How Anti-Two-Blocking Systems Mitigate Dangers

Written by Staff Writer

A lift can look smooth from the ground. The boom’s steady, the load is behaving and the operator is making small, clean movements. Then the line runs up a little too far, the hook block closes the last few inches and the day changes fast.

That event is called two-blocking. It’s one of those crane hazards that can start quietly and end loud, with snapped wire rope, damaged sheaves, dropped loads and injuries that were never on the plan.

An anti-two block (ATB) system exists for this exact moment. It’s a crane safety device designed to warn the operator and, on many cranes, stop the motion before the block contacts the tip of the boom. 

What Two-Blocking Really Means

Two-blocking happens when the load block, overhaul ball or similar component contacts the boom tip or upper block area. In simple terms, the hook assembly is pulled too far upward. 

When that contact happens under tension, forces spike in the line and components and the crane can experience a sudden shock load. If the upper travel isn’t controlled, there are risks that could endanger your worksite.

Why Two-Block Protection Matters

Two-blocking is not only a problem in terms of a crane being damaged. It can create a load drop hazard, a struck-by hazard and a swing hazard all at once.

The mechanical reasons are straightforward. When the block contacts the boom tip, the line can lose proper spooling, the rope can part or the crane can experience unintended load effects. That’s why crane standards and rules from the Occupational Safety and Health Administration (OSHA) treat upper-limit protection as a serious control, not a nice add-on.

How Anti-Two-Blocking Systems Work

Most systems that prevent two-block problems use a sensing method to detect when the block is approaching the upper limit. When the system detects a two-block condition, it typically triggers an alarm first. 

Common designs include: 

• Weighted switches hanging from the boom tip
• Proximity sensors with an alarm system
• Electronic limit systems integrated into the crane’s control package 

Some systems are wired, others are wireless and many modern cranes integrate the device into broader operational aids like load moment indicators and load monitoring.

Common Features

Systems vary by crane type, age and manufacturer, but the feature set usually falls into a familiar pattern. A crew will often see these functions in the field:

• Audible and visual warning when the hook block nears the limit
• Motion cutout to stop hoist-up before contact occurs
• Coverage for multiple two-block points such as boom, jib and extensions
• Integration with load monitoring or rated capacity indicator systems
• Warnings when something isn’t working

What Industry Standards Expect from Crane Operations

OSHA treats anti-two-blocking devices as a required safety feature on many cranes. Here are four important standards to keep in mind:

1. Modern Cranes Must Prevent Two-Blocking Automatically

Telescopic boom cranes manufactured after February 28, 1992 must be equipped with a device that automatically prevents damage. The device must prevent such damage at all points where two-blocking could occur.

2. Lattice Boom Cranes Follow Similar Rules

Some models rely on warnings instead of automatic shutdowns, depending on age and configuration.

3. Manufacturers’ Instructions Must Be Followed

If the manual says the system must be tested a certain way or shut down after a fault, that guidance becomes part of compliance.

4. Personnel Hoisting Has Stricter Requirements

When lifting people, OSHA generally requires a device that automatically prevents damage or load failure, with only narrow exceptions.

The message is consistent: If two-blocking is possible, protection should be built in.

Crane Operator Training for Safer Worksites

Safety equipment only helps if people know how to use it and how to respond when it activates. Good training in the construction industry covers:

• What the device does (and what the device doesn’t do)
• What alarms or shutdowns actually mean
• When work must stop
• How to reset safely

Focused crane training pays off. OSHA Education Center’s crane operator courses connects federal requirements to practical jobsite scenarios.

Rigging matters, too. Two-blocking risk climbs when lifts are improvised or communication breaks down. Basic rigging training helps crews choose proper gear, define roles, work with efficiency and keep lifts predictable, instead of chaotic.

Certificate Programs that Consider Compliance

Certificate-based programs help standardize expectations and keep clean records of who completed what and when. Programs like the Cranes and Derricks certificate simplify audits and internal tracking.

Online delivery helps, too. Crews train around weather delays, travel and shift schedules. Certificates of Completion are available immediately, which keeps compliance moving instead of stalling.

Simple systems reduce friction, and safer crews work with more confidence. So, if you’re working around cranes — or moving toward an operator role — getting formal crane and derrick training now can help you support safer lifts on the jobsite.

Safer Lifts Today

Crane safety systems are often a strong safeguard, but it only works when crews treat them as part of a bigger plan. That plan should include:

1. Routine inspections before every shift
2. Clear communication between operators and spotters
3. Thoughtful lift planning, not rushed decisions
4. Proper crane operator training so everyone knows what to watch for

A successful crew of construction workers will respect a system built to stop crane dangers from happening. Worksite protection is important, and everyone plays a role.