🔍 Key Concepts

• Purpose of a fusible plug as a safety device to prevent overheating when water level is too low • Where overheating is most dangerous in a Scotch or auxiliary boiler (think crown sheet vs. shell vs. furnace) • Relationship between water level and protection of metal surfaces exposed to flame

💭 Think About

• Ask yourself: Which part of the boiler metal is most at risk if the water level drops and the metal is directly exposed to furnace heat? • Consider: Should the fusible plug sense metal temperature where the flames are hottest, or water temperature in the shell? • Which location would give the earliest warning (or relief) if the water level becomes unsafe, before serious damage occurs?

✅ Before You Answer

• Identify which areas are directly exposed to furnace/flame heat when not covered by water. • Check which option mentions a position related to the lowest safe water level, not just the normal waterline. • Verify which part (shell, crown sheet, or furnace) is traditionally protected by a fusible plug in fire-tube boilers like Scotch boilers.