What is one function of a fusible plug in a boiler?

A fusible plug in a boiler serves a critical safety function by melting under high temperatures, which helps to alleviate pressure caused by overheating. When the temperature in the boiler exceeds safe operational limits, the plug melts, allowing steam and pressure to escape. This prevents the dangerous potential of a boiler explosion by relieving the excess pressure that builds up from overheating conditions.

The other options do not align with the specific purpose of the fusible plug. For instance, regulating steam pressure is typically managed by different control mechanisms, such as pressure relief valves or regulators. A backup water level indicator is not a function associated with fusible plugs; this role is fulfilled by sight glasses or level control technology, which ensures accurate monitoring of water levels in a boiler. Likewise, automatically filling the boiler with water is the responsibility of automatic feed systems or control valves, not the fusible plug. Therefore, the correct choice highlights the essential safety function of the fusible plug in preventing boiler hazards.