At what methane concentration must the detector automatically trip the electricity supply to the auxiliary fan?

• A. 1%
• B. 2%
• C. 3%
• D. 4%

Answer: (B)

When methane in the return air hits a relatively low but still significant level, the safety system is designed to shut down non-essential electrical equipment to reduce ignition risk. In this setup, the detector is set to trip the electricity supply to the auxiliary fan at a concentration of 2% methane. That threshold is chosen because it provides an early, decisive action point—low enough to intervene before hazards escalate, but high enough to avoid nuisance trips from normal fluctuations.

If the level were set much lower (like at 1%), small spikes could cause frequent shutdowns, disrupting ventilation unnecessarily. If it were higher (3% or 4%), more methane could accumulate before action is taken, increasing risk. Since methane becomes flammable around 5% in air, the 2% trigger acts as a precautionary interlock to limit ignition risk by removing energy from equipment in an area showing rising gas levels, while still allowing practical operation and timely response by the crew.