Switch thermal cutout

Thermal and Overspeed Cutouts The temperature of an electric motor will rise during use. If the temperature exceeds a certain value, a dangerous condition may exist. A temperature-sensitive switch with a preset temperature limit can interrupt power. Some electrical motors, equipment with resistance heaters (such as hair dryers), and other electrical equipment have thermal cutout switches. 

An associated and necessary modification is that the supply of heat must have a thermal cutout switch which is installed in the cleaning sirmp. The purpose of the switch is to stop the supply of heat to the cleaning sump when the temperature in it exceeds a preset value. That preset temperature value is one which doesn t exceed the normal boiling point (otherwise the work would not be cold cleaning and more importantly, the freeboard zone would be expelling solvent vapor). 

Critical components are any components that may influence the safety of a product, such as those that operate at mains supply (120/230/400 Vac) or hazardous voltages (> 50 Vac or 60 Vdc). Examples of critical components are inlets, filters, switches, motors, circuit breakers, power supplies, and transformers. Components that may operate at lower voltages (i.e., 12 or 24 V) and may affect safety are also considered critical components and examples include emergency stop switches, door interlocks, relays, secondary fuses, thermal cutouts, fans, and sensors. 

Bimetallic switches (Fig. 18.20) are widely used in on-off temperature control systems. If two metal strips with different coefficients of thermal expansion are bonded together while both strips are at the same temperature, the bimetallic structure will bend when the temperature is changed. Although these devices are often called thermal cutouts, implying that they are used in normally closed switches, they can be fabricated in either normally closed or normally open configurations. The bimetallic elements can also be fabricated in coil or helical configurations to extend the range of motion due to thermal expansion. 

Thermal and overspeed cutout devices are coimnonly used to protect electrical equipment (and thus the operator). A thermal cutout is simply a temperature-sensitive switch with a preset limit designed to interrupt power when the temperature exceeds a certain value. As its name implies, an overspeed switch operates when it senses that a motor or other device is operating at too fast a speed. Obviously, excessive speed may create dangerous conditions and indicate failure of equipment. The overspeed switch operates to shut down an overspeeding device by interrupting power to it. 

I must emphasize that before a detector is warmed, the HV must be switched off. If, as should be the case, the preamplifier HV cutout is connected, that will automatically switch off the HV as the temperature rises. However, when the detector cools down again, it would be preferable that the HV did not switch on again automatically without the user being satisfied that thermal equilibrium is established. (There is a hazard that if a pressure rise occurs, due to desorption of gas, and this happens with HV applied, there could be damaging electrical discharges.) A recommended procedure is as follows ... 

There are two shock-hazard locations to watch for the wiring side of the input line cord connector block, and the two thermal switches (SI for thermal cutout, S2 for thermal warning) mounted against the heatsink. These locations are exposed to the ac-line voltage. 

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