During which operation can two blocking not occur?

The correct choice indicates that two blocking cannot occur during all boom up operations. This is grounded in the operational principles guiding crane movements. When a crane is lifting a load, the dynamics of boom up operations necessitate careful management of load, counterweights, and the crane's structural limits.

In boom up operations, the load is raised, and the system is subject to specific forces that prevent any form of blocking, which can lead to unsafe operations or instability. Blocking typically refers to conditions where the rigging or load can become inadvertently fixed or jammed, creating hazards.

Contrastingly, during boom down operations, it is often safer to manage the load and effectively control its descent. Therefore, certain operations where the boom is moving down may allow for blocking under specific controlled conditions.

Understanding the mechanics of how lifting and lowering affect a crane’s operation is essential for ensuring safety practices in lifting operations. Safety protocols mandate that signal persons should be aware of these dynamics to prevent accidents, particularly emphasizing that during boom up operations, the focus must remain on maintaining clear and correct load handling without engaging in any blocking behaviors.