How soon must fuel and ignition circuits be de-energized following flame failure?

In the event of a flame failure in a high-pressure boiler system, safety protocols dictate that the fuel and ignition circuits must be de-energized promptly to prevent hazardous situations such as an explosion or fire. The requirement to de-energize within a specified timeframe is crucial for quick intervention to stop the flow of fuel, ensuring that any potentially dangerous conditions are mitigated swiftly.

The correct answer indicates that the circuits should be de-energized within four seconds of a flame failure. This timeframe is based on safety engineering and operational standards that have been established to minimize risks associated with the combustion process. A rapid response helps prevent the accumulation of unburnt combustible gases and mitigates the risk of catastrophic failure.

Given this context, the other timeframes mentioned are less optimal. For instance, requiring immediate shutdown might be too abrupt and impractical in certain systems, while longer periods like ten or even two seconds could potentially allow an unsafe condition to develop. The four-second window strikes an effective balance between safety and operational feasibility, allowing for a swift response without compromising the effectiveness of the system's safety mechanisms.