Which of the following lists the three activating devices used on sprinkler heads?

• A. Fusible Link, Glass Bulb, Chemical Pellet
• B. Fusible Link, Glass Bulb
• C. Glass Bulb, Chemical Pellet
• D. Thermal Switch, Fusible Link, Glass Bulb

Answer: (A)

Sprinkler heads rely on heat-responsive elements to release water when the environment gets hot. The three activating devices you’ll typically see are a fusible link, a glass bulb, and a chemical pellet.

• A fusible link is a small metal piece that melts at a predefined temperature. Once it melts, the mechanism frees the sprinkler’s valve or cap, allowing water to spray.

• A glass bulb contains a colored liquid that expands with heat; when the temperature reaches the bulb’s rating, the liquid pressure breaks the bulb and releases the mechanism.

• A chemical pellet sits in the mechanism and expands or changes strength with heat, weakening the hold on the sprinkler and triggering release at its rating.

A thermal switch isn’t a standard activation device for typical sprinkler heads, so it isn’t listed among the usual activating elements.