Fall arrest
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The U.S. Dept. of Labor's Occupational Safety and Health Adminsistration specifies under Title 29 of the Code of Federal Regulations that individuals working at height must be protected from falls. Fall arrest is one of several forms of fall protection as defined in 29CFR. The most common manifestation of fall arrest in the workplace is the Personal Fall Arrest System, or PFAS. A personal fall arrest system is a series of components designed to arrest a worker's fall, preventing him from striking the next lowest level and minimizing the possibility of serious injury.
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[edit] Personal Fall Arrest Systems
These systems must include 4 elements referred to as ABCD's of Fall Arrest:
A - Anchorage - a fixed structure or structural adaptation, often including an anchorage connector, to which the other components of the PFAS are rigged.
B - Body Wear - a full body harness worn by the worker.
C - Connector - a subsystem component connecting the harness to the anchorage - such as a lanyard.
D - Deceleration Device - a subsystem component designed to dissipate the forces associated with a fall arrest event.
Each of these elements is critical to the effectiveness of a personal fall arrest system. There are many different combinations of products that are commonly used to assemble a personal fall arrest system. The specific environment or application generally dictates the combination or combinations that are most appropriate.
[edit] Energy Absorption
To arrest a fall in a controlled manner, it is essential that there is sufficient energy absorption capacity in the system. Without this designed energy absorption, the fall can only be arrested by applying large forces to the worker and to the anchorage, which can result in either or both being severely effected.
An analogy for this energy absorption is to consider the difference in dropping an egg onto a stone floor or dropping it into soft mud. Even for the same fall distance and weight of egg (the input energy), there will be more damage with the stone floor as the arrest distance is smaller and so forces must be higher to dissipate the energy. For the soft mud, the arrest distance is longer and so arrest forces are lower but the egg is still stopped and is hopefully undamaged.
Most fall arrest anchorages are designed on the basis that all workers will have a connector which includes an energy absorber. Hence, when using such systems, it is essential that the PPE used is rated for Fall Arrest and includes an energy absorber.
A common form on anchorage device is an HLL (Horizontal Life Line). These are linear anchorage devices, which allow workers to move along the whole length of the anchor, usually without needing to disconnect and fixing points of the anchorage.
It is normally essential to include energy (or shock) absorbers within HLL in addition to those within the workers' PPE. Without such absorbers, the horizontal life line cannot deform significantly when arresting the fall. This in turn results in large resolved forces being generated within the anchorage system, which can readily be sufficient to cause failure of the anchorage system. This can occur even with energy absorbers being included in the PPE of the worker.
[edit] Fall clearance
In arresting a fall in a controlled manner the distance required to arrest the fall must be considered. This is a function of the "fall factor" and the deployment of the "energy absorbers". As a rule of thumb for a factor 2 fall, a fall distance of approx 6 metres will be required. This is equivalent to 2 storeys of a building. If the fall clearance is less than this the worker may strike the ground before his fall is arrested.
[edit] Design of HLL Systems
This is a complex process. The designer should always perform a design calculation and the results of this calculation should be presented in any proposal and verified as acceptable. The loads applied to the structure and the fall clearance required should be checked.
