Anti-Two-Block Devices and Safety Systems Flashcards

An anti-two-block (ATB) device activates on a lattice boom crane during a pick. The operator overrides the device using the manufacturer-provided bypass switch to complete the lift. Under OSHA 1926.1416, which statement best describes the legality and required follow-up?

Answer: The bypass is permissible if a competent person deems it safe, and the device must be repaired before further use unless an alternative safeguard is in place.

OSHA 1926.1416(d)(1) requires that safety devices be operational, but it also recognizes that bypasses may exist. A competent person can authorize continued limited use with an equivalent safeguard in place, but the ATB must be repaired before returning to normal operations without that alternative protection. Simply completing 'one lift' without an alternative safeguard or competent person authorization is not compliant.

A crane is equipped with both a primary ATB switch (weight-actuated bob) on the main block and a secondary ATB limit switch on the auxiliary hook. During inspection, the auxiliary ATB is found inoperative. The crew only plans to use the main hoist that day. What is the correct course of action per ASME B30.5 and OSHA standards?

Answer: The auxiliary line must be physically prevented from being used (e.g., blocked, flagged, de-rigged) AND the deficiency must be documented and scheduled for repair before the auxiliary is used again.

When an ATB device on one hoist line is inoperative, the safest compliant approach is to ensure that line cannot be inadvertently used — through physical means such as blocking or de-rigging — while documenting the deficiency for repair. ASME B30.5 and OSHA both require that protective devices are either operational or that the hazard is otherwise controlled. Verbal notification alone does not constitute an adequate safeguard.

Which of the following conditions would MOST likely cause a weight-actuated ATB device to give a FALSE 'safe' indication (fail to trigger) even when two-blocking is imminent?

Answer: The ATB weight bob is frozen or stuck in its raised (depressed) position due to corrosion or ice accumulation.

A weight-actuated ATB bob that is frozen or stuck in its raised/tripped position would fail to drop down and actuate the switch as the block approaches the boom tip — creating a dangerous false 'safe' indication. An open circuit (broken wire) would typically cause the ATB circuit to fail to a safe (tripped) state in properly designed systems, halting hoist function. Operating radius and parts of line do not affect ATB functionality in this failure mode.

A crane operator is working with a jib attached and notices the ATB indicator light on the jib line is illuminated (ATB circuit open/tripped) before hoisting begins, even with the auxiliary block fully lowered. After resetting, the light immediately returns. What is the MOST likely cause and correct action?

Answer: The ATB weight on the jib head sheave is likely resting on the limit switch due to a broken suspension cord or wire. The crane must be taken out of service and inspected.

If the ATB circuit reads as tripped (open) with the block fully lowered and immediately returns after reset, the most probable mechanical cause is that the ATB weight bob has fallen or its suspension cord has broken, causing it to rest on and continuously actuate the limit switch. This is a physical defect requiring immediate removal from service and inspection — not an operational condition. ATB systems do not have warm-up periods, and a suspended block with no load should not trip the ATB.

According to ASME B30.5, when is it permissible to use a crane WITHOUT an anti-two-block device installed on a hoist line that has a hook, shackle, or similar attachment?

Answer: It is never permissible to operate a crane covered by ASME B30.5 without an ATB device on any hoist line equipped with a hook.

ASME B30.5-2018 (and subsequent editions) requires anti-two-block devices on all hoist lines with hooks or attachments — there is no tonnage exemption, no 'commercially unavailable' exception, and no grandfather clause for older cranes in covered operations. If a crane is subject to B30.5, all equipped hoist lines must have functional ATB protection. The other options describe common misconceptions used to rationalize non-compliance.

During a tandem lift using two cranes, Crane A's ATB activates unexpectedly while both cranes are holding the load. The signal person is unaware this has occurred. What is the MOST hazardous immediate consequence specific to the tandem lift scenario, and what should the operator do?

Answer: Crane A's hoist motion is locked out, potentially causing load shift and dynamic load redistribution to Crane B, possibly exceeding its rated capacity. The operator must immediately signal an emergency stop to all crane operators and the signal person.

In a tandem lift, any unexpected interruption to one crane's hoist — including an ATB lockout — creates an immediate risk of unplanned load redistribution. If Crane A's hoist freezes at that position while the lift is ongoing, the load geometry may shift, dynamically transferring load to Crane B and potentially exceeding its rated capacity. The correct immediate action is an emergency stop communicated to all parties. Tandem cranes do not share automatic lockout systems; the operator must not independently boom down or lower, which could worsen load dynamics.