What happens when a thermal switch activates in its circuit?

• A. It breaks the circuit to isolate the fault.
• B. It completes a circuit and triggers a warning.
• C. It reduces the current in the loop.
• D. It disconnects the power supply.

Answer: (B)

A thermal switch is a temperature-sensitive device that changes its electrical state when the temperature reaches a preset point. In many warning circuits, the switch is normally open, so when it reaches the set temperature the contacts close, completing the circuit and energizing a warning indicator or alarm. That activation effect—closing the circuit and triggering a warning—is exactly what happens in this scenario. (Some switches are normally closed and open on trip, but the described behavior—completion of the circuit and a warning—aligns with the normally open design used for alerts.)