How close to the face must rock dust be applied for safety?

The application of rock dust is a critical safety measure in coal mines, primarily to control the risk of dust explosions and to reduce the risk of ignition of combustible materials. The correct answer, which indicates that rock dust should be applied within 40 feet of the face, is based on regulations and best practices that aim to ensure an effective layer of inert material is present in areas prone to ignition.

Applying rock dust up to 40 feet from the face helps create a safer environment for miners by mitigating the dangers posed by explosive dust accumulations. This distance ensures that both the initial workings and the ventilation systems are protected, as rock dust can help minimize the intensity of a potential explosion and limit the propagation of flames if an ignition occurs.

While the other distances mentioned in the choices may be relevant in different contexts, the 40-foot guideline specifically addresses the need to cover the area where explosive dust can accumulate in a more immediate proximity to the active working face. This proactive measure is aligned with safety regulations designed to protect miners and maintain operational safety within coal mining environments.