What function does the fusible plug serve in a boiler?

The fusible plug serves the important function of preventing overheating in a boiler. It is designed to melt at a predetermined temperature, which is typically set above the normal operating temperature of the boiler. When a low-water condition occurs, the water level in the boiler drops, exposing the fusible plug to heat without sufficient cooling by water. If the temperature rises to the point where the fusible plug melts, it creates an opening that allows steam or gas to escape, which can prevent an explosion or catastrophic failure of the boiler. This safety mechanism is critical in alerting operators to a potentially hazardous condition related to low water levels.

While the other options reference important safety measures or indicators within a boiler system, the primary role of the fusible plug specifically focuses on mitigating the risk of overheating and preventing boiler damage by providing a fail-safe mechanism.