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A thermal switch (sometimes thermal reset or thermal cutout (TCO)) is a device which normally opens at a high temperature (often with a faint "plink" sound) and re-closes when the temperature drops. The thermal switch may be a bimetallic strip , often encased in a tubular glass bulb to protect it from dust or short circuit .
The infinite switch itself was described in a 1975 patent of George F. Esker Jr. and Otto J. Cousins of the Harper Wyman Company. This switch would reset itself as the bi-metallic strip cooled, provided a cycle of calibrated opening and closing. It also provided an indicator light that remained on as long as the knob was not in the closed position.
The metals involved in a bimetallic strip can vary in composition so long as their thermal expansion coefficients differ. The metal of lower thermal expansion coefficient is sometimes called the passive metal, while the other is called the active metal. Copper, steel, brass, iron, and nickel are commonly used metals in bimetallic strips. [6]
The resetting will often not take place even if the fault alone has been removed with the power still flowing as the operating current may be above the holding current of the PPTC. The device may not return to its original resistance value; it will most likely stabilize at a significantly higher resistance (up to 4 times initial value).
A high-resistance connection (HRC) is a hazard that results from loose or poor connections in traditional electrical accessories and switchgear which can cause heat to develop, capable of starting a fire. [1] Glowing connections occur when relatively high current exists in a relatively large resistance object. Heat comes from power dissipation ...
Thermal shock resistance measures can be used for material selection in applications subject to rapid temperature changes. The maximum temperature jump, , sustainable by a material can be defined for strength-controlled models by: [4] [3] = where is the failure stress (which can be yield or fracture stress), is the coefficient of thermal expansion, is the Young's modulus, and is a constant ...
The SI unit of absolute thermal resistance is kelvins per watt (K/W) or the equivalent degrees Celsius per watt (°C/W) – the two are the same since the intervals are equal: ΔT = 1 K = 1 °C. The thermal resistance of materials is of great interest to electronic engineers because most electrical components generate heat and need to be cooled.
In mechanics and thermodynamics, thermal stress is mechanical stress created by any change in temperature of a material. These stresses can lead to fracturing or plastic deformation depending on the other variables of heating, which include material types and constraints. [ 1 ]