To detect and open the circuit if a thermal overload is present and to detect and open the circuit if a short circuit caused by a magnetic increase of flux in the circuit.
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In the electrical trade, this device is known as a circuit breaker.
An electrical safety device that can be reset and used again fits the description of an electrical circuit breaker.
You can reduce the breaker size because that limits the current that can reach the main panel. There is no safety issue other than the significant danger in an unskilled person changing out a main breaker.
Residual Current Device. Also known as an Earth Leakage Circuit Breaker or a Safety Switch
Usually the breaker's shunt trip coil is tied to a corresponding current transformer that is sized to the amperage that is allowed to be passed through the breaker. These types of breakers can also be connected into a distribution monitoring device. If the monitor detects a phase reversal or phase loss or voltage rise or drop the breakers shunt trip coil is remotely energized and isolates equipment down stream from the fault. Shunt trip coil circuits are also used as safety circuits where the situation calls for only one breaker to be energized at a time. If the second breaker is inadvertently closed, this would allow both breakers to be on, the second breaker's auxiliary contacts that are an internally part of that breaker will close the safety circuit and energize the shunt trip in the first breaker to causing it to open. So as you can see the two wires could be part of many wiring configurations depending on what situation calls for.