Thermal switches and thermal protectors are thermally-actuated electro-mechanical on/off switches. They differ from thermal fuses as they are reusable, and suited to protecting against temporary situations that are common and user-correctable. The switch is activated by a temperature differential, at which point there is a change in state either from normally open to closed or from normally closed to open. The contacts movement may be accompanied by a faint audible as power to the electrical circuit is interrupted.

Applications

They are often used as thermal protection devices to prevent damage from over-heating of electrical circuits, although they can also be used as a temperature control device. They prevent overheating in a variety of consumer, industrial, and commercial products by controlling power to circuitry in power supplies, electric motors, transformers, lighting fixtures, ballasts, and battery packs. Temperature control applications include operation of heat pumps, electronic cooling fans, low voltage relays that 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.

Types

There are several basic types of thermal switches and thermal protectors: bi-metal disc or snap action, thermal reed switch, mercury switch, rod and tube with different temperature coefficients, and gas-actuated or vapor-tension.

• Bimetallic disc or snap action switches operate due to the phenomenon of thermal expansion. Two dissimilar metals expand at different rates and when the temperature threshold is reached the discs' snap action forces the switch to activate.
• Thermal reed switches consist of a pair of contacts on ferrous metal reeds which are hermetically sealed. They are actuated by a magnetic field. The contacts may be normally open or normally closed and change state when a ferromagnetic substance reaches its curie point and alters the magnetic field surrounding the reed switch.
• Mercury switches have contacts sealed in a glass envelope with a small amount of mercury. Mercury, which is in a liquid state at temperatures above -40°C, is an electrical conductor that makes or breaks the contacts depending on the angle of inclination. The thermal switch is typically actuated by mounting the switch on a metal coil. Thermal expansion then causes movement in the coil to tilt and actuate the switch.
• Rod and tube thermal switches are composed of an outer tube and internal rod, each with dissimilar coefficients of thermal expansion to allow for a thermally-induced, plunger-style contact movement. They are characterized by rapid response times and high operating temperatures.
• Gas-actuated, also referred to as vapor-tension, thermal switches use the thermal expansion of a gas or vapor in a sensing bulb to create a proportional pressure on a diaphragm or piston assembly that actuates the electrical switching element. 

Specifications

Selecting thermal switches and thermal protectors requires an analysis of pole-and-throw specifications, set-point temperature, hysteresis, maximum current rating, maximum AC voltage rating, maximum DC voltage rating, and switch life.

Pole-and-throw specifications

• Single pole, single throw (SPST) switches 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. 

Set-point Temperature

Thermal switches and thermal protectors have a set point temperature that is either fixed or field adjustable. The set point is the rated temperature at which the device undergoes a change in state associated with movement of the electrical contacts. Fixed-point devices are rated at a given temperature while field-adjustable devices have a temperature range over which the set point can be adjusted.

Hysteresis

Hysteresis, also referred to as the temperature differential, is a finite temperature range over which switching takes place. It is designed to prevent circuitry from constantly changing between states. A hysteresis of 0.5 C will typically actuate 0.5 degrees past the set-point temperature and not change state again until the temperature drops to 0.5C below the set-point temperature.

Maximum current

The maximum current rating is the maximum electrical load in amperes that the contacts of the electrical switch is designed to carry.

Maximum voltage

The maximum voltage rating is the greatest potential difference that the device is designed to hold across open switch contacts without arcing or development of a current. The maximum voltage is specified for alternating-current (AC) and direct-current (DC), with AC exhibiting a greater self-extinguishing tendency of an arc across an air gap and therefore able to hold a greater potential difference.

Life Cycles

Expected number of cycles the switch can undergo before contact wear causes failure.

Standards

UL 60730-2-2 - UL Standard for Safety Automatic Electrical Controls for Household and Similar Use; Part 2: Particular Requirements for Thermal Motor Protectors

IEC 60034-11 - This part of IEC 60034 specifies requirements relating to the use of thermal protectors and thermal detectors incorporated into the stator windings or placed in other suitable positions induction machines in order to protect them against serious damage due to thermal overloads. 

Resources

Working Principle Of Thermal Motor Protection Relay

Wikipedia - Thermal Switch

Image Credits:

Thermik Corporation | Honeywell Sensing and Control