Even if you aren’t an electronics person, you probably have a working knowledge of circuit breakers. When the lights go out, you find the circuit breaker and flip it back to the on position. Most people understand, too, that the breaker will trip if you overload the circuit with too many things plugged in or with an accidental short circuit. But how does this common device actually work? Keep in mind, circuit breakers need to be super reliable and have been around long enough that you can imagine they are pretty low tech. [Learn Engineering] has a very clear video about what goes on inside a circuit breaker that’s worth the eight minutes to watch. You can see the video below.
The handle is a mechanical engineering marvel, using two springs and a special design so that even a tiny force will cause it to snap to the off position. That takes care of a human tripping it. However, you have two other cases where you want to turn it off: overload and short circuit.
The short circuit case is easy to detect with an electromagnet that will have a strong magnetic field when so much current flows. However, the overload case isn’t that abrupt. For that, the breaker uses a bimetalic coil that expands as it heats.
There is one other detail. When the breaker opens at high currents, an arc forms. There’s a special component inside to help dissipate the arc. There’s a lot going on in that little box.