European regulations for work at height
In 2005, new European regulations were introduced for work at height (reference 1). These regulations introduced a simple hierarchy for planning managing work at height.
1. Avoid working at height if possible.
2. Where work at height cannot be avoided, equipment or work systems should be used to prevent falls.
3. Where the possibility of a fall cannot be eliminated, equipment or work systems should be used to minimise the consequences of any fall.
Guarding the hazard is considered to be preferable (and therefore higher up the hierarchy) to guarding the worker. For example, a barrier around an edge would be preferable to using a restraint harness and lanyard. Similarly, HSE guidance (reference 2) considers collective passive protection to be better than individual protection, as this removes the reliance on each individual taking specific actions (such as remembering to clip on a safety harness lanyard) for protection themselves. Safety nets are mentioned as an example of the type of collective protection that is the preferred option.
While information on major injuries is available, there was little historical information on the number of times personnel have entered nets without suffering significant injury (ie the number of injuries prevented by the employment of nets). Without this information, it is difficult to accurately determine the effectiveness of nets.
Given the increased emphasis on safety nets as a result of these regulations, the aim of this work was to evaluate the effectiveness of safety nets, as used in the UK, and identify the risk (if any) of premature failure in the less than ideal situations that could occur in the workplace. The project was developed as a sampler for these non-ideal loading situations and included a range of variables. These included:
a) Polymeric material and method of manufacture (knotted or knotless).
b) The effect of items falling onto different positions in the net such as edges and corners.
c) The spacing of the attachment points between the net and structure.
d) The effect of repeated dynamic loading on one position in the net, simulating multiple
e) The effect of differently shaped items falling into the net.
f) The effect of sag in the net.
g) The use of alternative techniques to control sag in oversized nets.
h) The presence of defects.
In common with other polymeric fibre based materials, safety nets are susceptible to chemical, thermal and (most significantly) ultra-violet degradation. This degradation is collectively referred to as “ageing”. Additives packages are added to the base material to inhibit this degradation but it is not possible to entirely eliminate these effects. Hence nets undergo an annual tensile test to evaluate reduction in strength, however these tests are carried out on spare test meshes attached to the net at one corner and remote from sources of potential damage.
i) The effects of ageing and degradation due to ongoing service damage and the effectiveness of test meshes in monitoring degradation was evaluated.
To evaluate these variables a range of tests were carried out using a purpose built drop facility at HSL, Buxton. The resulting loads were measured at various positions around the periphery of the net and damage to the net monitored.
The work was carried out in three phases; phase 1 covered variables (a) to (d), phase 2 covered variables (e) to (h) and phase 3 covered variable (i) and involved small scale mesh tensile testing of meshes only.