Two men would have no problem at all picking up crate of beer. But if the beer was suspended in the centre of a rope, and they could only pick it up by pulling each end of the rope, this would be far more difficult: and become more and more difficult as the rope length increased.
Exactly the same principle applies with fall arrest.
This basic post is more than adequate to hold the weight of this small worker high above this famous city.However when he is suspended from a string between two posts, it can be seen that exactly the same weight exerts more force on the posts and that this effect increases as the posts are placed further apart.
In this basic desk top example, we are only considering supporting the Lego character, not arresting his fall.
In real life the fall arrest force exerted by a man falling is 6kN as discussed in technical paper 1.
The force required at each end (or reaction) if the wire is to resist this fall, can be worked out by resolution of forces.
It can be seen that reactions, are a tan function of the 6kN force which gives a very narrow range of angles which will be practical.
The angle is dependant on the distance between the posts, the pretension in the wire, the deflection of the wire and lengthening of the wire under load.
The distance between the posts has an exponential effect on the force applied as it increases.
Basically if proper consideration is not given to all the following :
• The post centres
• The cable tension
• The deflection of the cable
• The extension of the cable at impact
• The load absorption of the cable and shock absorbers
The load applied to the structure the system is fitted to can increase exponentially.
For a system with posts at 10 m apart and no shock absorption, and incorrectly tensioned wire, a force up to and over 30kN – equivalent to the weight of two 4 wheel drive vehicles will be exerted on the posts.
The only way to absorb energy and reduce the load is through calculable and proven energy absorption.
A system with controlled energy absorption and correctly tensioned wire can reduce the load on the post to around 10kN.
Wire systems the facts
STOP ! A cable system is NOT class A anchors to EN795 with a wire connecting them!
- Cable systems are covered under clause 4.3.3 Class C Anchor devices employing horizontal flexible anchor lines. According to the standard the manufacturer must be able to calculate the loads their system will induce, and prove the accuracy of their calculations by testing.
- All component parts of the system should be CE marked.
- Never use a manufacturer who is unable to provide calculations to prove their products, or who states that post centres do not matter.
There are some systems being sold, that quite simply do not work and have not been correctly tested.