The anti two block (A2B) device is an important topic for interviews and for every safety officer who inspects cranes on site. When you fill or verify a crane checklist, you must understand what this device does and how to check it.
What is two blocking?
Two blocking happens when the hook block is raised too high and touches the boom tip or upper block. When the two blocks touch, the wire rope can break and the load can fall, injuring people below and damaging property and equipment.
What is an anti two block device?
An anti two block device is a safety system installed on cranes. It includes a weight and a switch near the boom tip. When the hook approaches the boom tip, the weight lifts and triggers the switch. The device then:
- Gives a warning to the operator that the hook has reached the boom tip, and
- Stops hoisting automatically if the operator does not stop.
Without this device, the hook can be pulled into the boom tip, the rope can break and the load can fall, or it can damage the boom itself.
Why is the A2B device important?
- Prevents accidents and falling loads.
- Protects the crane and other equipment from damage.
- Saves repair cost and downtime.
- It is required by safety standards.
Responsibilities
- Operator: check the A2B device during the pre-use inspection and on the checklist: is it fitted, is the wire undamaged, is it working?
- Rigger: keep clear communication with the operator. If the A2B wire is damaged or the device is missing, inform the operator and stop the activity until it is fixed.
- Foreman and Work Permit Receiver (WPR): stop the lifting if the A2B is not working. They sign the permit, so they share the responsibility.
- Workers: stay clear of suspended loads. A sling can break, a hook can crack or the crane can fail mechanically, so never stand under a suspended load.
Final message
The anti two block device is small, but it prevents major accidents. Never operate a crane with a damaged or bypassed A2B device. Safety first.