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A thermal circuit breaker is a safety device which breaks an electrical circuit if the temperature gets too high. These devices are used to prevent fires, damage due to voltage fluctuations, and other dangerous electrical situations. Many people are familiar with thermal circuit breakers because they may have them in their homes, or they may use a thermal/magnetic circuit breaker, which contains two failsafes to open a circuit in the event that a problem develops.
Circuit breakers work by detecting hazardous conditions in an electrical circuit and opening that circuit so that it cannot be completed. As long as the circuit is open, electricity cannot flow through it, and thus the opening of the circuit eliminates a hazardous situation. Once the problem which caused the breaker to open has been addressed, the breaker can be reset, closing the circuit and allowing it to work again. If the breaker is reset without addressing the problem, there is a high probability that it will trip again.
In the case of a thermal circuit breaker, the breaker is designed to detect elevations in temperature. The temperature triggers a mechanical reaction which trips the breaker, breaking the circuit. Classically this is accomplished by putting two pieces of metal with different rates of expansion together. As the metals expand in response to heat, they pull on the breaker, snapping it open. Circuit breakers are designed to be fully open or fully closed, preventing partial flows of electricity, and when they trip, they usually come to rest in the middle of their range of motion, so that someone can easily identify the breaker which tripped. To reset the breaker, it has to be pulled all the way off and then put back in the on position.
The mechanical action involved in opening the circuit often generates an audible snapping or popping noise. This can alert people to the fact that a thermal circuit breaker has tripped in the event that they do not notice an electrical problem. When a breaker trips, people should determine which circuit it is associated with, and check the circuit for signs of a problem. For example, running multiple electrical heaters on a single circuit can trip a thermal circuit breaker, and it may be possible to resolve the problem by unplugging the heaters and moving them to different circuits. In other cases, there may be a serious problem with the circuit which requires the attention of an electrician.
With a magnetic/thermal circuit breaker, the circuit breaker has a thermal component and an electromagnetic component which responds to currents which are too high by tripping the breaker and opening the circuit. This type of circuit breaker can respond quickly to issues such as power surges in addition to problems like poorly grounded electrical circuits.
Think about that the next time a circuit breaker trips and you're cussing at it. The design of that circuit breaker may have saved your house from catching fire. Those things trip for a reason, you know.