At what concentration must the visual alarm be automatically activated by an automatic methane detector located in the intake airway at the interface between a NERZ and ERZ1?

• A. 0.25%
• B. 0.5%
• C. 1.0%
• D. 1.5%

Answer: (A)

Falling back on an early warning approach for methane in the intake air is the main idea here. In underground coal mining, detectors in the intake airway—especially at interfaces between zones like a NERZ and ERZ1—are set to give a warning well before gas levels become dangerous. The visual alarm at the interface is meant to alert people and trigger control actions before methane concentration gets higher.

Setting the visual alarm to 0.25% CH4 gives an early cue that gas is entering or accumulating in the intake air, so ventilation can be adjusted, sources of methane can be checked, and work can be stopped or redirected if needed. This threshold is low enough to provide a safety buffer because 0.25% is a small fraction of the methane’s lower explosive limit (5%), so it acts as a precautionary signal rather than waiting for a high concentration. Using a higher threshold would delay awareness and increase risk at this critical junction, whereas 0.25% aligns with the goal of prompt action to maintain safe conditions.