A freezestat is best described as which type of switch?

• A. Normally open bimetal switch
• B. Mercury switch
• C. Thermal fuse
• D. Normally closed bimetal switch

Answer: (D)

A freezestat is a temperature-activated protective switch that opens when the evaporator surface reaches the freezing point to prevent frost buildup. It uses a bimetal element that is normally closed, so at typical operating temperatures the circuit stays complete and the cooling system runs. When the surface temperature hits freezing, the bimetal bends and opens the contacts, interrupting the circuit to stop the cooling cycle until the temperature rises again. This resettable, normally closed behavior is what makes a freezestat different from a one-time thermal fuse or a mercury switch, and from a normally open contact that would only close when cold.