Monday, 22 October 2012

A shocking tale

A lot of rope rescue technicians still don't understand the principles of fall factor. However, it's quite simple, even if you hated arithmetic. Fall Factor is simply the distance of the fall divided by the length of the rope or lanyard from the person falling to anchor point.

The equation looks like this;

Fall Factor = Length of Fall / Length of Rope/lanyard

A Fall Factor 2 is the maximum you will encounter in a typical un-arrested fall, since the height of a fall can't exceed two times the length of the rope or lanyard. Normally, a Fall Factor 2 can only occur when a technician is working above his anchor point and at the maximum length of the rope or lanyard. The force is the same if you fall 2 metres or 20 metres.

Your well being depends on several factors, strength of your anchor, the stretch of the rope or deployment of the energy absorber, strength of your connectors and finally the fit of your harness.

Shock load is the result of three factors; the rope or energy absorber, the fall factor, and the weight of the falling object.

Obviously, the only part of this equation that can drastically reduce the force of a fall is the deployment of the energy absorber. So, safety systems are designed around the shock-absorbing quality of the energy absorber. It cushions the fall, reducing the impact force and the chance of system failure. The energy absorber is the one element in the whole system that is designed to limit the force of a rescue technicians’ weight in a worst-case fall (Fall Factor 2) to not more that 6 kN. The rest of the system can be designed to work with this known maximum force.

Static ropes are designed to minimise stretch. Their ability to absorb shock is marginal, particularly along short lengths of rope, and they transmit virtually all the shock load equally to the anchor system and to the body. In a rescue situation, a very short fall can develop enough force to be critical, especially with a casualty on board. Therefore rope of choice for rescue applications must be a semi static conforming to EN1891A and be of 11mm or 10.5mm in diameter. The compromise of the stretch will be very useful when arresting a fall and one thing often overlooked is the action of the various knots in the system to also tighten and absorb energy.

Webbing slings perform like static rope. Used for anchoring and extending a connection, slings are just as rigid as static rope. A Fall Factor 2 develops enough shock load to risk failure of the sling, the wearers harness, karabiners, not to mention damage to the rescue technicians internal organs.

In conclusion

A fall of less than four feet on a static rope or sling can create enough shock force to cause serious injury. The human body can only sustain, a shock force of 12 kN and loads of 18 kN are not only highly undesirable but very dangerous. In addition 18 kN is getting close to the minimum limits of all the items in your rope system. A fall factor 1.9 attached directly to a static rope running over a karabiner or pulley, with is normal shock force of 18 kN, becomes a shock force of 30 kN. Would your anchor hold? It is academic, because something else would undoubtedly fail.

Always ensure you’re protected by an energy absorber, protect yourself from falling by the use of restraint techniques or if safe to do so, use a fall arrest lanyard. Never work above your anchor point unless you are very well protected. Use only semi static ropes and ensure the equipment you use in the system is rated accordingly, e.g. sports climbing equipment has very limited use in rescue.

The most valuable item you carry is your brain, look at the problem and be sure you fully understand the consequences of your actions and results of an accidental arrested fall.


  1. It cushions the fall, reducing the impact force and the chance of system failure. The energy absorber is the one element in the whole system that is designed to limit the force of a rescue technicians,

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    1. Thanks for your comment. Manufacturers now understand that equipment designed for the individual, may have to take the combined load of a rescuer and casualty that have fallen onto their safety rope, as a result of a catastrophic failure in their working rope or systems. The Petzl ASAP with the Absorbica is a classic example. The ASAP’sorber has insufficient resistance for a two person load. As always, seek guidance from the manufacturers, as only they know the limitations of their products.