All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. However, to make things easy, we are going to cover the small, self contained, lower voltage and amperage breakers the public uses. There are very high current and voltage circuit breakers that are intricate to say the least. These includ solenoids, batteries, motors, and so on, but we will not cover these as you should have an electricians training. To the public these are dangerouse items. Current and Voltage at these levels can be very dangerous.
Again, a circuit breaker detects a fault condition or higher amperage than it's rating. To detect a fault, a bi-metallic strip between the contacts heats up, literally bending as it changes temperature. Once it bends far enough, a fault is detected, and the contacts physically swing open as the power goes off. Most circuit breakers use mechanically-stored energy, by way of spring(s) or compressed air to help trip the breaker. The energy is provided by the user as the breaker is "SET" to the "ON" position with force.
How does the bi-metallic strip work?
A bimetallic strip is used to convert a temperature change into mechanical displacement. The image to the right shows a bi-metallic strip from a thermometer. Notice how far it moves with a little heat from the lighter. The bi-metallic strip consists of two strips of different metals which expand at different rates as they are heated. The strips are joined together throughout their length either riveting, brazing or welding. The different expansion rates of the two metals force the flat strip to bend one way if heated, and in the opposite direction if cooled below its normal temperature. Typically steel and copper are used for a bi-metallic strip. In some more corrosive conditions, brass is used in lieu of copper.
The circuit breaker contacts on either side of the bi-metallic strip connect the strip to the circuit surrounding. These contacts are much more robust than the stip itself as they carry the load current without excessive heating. Additionally, the contacts must be sizeable enough to dissipate the the heat of the arc produced when interrupting the circuit. Each time a breaker "Breaks," a small electrical arc scars both the bi-metallic strip and the contactor on the side of the interruption. Service life of the contacts is limited by the erosion due to this interrupting arc. Miniature and molded case circuit breakers are discarded and replaced as they are cheap, and this is a very safe way of doing things.
There is an electical arc generated?
YES, When current is interrupted in a circuit, an arc is generated as the circuit opens. Similar to removing a wire from a battery, you can literally see the electrical arc, and the light that is generated from the heat. This arc, and associated heat, is contained, cooled, and dissipated in a controlled way inside the breaker. However, this arresting of the arc cannot be done indefinitely. The gap between the contacts can withstand only so many of these scarrings from interruption. To elongate the lifespan of many breakers, this arc arrest is the focus of much scrutiny. Each manufacturer has their own way of reducing the wear and tear of the arc, elongating the service life of their breakers. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms.