What CH4 concentration threshold triggers an alarm on detectors based on PHMP guidelines?

• A. Above 1%
• B. Above 3%
• C. Above 5%
• D. Above 0.5%

Answer: (A)

Detectors in coal mines are set to trigger alarms at a low, conservative methane level to give workers time to identify the source and take action before gas becomes explosive. CH4 is flammable in air from about 5%, so sounding an alarm above 1% provides a safety margin for ventilation adjustments, source control, and evacuation if needed. Higher thresholds like 3% or 5% reduce reaction time and bring you closer to the flammable limit, increasing risk, while a very low threshold such as 0.5% could cause nuisance alarms and unnecessary interruptions. Thus, the alarm threshold is placed at 1% to ensure timely safety responses.