ANS:B - put off fire in the furnace when water level in the boiler falls to unsafe limit.

The statement "put off fire in the furnace when water level in the boiler falls to unsafe limit" does not accurately describe the function of a fusible plug. Rather, it refers to the function of a different safety device in a boiler system known as a low-water cutoff. A low-water cutoff is a critical safety device installed in a boiler system to prevent damage to the boiler or a potential explosion in the event of low water levels. Here's an explanation of how it works:

1. Function: The primary function of a low-water cutoff is to shut off the burner or fuel supply to the boiler when the water level in the boiler falls below a safe operating level. This is crucial because water acts as a coolant and provides a protective layer between the metal surfaces of the boiler and the intense heat of the furnace. Without an adequate water level, the metal surfaces can overheat, leading to damage or failure.
2. Operation: Low-water cutoff devices are typically installed at a predetermined water level in the boiler, usually slightly above the lowest permissible water level. They may use different mechanisms to detect the water level, such as float switches, probe sensors, or conductivity sensors. When the water level drops below the set point, the low-water cutoff triggers an alarm and/or shuts off the fuel supply to the burner, preventing further firing and extinguishing the fire in the furnace.
3. Safety: The low-water cutoff is a critical safety feature in boiler systems, especially in situations where the boiler operates unattended or in large industrial settings. It helps prevent catastrophic boiler failures, explosions, and fires that can result from overheating due to low water levels.

In summary, the function of a low-water cutoff in a boiler system is to shut off the fire in the furnace or the fuel supply to the burner when the water level in the boiler falls to an unsafe limit, thereby preventing overheating and potential damage to the boiler.