Temperature Switches are designed to work in two different ways, bi-metal switches and thermal expansion.
both of these methods require no electrical power for the switch to sense temperature and mechanically actuate the switch mechanism. Of course power has to b present to actually be switched.
A bi-metal switch uses two dis-similar metals. Metallic objects expand and contract when heated and cooled, by attaching or fusing or combining two different metals together, they will expand or contract, or bend or otherwise cause a movement that can be used to make a switch contact. By restricting the movement with a spring or tension, the switch can be "set' for desired temperatures. The switch can be calibrated at the factory or by the user.
A thermal expansion switch uses a liquid/gas filler that will expand and move a plunger that will actuate the switch.
A liquid is stored in the "bulb" of the switch and when heated will boil and turn to gas and expand. The expansion will move up the capillary and create pressure to actuate the switch mechanism. As with bi-metal switches these switches ca be calibrated by the users to actuate at different set points. Also, thermal expansion switches can have two or more set points by use of two or more switches.
A few examples of temperature switches that use this technology are the thermostats in cars to allow coolant to flow, and the old style round house thermostats that used bi-metal and mercury to make the switching. Most over temperature safety switches found in appliances are bi-metal switches as they can be small and inexpensive. This Style of switch is factory set to a certain temperature. Some of these switches can have a manual reset mechanism and some will reset when they cool to a lower temperature.
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