Weighty issues for harness wearers

Designing good harnesses for people working at height has meant facing some difficult challenges, not the least of which has been that, over the last 10 years, there has been a gradual change in the body size and weight of the people who work at height that we are working to protect.

In Australia and New Zealand, fall protection equipment is manufactured, tested and used according to the AS/NZS 1891 standard - Industrial fall-arrest systems and devices. This standard is subdivided into four specialist parts:

• Part 1 covers the manufacture of harnesses, lanyards and ancillary equipment;
• Part 2 covers horizontal lifeline and rail systems;
• Part 3 covers fall arrest devices; and
• Part 4 covers the selection, use and maintenance of industrial fall arrest systems and devices.

Like all of the world’s fall arrest systems standards, there is a provision for ‘dynamic testing’ of harnesses in the AS/NZS 1891 standard. A dynamic test is a test where the harness is fitted to a dummy torso and the assembly is vertically dropped 4 m, firstly foot-first and secondly head-first. The AS/NZS 1891 standard uses the European style of fall arrest testing torso that weighs 100 kg and has square shoulders and no waist or buttocks.

In light of the fact that this dynamic test uses the European style of torso, a question that fall protection users in Australia and New Zealand regularly raise is: “How can a manufacturer expect their harness to perform properly if it is being used by person whose combined weight, including equipment and tools, weighs more than the 100 kg test dummy?”

This is an understandable question, as the harness is the part of the fall arrest system that is closest to the wearer, and may at first glance appear to be the most vulnerable part. Both the dynamic (drop) and static (pull) tests on an AS/NZS 1891 approved harnesses are much harsher than any of the other fall protection standards (eg, ASNI, EN, GB, JIS, KOSHA, ISO or CSA).

A harness certified under AS/NZS 1891 Part 1 has to be more robust than any harness made to the above overseas standards.

Although it varies from model to model, it would take forces in excess of 2 tonnes (approximately 20 kN) to cause an AS/NZS 1891-compliant harness to fail under load. As any fall arrest system set up since 1995 must have an energy-absorbing component to keep the deceleration force below 6 kN, there is no risk that the weight of the person alone (even with tools) will create significant damage to the harness.

Energy absorbers

Energy absorbers fall roughly into one of three types:

• Tear webbing is the most common type of energy absorber used with lanyards and other connecting components. This is a piece of ‘sacrificial’ webbing, designed to tear down the middle, smoothly taking up energy in the process. The two parts of the webbing peel apart, dissipating the impact of the fall by tearing literally thousands of fibres.
• Core energy absorbers are a very compact style of energy absorber that runs the length of the lanyard. These devices have the distinct advantage of providing a very soft arrest force.
• Friction energy absorbers are frequently the energy absorber component of mechanical devices such as Type 2 and Type 3 fall arresters. These devices have a retractable component (often galvanised or stainless steel wire or, in some cases, webbing) that a person working at height would connect to. The unit pays out and retracts as the person moves. Should the wearer fall, it will lock up and engage an energy absorber to minimise the impact on the person and the system.

At the time of impact, an energy absorber will lengthen as it starts the deceleration process in the event of a fall. The distance it will take to bring a falling person to a complete halt will vary depending on the distance the person has fallen before the lanyard has taken up the load. The weight of the person is also a determining factor in the distance it will take to come to a complete halt. With both tear- and core-style energy absorbers, what can happen with heavier people is that if they are connected to an anchorage point with a lanyard and they do take a 2 m fall, they may cause the energy absorber to deploy further than (say) a 100 kg person.

Harness weight testing

Using AS/NZS 1891 Part 1 2007-compliant energy absorbers, Miller Fall Protection recently carried out a succession of fall arrest tests with increasingly heavier test dummies. The results of these tests revealed exactly what was expected - as the dummy increased in weight, the amount of energy-absorbing tear webbing deployed also increased. Even with the heaviest dummy, there was no sign of damage to the harness (in these tests, a full body ‘tower worker’ harness was used).

As these tests were carried out with a solid torso dummy, the impact on the system (harness, lanyard and anchor point) would have been significantly more than would be applied with a human body.

So, in the event of a person heavier than 100 kg falling up to 2 m and their fall being sustained by a conventional fall arrest lanyard, the fall will be arrested, but the deceleration distance will be greater than a lighter person. When a 100 kg dummy is dropped, the amount of energy-absorbing capacity used is well below the 50% capacity of the energy absorber. The Miller Fall Protection testing showed that, on average, the point at which more than 50% of capacity was used was at 135 kg (assuming a 2 m fall). Consequently, for conventional harness and lanyard configurations, a maximum weight of 130 kg is recommended.

If the person is being anchored with a Type 2 or Type 3 fall arrester (inertia reel) fall arrest block, the situation is slightly different. As most blocks use a friction process to provide the necessary energy absorption to keep the forces on the wearer below 6 kN, this enables them to carry a higher rating (usually up to 140 kg).

It should be understood that working with fall protection equipment requires good planning and a good understanding of the guidelines that are set out in AS/NZS 1891 Part 4. This sort of information provides the basis to enable a supervisor to make good quality decisions on the selection, usage and positioning of fall protection equipment. Thoroughly understanding the guidance that is outlined in this standard is recommended to all people who have to make decisions on height safety equipment and/or ensure that they are adequately maintained.

Most manufacturers also provide detailed instructions with their equipment. These should be read and understood before they are to be used in any work at height.

Health aspects

The other side of this issue is one of health. Working at height is usually associated with considerably strenuous work. Consequently, it could end up being work that an overweight person may find difficult or uncomfortable to perform. This aspect should also be taken into consideration by an employer when selecting people to perform work that has to be carried out at heights requiring fall arrest equipment to be worn.