Under what condition should power to an auxiliary fan be switched off?

The correct choice highlights a critical safety consideration in mining operations. When an auxiliary fan's air supply exceeds a concentration of 1% methane (CH4), it signifies a potentially explosive atmosphere. Methane is a flammable gas that, in sufficient concentrations, can create hazardous conditions, making it essential to turn off power to the fan to mitigate the risk of ignition or explosion.

The focus on methane levels is crucial because it directly relates to the health and safety protocols in underground mining environments. Maintaining a close watch on gas levels, including methane, is a standard safety practice to ensure that workers are not exposed to dangerous conditions.

In contrast, factors like humidity, unusual noises, or operational status may be important to monitor but do not pose the immediate threat that high methane levels do. Humidity might affect comfort or equipment performance but is not directly dangerous on its own in the same way that elevated methane levels are. Unusual noises from a fan can suggest mechanical issues that require attention but do not necessitate immediate power shutdown unless they are indicative of an escalating problem tied to gas concentrations. Lastly, if the mine is not fully operational, this status does not inherently relate to the ventilation or air quality management that the auxiliary fan is designed to address.