“We’ll just stretch a line and hook onto it.”
This statement is very often heard by lifeline manufacturers and suppliers from potential clients attempting to improvise fall protection systems.
However, fall arrest systems must comply with clearly defined performance requirements, as specified in applicable technical standards such as EN 795:2012.
First of all, the equipment provided to workers must include an energy absorber, designed and manufactured in accordance with EN 355. The purpose of the energy absorber is to limit the maximum arrest force transmitted to the worker during a fall to 600 kg.
This reduces the loads acting on the human body to approximately half of its maximum tolerable limit. Studies conducted by military aviation on parachutists have demonstrated that an 80 kg human body can withstand maximum accelerations of 15 g, corresponding to a force of approximately 1200 kg. On this basis, the strength requirements for fall protection devices are defined under two conditions: dynamic and static.
Under dynamic conditions, a maximum load of 900 kg is applied for one worker or 1200 kg for two workers.
This load may be reduced if the device is capable of dissipating energy through controlled plastic deformation, ensuring a safety factor of 1.5 under service conditions.
In addition, a second requirement, known as the integrity requirement, specifies that the device must withstand a static load of 1200 kg applied for at least 3 minutes.
To provide a practical comparison, a mid-sized passenger car has an unladen weight of approximately 1200 kg.
The situation is further complicated by catenary theory, which applies to flexible horizontal lifeline systems. A load applied at the midpoint of a cable generates significantly higher forces at the end anchors, depending on the cable deflection (and therefore its length). In the absence of line-integrated energy absorbers, forces of up to 2500 kg can be reached and transmitted to the supporting structure.
It should be emphasized, however, that these forces are impulsive in nature. In most cases, the structure is capable of withstanding such loads, although it will likely be damaged after a fall and require repair. A “small” price to pay for saving a human life.
“More effective prevention strategies would save not only tens of billions of dollars, but save tens of thousands of lives… THEY ARE THE DISASTERS THAT DID NOT HAPPEN.”
— Kofi Annan, United Nations Secretary-General’s Annual Report, 1999
