In a booster fan methane detection system, at what concentration does the automatic detector trigger an audible and visible alarm?

• A. 1.25%
• B. 0.5%
• C. 2%
• D. 3%

Answer: (A)

The key idea here is understanding the alarm threshold set for methane detectors in booster fan ventilation. The automatic detector is calibrated to trigger an audible and visible alarm when methane reaches 1.25% in air. This level gives a timely warning that gas is entering or accumulating near the booster fan, prompting immediate action such as adjusting ventilation or following gas control procedures, before concentrations get closer to explosive levels.

Why this makes sense: 1.25% is high enough to avoid nuisance alarms from normal fluctuations, yet low enough to catch rising gas early so protective measures can be taken quickly. A lower setting like 0.5% would cause frequent alarms and reduce reliability, while higher settings like 2% or 3% would allow more gas to accumulate before alerting, increasing risk and delaying response.