Excerpt from MAIB SAFETY BULLETIN 1/201 ISO12401 (Small craft – Deck safety harness and safety line – Safety requirements and test methods), is the international standard applicable to this equipment. The standard contains detailed testing requirements that assume the tether and its hooks will be loaded longitudinally rather than laterally. The tether hook when loaded longitudinally, the tether can withstand a load of over 1 tonne. However, when loaded laterally a tether hook will deform at much less load. It is important that tether hooks remain clear of obstructions and are free to rotate to align the load longitudinally.
SAFETY LESSON To prevent the strength of a safety harness tether becoming compromised in-service due to lateral loading on the tether hook, the method used to anchor the end of the tether to the vessel should be arranged to ensure that the tether hook cannot become entangled with deck fittings or other equipment.