Crane Anti Two Block Device
How do you distinguish a compliant positive-acting anti-two-block device from a non-compliant warning-only system?
A compliant positive-acting ATB is wired into the crane's safety control circuit and automatically ceases hoisting (and often boom-up/retract) when the block contacts the sensor, requiring reset before resuming. A warning-only device provides alarms but allows continued motion if ignored. OSHA explicitly states that remote signaling alone does not meet the positive-acting requirement.
What is the difference between an anti-two-block device and a hoist limit switch?
A hoist limit switch provides general overtravel protection, typically stopping the hook block at mechanical limits without addressing two-blocking geometry or stopping boom motions. An anti-two-block device specifically targets two-blocking using a weighted ring or proximity sensor at the boom tip, disengaging all functions that could cause two-blocking, as required by OSHA/CSA/ASME for many cranes and personnel lifting.
How do WorkSafeBC rules interact with CSA Z150 and manufacturer requirements for anti-two-block devices?
WorkSafeBC OHSR Part 13 and WCB Standards require a prevention device for telescoping booms and a warning device for fixed/lattice booms when raising work platforms. CSA Z150:20 mandates anti-two-blocking devices for mobile cranes hoisting personnel. Manufacturers must integrate ATB per ASME/ANSI for post-1992 cranes. Any modification affecting ATB requires engineering review under OHSR 14.15.
A crane anti-two-block device (A2B) is a positive-acting safety system that automatically stops or disables all crane motions capable of causing two-blocking—where the lower load block contacts the upper sheave or boom tip—preventing hoist rope failure, component damage, or load drop. It typically uses a weighted ring or sensor at the boom tip to trigger a cut-out and warning, ensuring compliance with OSHA, ANSI, CSA, and WorkSafeBC standards.
In shop and industrial settings, anti-two-block devices are critical on service truck cranes, small mobile cranes, and tower cranes used for lifting heavy components or personnel. The sensor assembly, often a weighted ring near the boom tip, activates when the hook block approaches, cutting power to hoist-up and often boom-up or retract motions. This prevents wire rope failure, sheave damage, and load drops during tight maneuvers or blind lifts. Operators must lower the hook or adjust the boom to reset the system. These devices are mandatory for personnel hoisting under OSHA, CSA Z150, and WorkSafeBC, ensuring automatic prevention rather than relying on operator reaction.
Treating warning-only systems (alarms without cut-out) as compliant positive-acting devices, which fails OSHA and CSA requirements for personnel lifting and post-1992 cranes.
Bypassing or disabling the anti-two-block device via jumpers or mechanical holds to avoid nuisance trips, exposing the crane to two-blocking risks and serious regulatory violations.
Using cranes for personnel lifting without a dedicated anti-two-block device or with only a generic hoist limit switch, which does not address two-blocking geometry or stop boom motions.