For increased safety, a steam boiler that is not continuously attended requires what safety devices?

A steam boiler not continuously attended needs safety devices primarily designed to prevent dangerous conditions that could arise when the boiler is not under regular supervision. The correct answer highlights the importance of low water fuel cut-off devices, which serve a critical function in maintaining safe operating conditions.

Low water fuel cut-off devices are essential because they automatically shut off the fuel supply if the water level in the boiler drops below a predetermined safe point. This is crucial for preventing overheating or even a potential explosion that could occur due to low water levels, as the boiler components could become damaged or fail if they run dry. In unattended operations, where human oversight is absent, relying on this safety mechanism becomes paramount.

While pressure relief valves are also vital for protecting against excessive pressure buildup, they do not directly address the specific risks presented when a boiler isn’t attended, such as the risk of low water conditions. Temperature gauges help monitor the boiler's operation but do not provide the automatic action necessary for safety in an unattended scenario. Emergency shut-off switches serve a useful purpose in immediate danger situations but do not function proactively to prevent unsafe conditions in the absence of supervision. Hence, the low water fuel cut-off device stands out as the most critical safety feature for an unattended steam boiler.