Anti-Two-Block Devices and Safety Systems Flashcards

An anti-two-block (ATB) device is installed on a crane, but the load line has been reeved with an additional part of line since the last inspection. Which of the following best describes the likely effect on ATB system performance?

Answer: The ATB weight may not travel high enough to actuate the switch if the added line shortens the effective travel distance of the weight

ATB devices rely on a weighted ring or bob hanging on the load line beneath the hook block. When extra parts of line are reeved, the geometry changes — the weight may not rise high enough relative to the boom head to contact and actuate the limit switch before a true two-block condition occurs. This creates a dangerous gap where the ATB system fails to intervene in time.

During a lift, the operator notices the ATB alarm activates momentarily while lowering the boom, even though the load line is slack and the hook block is well below the boom head. What is the MOST likely cause?

Answer: Lowering the boom changes the angle of the load line, causing the ATB weight to swing and contact the switch

When the boom is lowered, the angle of the hoist line changes relative to the boom head. This angular shift can cause the ATB weight — which hangs freely on the line — to swing outward and momentarily contact the limit switch, triggering a false alarm even when no two-block condition exists. This is a known phenomenon operators must recognize to avoid incorrectly diagnosing a wiring fault.

A lattice boom crane is equipped with a functional ATB system. The operator is performing a pick-and-carry maneuver on a slope. Under which condition is the ATB system LEAST capable of preventing a two-block event?

Answer: When traveling downhill, causing a sudden surge of load line that momentarily unloads the ATB weight

Traveling downhill can cause the load and hook block to surge forward and upward relative to the boom head due to inertia and grade. This motion can cause the load line to go momentarily slack, reducing tension on the ATB weight. If the ATB system relies on the weight hanging taut on the line, a slack-line condition means the weight may not be properly positioned to actuate the switch before a two-block occurs.

ASME B30.5 requires that anti-two-block devices on mobile cranes prevent movement that would cause two-blocking. Which of the following crane functions must be DISABLED — not merely alarmed — when the ATB device is actuated?

Answer: Hoist up, boom down, boom extend, and any other function that decreases the distance between the hook block and boom head

Per ASME B30.5, the ATB device must prevent all crane motions that would decrease the distance between the uppermost load-handling device and the boom tip. This includes hoist up, boom down (which raises the tip toward the block), and boom extend on telescoping cranes. Simply alarming without disabling these motions is insufficient — the standard mandates automatic motion cutout for all movements that contribute to a two-block condition.

A crane operator is working with a whip line (auxiliary hoist) and a main hoist simultaneously. The crane has ATB devices on both lines. The whip line ATB actuates, but the main hoist continues to function normally. What is the correct immediate action?

Answer: Stop all crane operations immediately, lower the whip line hook block, and inspect the whip line ATB system before resuming any use of the whip line

When an ATB device actuates, it indicates the hook block has reached a dangerously close proximity to the boom head. The correct response is to immediately stop the affected function, lower the hook block to create clearance, and inspect the ATB system before resuming use. The ATB activation is not a routine event — it signals that the operator or system allowed the hook to approach a two-block condition, which requires investigation. Reduced speed does not prevent structural damage during two-blocking.

On a telescoping boom crane with a fixed jib attachment, where must the anti-two-block device be installed to comply with ASME B30.5?

Answer: At both the main boom head and the jib head, with independent actuation circuits for each

ASME B30.5 requires that an ATB device be installed at every point where a two-block condition can occur. When a fixed jib is attached, there are two independent load-handling points — the main boom head and the jib tip. Each requires its own ATB device with its own actuation circuit, because a two-block event can occur at either location independently. A shared single circuit would fail to protect against a two-block at whichever point is not directly monitored.