Choosing a Fall Arrest Block or Shock Lanyard
A fall arrest device is designed so that in the event of a fall the unit will ensure that your impact loading does not exceed 6kN or 600kgs. This can be done either with an external shock absorbing pack or with an internal ratchet system.
Upon impact the external shock absorbing pack tears and increases the lanyard length ensuring the force used to achieve this substantially decreases the direct impact loading upon the body.
With the internal ratchet system the clutch mechanism version rotates inside the housing which again absorbs the force by increasing the connection length which again decreases the impact force upon the body.
So without prior knowledge you could say they both do the same job. But shock lanyards have a distinct disadvantage over a fall arrest block. If a fall is taken wearing a lanyard the fall will amount to the entire length of the lanyard from the anchorage to the attachment point.
So if you are using a 2 metre lanyard that is anchored above your head and then to either the shoulder or chest ‘D’ ring, your drop distance will be the length of the lanyard that is slack. This could be very little or the full length of the lanyard if the anchorage point is only the same height as the attachment point in a fall factor 1.
So in a fall where the attachment is like this you would fall 2 metres and then the pack would deploy a set amount dependent upon body weight. So this could be 2.5 to 3 metres plus the length of your body and then 1 metre of safety. So even in a fall factor 1 the wearer could travel 5+ metres.
If a fall arrest block was used, because the wire or webbing is governed by a recoil spring. it automatically pays in and out with your movement. At the moment a fall takes place and the wire or webbing achieves a certain speed the unit will lock and engage the ratchet or absorption pack. The lock moment is almost instantaneous so we can state that the room required for this type of unit in a fall factor 1 would be 3.5 metres and quite possibly less in reality.
If the units are ever used in fall factor 2 instance then the lanyard with get progressively worse with the fall arrest block improving as the distance will be reduced as the unit will try to recover the wire or webbing during the fall until the unit engages due to having reached over speed.
Does this make either unsafe? No it just means that you should select the equipment you require determined by an in depth risk assessment of the task to be done. A shock lanyard used in the correct way will result in a drop of no more distance than the fall arrest block so long as it is only used in fall factor 1 instances.