Fall arrest
Encyclopedia
Fall arrest is the form of fall
Falling (accident)
Falling is a major cause of personal injury, especially for the elderly. Builders, electricians, miners, and painters represent worker categories representing high rates of fall injuries. The WHO estimate that 392,000 people die in falls every year...

 protection which involves the safe stopping of a person already falling. It is one of several forms of fall protection, forms which also include fall guarding (general protection that prevents persons from entering a fall hazard area e.g., guard rail
Guard rail
Guard rail or guardrail, sometimes referred to as guide rail or railing, is a system designed to keep people or vehicles from straying into dangerous or off-limits areas...

s) and fall restraint (personal protection which prevents persons who are in a fall hazard area from falling, e.g., fall restraint lanyard
Lanyard
A lanyard is a rope or cord exclusively worn around the neck or wrist to carry something. Usually it is used where there is a risk of losing the object or to ensure it is visible at all times. Aboard a ship, it may refer to a piece of rigging used to secure objects...

s).

The U.S. Department of Labor's Occupational Safety and Health Administration
Occupational Safety and Health Administration
The United States Occupational Safety and Health Administration is an agency of the United States Department of Labor. It was created by Congress of the United States under the Occupational Safety and Health Act, signed by President Richard M. Nixon, on December 29, 1970...

 specifies under Title 29 of the Code of Federal Regulations that individuals working at height must be protected from fall injury. Fall arrest is one of several forms of fall protection as defined in 29CFR.

Fall arrest is of two major types: general fall arrest, such as nets; and personal fall arrest, such as lifelines. The most common manifestation of fall arrest in the workplace is the Personal Fall Arrest System, or PFAS ("lifeline"). A personal fall arrest system is a series of components designed to safely arrest a worker's fall, preventing him from striking the next lowest level and minimizing the possibility of serious injury.

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
    Harness
    A harness is a looped restraint or support. It can also be referred to as an "hitcharness", especially by the Jordanian Armed Forces. Specifically, it may refer to one of the following harness types:* Bondage harness* Child harness* Climbing harness...

     worn by the worker.
  • C - Connector - a subsystem component connecting the harness to the anchorage - such as a lanyard
    Lanyard
    A lanyard is a rope or cord exclusively worn around the neck or wrist to carry something. Usually it is used where there is a risk of losing the object or to ensure it is visible at all times. Aboard a ship, it may refer to a piece of rigging used to secure objects...

    .
  • 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, and each must meet strict standards. The specific environment or application generally dictates the combination or combinations that are most appropriate.

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 force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...

s 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
Egg drop competition
The egg drop contest is an experiment usually performed by college or primary school students. Competitors typically attempt to create a device that can keep a raw chicken egg intact when dropped from a height...

 onto a stone floor or dropping it into soft mud. Even for the same fall distance
Distance
Distance is a numerical description of how far apart objects are. In physics or everyday discussion, distance may refer to a physical length, or an estimation based on other criteria . In mathematics, a distance function or metric is a generalization of the concept of physical distance...

 and weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

 of egg (the input energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

), 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
Mud
Mud is a mixture of water and some combination of soil, silt, and clay. Ancient mud deposits harden over geological time to form sedimentary rock such as shale or mudstone . When geological deposits of mud are formed in estuaries the resultant layers are termed bay muds...

, the arrest distance is longer and so arrest forces are lower but the egg is still stopped and is hopefully undamaged.

Because fall arrest designs require high-rate-energy capacity design methods, fundamental fall arrest design is tedious and esoteric. Thus, most fall arrest parts and systems are designed to the force standards contained in Federal OSHA 29CFR1910.66 appendix c, a force-type design standard which accounts for required energy considerations. The standard mitigates PPE
Personal protective equipment
Personal protective equipment refers to protective clothing, helmets, goggles, or other garment or equipment designed to protect the wearer's body from injury by blunt impacts, electrical hazards, heat, chemicals, and infection, for job-related occupational safety and health purposes, and in...

 interchangeability problems, allows wide use by designers not versed in high rate energy methods, and it limits the force into the worker to a survivable level.

Actual loads on the user and anchor-anchorage vary widely with user weight, height of fall, geometry, and type of line/rope. Excessive energy into the support and user is avoided by the use of energy absorbing PPE designed for the 1800 lbs maximum of the referenced Federal OSHA standard. (Designers should be cautioned that the force values of the standard are based on high rate energy system design and thus its force values are not necessarily inter-related.)

The most common fall arrest system is the vertical lifeline: a stranded rope that is connected to an anchor above, and to which the user's PPE is attached either directly or through a "shock absorbing" (energy absorbing) lanyard. Once all of the components of the particular lifeline system meet the requirements of the standard, the anchor connection is then referred to as an anchorage, and the system as well as the rope is then called a "lifeline".

Anchors used for lifeline anchorages are designed for 5000 lbs force per connecting user, and the standard permits an anchor to deform in order to absorb energy (adhesive anchors have higher design requirements because of aging loss).

The rope can be lifeline rope, which stretches to lengthen the fall distance as it absorbes energy; or static rope, which does not stretch and thus limits the fall distance, but requires the fall energy be absorbed in other devices. It is essential that the PPE be rated for Fall Arrest and PPE used with static line include an energy absorber. While the energy absorbing lanyards hold in excess of 5000 pounds when fully absorbed, most limit the load during the fall to under 1400 lbs.

Another common system is an HLL (Horizontal Life Line). These are linear anchoring 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. Because of the geometry of pulling across the horizontal line, this in turn results in large resolved forces being generated within the anchor system, sufficient to cause failure of the anchorage. This can occur even with energy absorbers being included in the PPE of the worker.

The load and horizontal line geometry in horizontal lifelines usually creates falls in excess of the 6 ft limit of the standard, limiting HLL design to standard-defined "qualified persons". (The recognition of these basic weaknesses have resulted in most temporary "wrapped structure" HLL anchors, which were anchors made from a wire rope wrapped around a structure and its ends fastened together by wire rope clips, being replaced by fixed-point anchors or HLL systems designed by defined "qualified" persons.)

Fall clearance

In arresting a fall in a controlled manner, the distance required to arrest the fall must be considered. Federal OSHA limits the fall distance to 6 feet unless the specific system is designed by a "qualified person" meeting the requirements of OSHA 29CFR1910.66 appendix c. The user also may not fall so as to strike protrusions or adjoining walls during the 6 ft fall.

The safe fall distance is a function of the fall factor
Fall factor
In climbing, using a dynamic rope, the fall factor f is the ratio of the height h a climber falls before the climber's rope begins to stretch and the rope length L available to absorb the energy of the fall....

 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 stories of a building
Building
In architecture, construction, engineering, real estate development and technology the word building may refer to one of the following:...

. If the fall clearance is less than this the worker may strike the ground before his fall is arrested.

Design of HLL Systems

This is a complex process. The designer should always perform a design calculation
Calculation
A calculation is a deliberate process for transforming one or more inputs into one or more results, with variable change.The term is used in a variety of senses, from the very definite arithmetical calculation of using an algorithm to the vague heuristics of calculating a strategy in a competition...

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.
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