3.0 metres is the minimum achievable clearance when falling from a standing position while using an automatic fall arrester with a 0.3 metre shock absorbing lanyard, even if the vertical life safety rope has negligible stretch and there is no potential for swing falls.
Note that the above calculation assumes full (1.07 metre) deployment of the shock absorber, which is the worst case scenario.
Situations may arise in which a personal fall arrest system must be used without a shock absorber. The most common circumstances encountered is a lack of adequate clearance distance. All else being equal, eliminating a shock absorber reduces the required clearance distance by up to 1.1 metres (3.5 feet) (see Figure 9.14).
If the shock absorber is removed from the personal fall protection system, then the worker’s free fall distance must be limited to 1.2 metres. Even with this fixed distance, employers and workers need to be aware that, depending on the type of lanyard selected, the maximum arresting force of 6 kN stated in subsection 151(3) can be exceeded. To determine the arresting force, the following equation should be used:
It is crucial that the employer carefully select the type of lanyard used in such situations and determine the maximum arresting force so that workers are not endangered.
Subsection 151(3) Maximum arresting force
Maximum arresting force is the short-duration (milliseconds to tenths of a second), peak dynamic force acting on a worker’s body as the worker’s fall is arrested. The maximum arresting force to which a worker can be exposed during fall arrest in Alberta is limited to 6 kN (1350 lbs).
Research studies have shown that the short duration forces that happen during fall arrest are unlikely to cause injury if they act vertically upwards through the buttocks and spine and are limited to no more than 9 kN (2000 lbs). The 6 kN limit is therefore considered safe, but as was discovered during the studies, is subject to the following conditions:
- the maximum arresting force is applied upwards through the pelvic area;
- the worker’s physical condition is sufficient to withstand such a jolt; and
- the duration of the maximum arresting force is limited to a fraction of a second.
A fall arrest system that correctly uses a shock absorber will limit the maximum arresting force under normal circumstances to either 4 kN (900 lbs) or 6 kN (1350 lbs), providing a margin of safety.
Maximum arresting force is determined by the worker’s weight, the length of the lanyard, and the ability of the fall arrest system to absorb the energy of the fall. The anchor should be above the work position, the length of the lanyard kept as short as possible (while still permitting the work to be performed safely) and the fall arrest system should almost always include a shock absorber. Readers are referred to the explanation for subsection 151(2) to see the equation often used to calculate the arresting force.
This edition of the OHS Code accepts a maximum arresting force (MAF) of 6 kN under normal circumstances because
- the 6 kN MAF value has been successfully used in Europe and other jurisdictions for many years. The 8 kN value previously cited in the OHS Code appears to have been a North American phenomenon,
- the lower MAF is technically achievable with today’s fall protection equipment, and
- the lower MAF means that workers are exposed to a lower arresting force, reducing the potential for injury.
Subsection 151(3) incorporates this change but includes a condition that reflects the fact that under worst case conditions (a wet and then frozen shock absorber), the MAF can be as great as 8 kN for a type E6 shock absorber. Readers are referred to section 142.3 for a discussion of E4 and E6 shock absorbers.
Subsections 151(4), 151(5) and 151(6)
As required by subsections 151(4) and 151(5), a worker must use the shortest length lanyard that still allows the worker to perform his or her work safely and the lanyard must be attached to an anchor no lower than the worker’s shoulder height unless an anchor at shoulder height is not available. When an anchor at shoulder height is not available, the lanyard must be secured to an anchor point as high as reasonably practicable.
Tying to an anchor at foot level is dangerous. A shock absorber approved to the CSA Standard for shock absorbers will safely absorb energy based on a 2 metre fall of a 100 kg worker. But tying a 1.8 metre lanyard at foot level can subject the shock absorber to a 3.6 metre free fall. Unless specifically designed for this type of free fall, the shock absorber’s webbing may fully extend without absorbing all the energy of the fall, resulting in a “bounce” at the bottom. The remaining energy (and there could be a great deal of it) goes into the worker, potentially causing serious injury.
Shock absorbers approved to CEN Standard EN 355: 2002 are currently available in the marketplace that will accommodate a 3.6 metre free fall and still limit the maximum arresting force on a 140 kg worker to 6 kN. Employers using these products must take into account the extra clearance that these products require. A European shock absorber will elongate up to 1.75 metres (5.75 feet) in a fall.
The problem of securing the lanyard to an anchor at an appropriate height may be solved by using a horizontal lifeline passing across the work area (see Figure 9.15), a hitching post that raises the anchor point, or a self-retracting device attached to an anchor located well above shoulder height. Other solutions may be possible.
Figure 9.15 Example of using a flexible horizontal lifeline to safely move through a work area