Safety Alert: Chain Tensioner – Incident involving a ZAZA2 CT chain tensioner

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Outdoors SA has received information from our fellow peak at Outdoors Queensland that Workplace Health & Safety QLD has advised about an incident involving a ZAZA2 CT chain tensioner and a safety recommendation regarding the ongoing use of these devices.

Re: Incident involving a ZAZA2 CT chain tensioner

An incident occurred at a high ropes facility in Queensland earlier in 2022 where a ZAZA2 CT chain tensioning device failed. The device included two chain connectors, a long threaded bolt, two collets through which the bolt passed, two nuts on the bolt and a steel washer. The threaded bolt allows for growth of the tree with the nuts being loosened as tree growth occurs. Please refer to information on the use of the ZAZA2 CT device that was previously available on a supplier’s website (Attachment).

During the incident, the bolt broke and the anchor chain around a tree pulled away, causing a fall arrest safety line to pull away from the tree so that it was no longer anchored to the tree. One person was completing a ‘lily pad’ obstacle course under the safety line and two other persons were waiting on a platform around the first tree prior to completing the obstacle course. It is understood that the person on the course has fallen, applied a shock load to the safety line, which has caused the bolt to fail. As a result, two persons on the platform were pulled off the platform as they were both attached to the safety line. It appears that all three persons became tangled in the obstacle course, and this prevented them from sustaining serious or fatal injuries.

Although the exact reasons why the bolt failed may be difficult to determine, the following is noted regarding the use of bolts in this application:

    • The design is unusual in that the bolt has both a tensile (pulling) load and bending load as the bolt sits against a block of timber and is also under tension from the chain around the tree. Bolts are typically either loaded in shear or tension.
    • Bolts normally do not have the same stringent testing requirements or quality control benchmarks as equipment that is specifically designed for safety critical operations (for example bolts do not normally require proof testing prior to being supplied nor the same quality assurance sampling methods). The quality control around the failed bolt is unknown.
    • It is possible for the bolt to be overloaded due to tree growth (i.e., if the nuts are not systematically adjusted for tree growth) or overtightening.
    • The design of the installed assembly is such that cracks and/or corrosion may be hidden under the collets and may not be visible for easy inspection while the device is in use.
    • The bolt may be susceptible to corrosion (e.g. coating type, dissimilar metals and environmental factors)
    • In the supplier’s data sheet for the product, the sum of the load for lines attached to the chain passing around a tree is not to exceed 30 kN (i.e. 3 tonne approx.). It may be difficult to limit the maximum potential load to 3 tonne or less in high ropes courses due to factors including:
      • shock loading from persons falling on the safety line (the load will be significantly higher from a person’s fall being arrested than the load from a static weight on the line).
      • the multiplier effect from more than one person falling at the same time.
      • the difficulty of being able to calculate the actual load (pre-tension, shock loading, load multipliers caused due to the relative angle between safety or zip line and the chain that passes around the tree).

      In view of the potential serious consequences of the recent failure of a ZAZA2 CT device and the information I have available, there is a need for facilities using this device to have additional measures in place to ensure the safety of these devices. Alternatively, facilities may wish to replace the ZAZA2 CT with another device that does not have these issues. Facilities should seek the advice of a professional engineer, experienced in the design of high ropes courses for assistance in this matter.

      For further information, please contact Stuart Davis at Stuart.Davis@oir.qld.gov.au

      Aaron Holman
      Chief Safety Engineer
      Engineering Unit
      Workplace Health and Safety Queensland