Y-systems are basically designed to move from anchor point to anchor point. This ensures continuous safety and in the event of a fall, the shock absorber works independently of the position of the two arms.
I-systems are designed for stationary use. Progressive movement is not possible with an I-system. If two I-systems are used in parallel to move, an additional risk may exist in the form of an overlapping of the impact force of the two shock absorbers:
s an example, we refer to the situation described above. A person with a total weight of 100 kg moves from anchor point to anchor point with two parallel I-systems. The vertical distance between the anchor points is 40 cm. The person now falls 1 m. The I-system attached to the top anchor point is tensioned first and the shock absorber starts to work. After 40 cm braking distance the fall is stopped. Since the anchor point of the second I-system is 40 cm lower, the second I-system becomes tensioned at exactly the same moment. If the free fall height was now 1.5 m instead of 1 m, the shock absorber would need 60 cm braking distance to stop the fall. After 40 cm braking distance the second I-system is tensioned and also the second shock absorber starts to work. The impact force of the two systems add up and forces of up to 12 kN can act on the person. This corresponds to twelve times the gravitational acceleration for a 100 kg person. Accelerations of this magnitude usually cause serious injuries.
The worst conceivable case occurs when two systems are attached to the same anchor point or at the same height. In the event of a fall, both systems are loaded exactly simultaneously and twice the fall arrest force occurs over the entire braking distance.
For the purposes of these considerations, it is assumed that both lanyards have the same length. If the lanyards are of different lengths, this can increase or reduce the problem, depending on the attachment scenario.
Whether there is a risk of superimposed impact force when moving with two I-systems depends on the following factor:
- User weight
- Freefall height
- Vertical distance between the attachment points, respectively length of the lanyards
The greater the fall height and the users' weight, the greater the braking distance and therefore the greater the vertical distance between the anchor points must be.
It is not practical to specify this distance for all possible shock absorbers available on the market, user weights and fall heights.
Thus, movement with two I-systems is possible in principle. However, due to the additional danger of superimposing the impact forces, it must be clear that this variant can only represent an emergency solution. If a lanyard with shock absorber is to be used for planned movement, the use of a Y-system is always recommended. If two I-systems are used in parallel, care must be taken to ensure that the vertical distance between the anchor points is as large as possible and that the upper lanyard is kept as tensioned as possible to reduce the freefall height to a minimum.