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A snap hook or latch is used to prevent accidental load disengagement.
OSHA doesn’t specifically address this issue in the General Industry or Construction standards. OSHA does require latches in the Marine Terminal standard at 1917.45(e)(2), “Crane hooks shall be latched or otherwise secured to prevent accidental load disengagement.”
However, there is a reference to safety latches in the ASME B.5-2001 Standard for Mobile and Locomotive Cranes, saying that hooks should have latches unless impractical.
Also, in ASME B.10-2001 Standard for Overhead and Gantry Cranes it says that latch-equipped hooks should be used unless impractical.
Earlier versions of both of these ASME standards are incorporated by reference in 1910.6(e)(18), (19) and (20).
OSHA inspectors could always cite under the General Duty Clause if they feel that an employee was in danger because there was no latch on a hook.