At what event must the low water fuel cut-off of a heating boiler shut the fuel off?

The correct answer is that the low water fuel cut-off of a heating boiler must shut the fuel off to an unsafe level. This is a critical safety feature designed to protect the boiler from operating when the water level drops below a safe threshold. If the water level falls too low, there is a risk of overheating and potential damage to the boiler or even a catastrophic failure. The cut-off serves to prevent the boiler from running under unsafe conditions, ensuring that the system operates within safe limits.

The other options do not capture the full essence of the requirement for the low water cutoff. For example, while shutting off at the lowest permissible water level appears reasonable, it is not sufficiently conservative to guarantee safety. Also, the emphasis on "quickly" does not address the fundamental requirement for safety, which is paramount. The safe operation of the boiler chiefly concerns the prevention of unsafe water levels, not just immediate action. Lastly, shutting off fuel below the top row of tubes would not align with the principle of safety since it does not specify a direct unsafe condition; rather, it describes an operational aspect that may vary depending on the boiler design. Therefore, the critical function of the low water fuel cut-off is to respond to potentially unsafe water levels, ensuring the boiler