At what concentrations does the detector alarm and trip for non-explosion-protected plant?

• A. Alarm 0.25%, Trip 0.5%, or within 3 minutes if detector fails
• B. Alarm 0.25%, Trip 0.75%
• C. Alarm 0.5%, Trip 1.0%
• D. Alarm 0.15%, Trip 0.3%

Answer: (A)

The main idea is how methane detector thresholds protect workers and equipment in a non-explosion-protected plant. The detector is set to raise an alarm at 0.25% methane by volume and to trip the plant at 0.5%. Those values correspond to about 5% and 10% of the methane’s lower explosive limit (LEL), respectively. That means you get an early warning well before the gas reaches explosive concentrations, and if levels continue to rise, the system automatically shuts down to prevent ignition risks. The clause about "or within 3 minutes if detector fails" provides a safety net: if the detector doesn’t alarm, the plant must trip within 3 minutes anyway to avoid dangerous buildup. Other option sets either move the trip into less protective ranges or use thresholds that don’t align with the standard safe response, so they’re not the best fit for this scenario.