How far must shielding or a trench box extend above the top of an excavation?

The correct choice, which states that shielding or a trench box must extend at least 18 inches above the top of an excavation, is based on safety regulations that aim to protect workers from potential hazards associated with trenching. The requirement for an 18-inch minimum helps ensure that the shielding or trench box effectively stabilizes the wall of the excavation, thereby minimizing the risk of collapse and falling materials.

Such a height provides an adequate safety buffer, ensuring that if soil or debris were to shift, the piece of equipment still offers protection to those working in the trench. The purpose of these regulations is to mitigate the dangers associated with trench work, where collapses can occur suddenly and with great force.

Other heights, such as 12, 24, or 30 inches, do not meet the established safety standards, which are designed to balance risk with effective employee protection. An insufficient height may not provide adequate protection, while a greater height could unnecessarily increase costs and impede efficiency without delivering significant additional safety benefits.