NCCCO Anti-Two-Block Devices and Safety Systems 4 — Questions and Answers
Question 1: On a crane with multiple part-of-line configurations, the A2B device must be:
- Installed only on single-part reeving
- Capable of functioning correctly regardless of the number of parts of line (Correct answer)
- Bypassed when using four or more parts of line
- Tested only when reeving is changed
Correct answer: Capable of functioning correctly regardless of the number of parts of line
The A2B system must protect against two-blocking in all reeving configurations used on that crane.
Question 2: What immediate danger does two-blocking create for the wire rope?
- The rope may unspool from the drum
- The rope can be pulled over the sheave causing catastrophic failure (Correct answer)
- The rope may freeze to the sheave in cold weather
- The rope becomes unbalanced across the sheave
Correct answer: The rope can be pulled over the sheave causing catastrophic failure
When two-blocking occurs, continued hoisting pulls the rope over or through the sheave, causing sudden rope failure and uncontrolled load drop.
Question 3: An A2B device must interrupt which crane functions when activated?
- Slewing and traveling only
- All crane motions simultaneously
- At minimum, hoist up and boom up and telescope out functions (Correct answer)
- Only the main hoist function
Correct answer: At minimum, hoist up and boom up and telescope out functions
ASME B30.5 requires A2B devices to interrupt hoist-up, boom-up, and boom-extension functions since each of these can worsen a two-block condition.
Question 4: Which condition can DEFEAT a weight-actuated A2B switch even when it is mechanically intact?
- High ambient temperature
- A kinked wire rope on the hoist
- Wind blowing the hanging bob-weight sideways causing it to contact the boom (Correct answer)
- Operating on a slope greater than 3 degrees
Correct answer: Wind blowing the hanging bob-weight sideways causing it to contact the boom
Strong wind can deflect the hanging bob-weight into contact with the boom structure, falsely triggering the A2B switch and halting operations.
Question 5: When a crane is equipped with a fly jib (boom extension), the A2B device must:
- Be disabled on the main boom when the fly jib is in use
- Protect both the main boom tip and the fly jib tip sheaves (Correct answer)
- Only protect the fly jib tip when in use
- Be manually repositioned to the fly jib by the operator
Correct answer: Protect both the main boom tip and the fly jib tip sheaves
Separate A2B protection is required at the fly jib tip because the hook block can two-block against the fly jib sheaves independently of the main boom.
Question 6: Under OSHA 1926.1416, if a required safety device such as an A2B system is not functioning, the crane:
- May continue operations if a spotter is used
- Must be immediately removed from service (Correct answer)
- May operate at 50% of rated capacity
- May operate only for pick-and-carry operations
Correct answer: Must be immediately removed from service
OSHA 1926.1416(d)(1) requires that any crane with a malfunctioning safety device be removed from service until repaired.
Question 7: The A2B device's electrical circuit is typically designed as:
- Normally open — current flows only when triggered
- Normally closed — tripping the circuit opens it and cuts power to controlled functions (Correct answer)
- A parallel circuit that adds redundancy
- A wireless Bluetooth signal circuit
Correct answer: Normally closed — tripping the circuit opens it and cuts power to controlled functions
A normally closed (fail-safe) circuit design means any break in the circuit — whether from triggering or wire failure — cuts power to the hoisting functions.
NCCCO Anti-Two-Block Devices and Safety Systems 4
On a crane with multiple part-of-line configurations, the A2B device must be: