Thermostats and thermal switches are electro-mechanical on/off switches that are activated by temperature changes. They are used to control heating and cooling systems. The sensing element is usually a spiral bimetallic strip that coils and uncoils in response to temperature changes because of the differential expansion of the two bonded metals. The switch element is either a set of electrical contacts or a glass-encapsulated mercury switch that controls a low-voltage relay. The relay can actuate a motor starter and igniter for an oil burner, a heavy-duty switch for electrical units, or a solenoid-operated valve on a gas furnace. Thermostats and thermal switches may also control residential air conditioners or industrial heat pumps. To reduce temperature swings, a small electrical heater may be energized during the warming period, causing the switch to break prematurely in anticipation of room-heater override. 

There are several basic types of thermostats and thermal switches: bi-metal disc or snap action, reed switch, magnetic reed switch, mercury switch, rod and tube with different temperature coefficients, and gas-actuated or vapor-tension. Important performance specifications to consider include maximum current rating, maximum AC voltage rating, maximum DC voltage rating, and number of life cycles that thermostats and thermal switches can withstand.   

Selecting thermostats and thermal switches requires an analysis of pole-and-throw specifications. Single pole, single throw (SPST) devices make or break the connection of a single conductor in a single branch circuit. They have two terminals and are commonly referred to as “single pole” switches. Single pole, double throw (SPDT) switches make or break the connection of a single conductor with either of two other single conductors. Often called “three-way switches”, SPDT devices have three terminals and are used in pairs. Double pole, single throw (DPST) switches make or break the connection of two circuit conductors in a single branch circuit and usually have four terminals. Double pole, double throw (DPDT) switches make or break the connection of two conductors in two separate circuits. Most DPDT switches have six terminals and are available in either momentary or maintained contact versions. Thermostats and thermal switches with more than two poles are designed to split loads into separate circuits.

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