At what concentration in the general body must the electricity supply to non intrinsically safe plant in the adjacent ERZ1 be automatically switched off by an automatic methane detector located in the intake airway at the interface between a NERZ and ERZ1?

• A. 0.25%
• B. 1.0%
• C. 0.5%
• D. 2.0%

Answer: (C)

The key idea is that the automatic methane detector at the boundary between the non-explosive risk zone (NERZ) and ERZ1 is set to trip and switch off power to non intrinsically safe equipment when methane in the general body reaches a level that signals a safe-but-protective response. At 0.5% methane, the detector triggers the automatic shutdown of electricity to non-IS plant in ERZ1 to remove potential ignition sources just as the gas starts to rise toward a flammable range. This threshold is chosen because it provides early protection well below the flammable limit (methane has a 5% LEL, so 0.5% is about 10% of LEL), giving a safety margin without causing excessive nuisance trips. Lower thresholds could cause unnecessary shutdowns, while higher thresholds would allow more methane to accumulate before action, increasing explosion risk.