For non-explosion-protected plant, which thresholds describe the required alarm and trip?

• A. Alarm at 0.25%, Trip at 0.5%
• B. Alarm at 0.25%, Trip at 0.5%, Or within 3 minutes if the detector fails
• C. Alarm at 0.5%, Trip at 1.0%
• D. Alarm at 0.1%, Trip at 0.2%

Answer: (B)

The key idea is setting gas-detection limits to give a safe, prompt response in a non-explosion-protected plant: an early warning, a rapid automatic shutoff, and a fail-safe if the detector stops reporting.

Alarm at 0.25% LEL provides an early warning so personnel can take action before gas levels rise further. Trip at 0.5% LEL ensures automatic shutdown to remove ignition sources and prevent an ignition risk as gas concentration increases. If a detector fails, there must still be a safety path to trip within a short window—here, within three minutes—to avoid relying solely on a potentially failed sensor.

This combination—0.25% LEL alarm, 0.5% LEL trip, and a 3-minute trip if the detector fails—embodies the required safeguards for non-explosion-protected plant operation. The other options either omit the fail-safe timing or use thresholds that don’t align with this safety approach.