Shock absorber technology in fall arrest lanyard
By Michael Biddle, Managing Director, Capital Safety
Wednesday, 12 June, 2013
You have heard of a ‘shock absorber’ before, but what does it have to do when it comes to fall protection? Or, maybe you have heard of a shock absorber but have you ever wondered how it works?
When people originally designed equipment to protect you from a fall, they designed body belts that would wrap around the waist, fitted with 6 foot (1.86 m) 3-strand polyester rope without any energy-absorbing properties, which were designed to either prevent you from reaching an edge where you could fall or, in the worst case, suspend your body after a fall.
The result of a fall in a body belt is never pretty - the fall forces applied in a concentrated area of the body can bring about severe internal injuries. So much so that belts were removed from recognition in fall arrest applications, along with the polyester rope lanyards without energy absorbers, some time ago. They were replaced with full body harnesses and fall arrest lanyards or self-retracting lifelines with energy-absorbing systems, both of which offer a higher degree of user safety during and after the fall, as well as lower impact on the body as a result of the fall itself.
The inclusion of shock-absorbing systems has now been part of fall protection for over 20 years. As their name suggests, they are designed to absorb energy that is created as the body falls towards the ground under that almighty force - gravity. The ‘shock’ of the fall is reduced with an energy-dissipating system that starts to take up energy applied to it over 200 kg of force. Under the Australia/New Zealand design standard AS/NZS1891.1, this must not allow the body to receive a force exceeding 6 kN (kilo Newtons - a measure of force named after the famous scientist Sir Isaac Newton).
In estimated terms, 1 kN of force is approximately equal to 100 kg of force. So even in the event of a fall where a person is subjected to the full force of gravity, this is a lot of force, but the average human can well survive such an event provided that they correctly use the equipment that has been designed for such a purpose.
So how does a webbing shock absorber actually work?
The predominant methodology of achieving shock-absorption properties is the use of sacrificial ‘tear webbing’ as a component of a total lanyard assembly. In such applications where a person uses the lanyard in ‘restraint technique’, the device will not deploy. This means that anyone can easily ‘lean’ into a lanyard and place their full body weight against the device at its full length and there will be no effect on the lanyard. It is only when a significant, sustained dynamic force exceeding 200 kg is applied to the lanyard (such as would occur in a fall) that you will start to see the device deploy.
Tear webbing is designed differently to other webbing as might be used in a harness. It generally involves using two strips (or halves) of webbing that are then woven together again a second time to form a single piece of webbing, but with two end ‘ears’. The ears are then sewn together with a piece of regular webbing in a circular loop, which is designed to be a ‘back-up’ device. This is very important as if the tear webbing were to separate completely in the event of a major fall, the separation could allow the person to fall right through and not be saved. This was actually typical of the first shock-absorbing lanyard designs in the 1980s where the length of tear webbing was quite short. However, these were soon replaced by those with the back-up strap feature once the consequences were fully understood.
Testing has shown that in most fall events, it is very unlikely that a full deployment of the shock absorber will occur. Along with testing, the laws of science also prove that the shorter the fall distance, the lower the amount of deployment (or tear-out) that actually occurs. This is, therefore, a reminder that you can minimise fall distance and impact on the body from a fall by using an adjustable lanyard or connecting at the highest anchorage point possible.
Energy-absorbing lanyards are provided with a worker weight limit and care should be taken to ensure that the user does not exceed these levels.
In the event a fall arrest lanyard is involved in a fall, it must be immediately removed from service and never used again. There is no such thing as keeping it for a ‘second use’.
And when in doubt, tag it out!
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