What does the term "fused" refer to in the context of fire sprinklers?

In the context of fire sprinklers, the term "fused" refers to the sprinkler heads that have opened. This occurs when the thermal element within a sprinkler head reaches a specific temperature due to heat from a fire, causing it to break and allowing water to flow out. The mechanism relies on a glass bulb or a fusible link that is designed to melt or shatter at certain temperatures, effectively triggering the sprinkler head to activate.

Understanding the function of fused sprinkler heads is crucial in fire safety, as these activated heads are vital for suppressing fires and minimizing damage. The other terms do not accurately describe the process that happens with fire sprinklers; for instance, sprinkler heads that remain closed or are defective would not contribute to fire suppression, while those requiring maintenance does not indicate their operational status.