Circuit breaker

A circuit breaker, sometimes simply called a breaker, is an electrical switch that can be automatically or manually operated and reset. Electrical switches either connect or disconnect electrical circuits. An open switch disconnects a circuit, while a closed switch connects a circuit.

The function of a circuit breaker is to automatically interrupt current flow when an overload or short circuit is detected. Circuit breakers trip (or open) when the current in a circuit exceeds safe limits. They typically can also be operated manually when it is desired to purposely interrupt current flow. Circuit breakers are differentiated from fuses, which are also used to interrupt flow, in that breakers can be reset either manually or automatically to resume current flow. Fuses must be replaced after tripping. 

Circuit breakers are used throughout the electrical system. Consumers are familiar with the breakers in their breaker box, which protect their home circuits from overload. Many also have ground fault circuit interrupters (GFCI), also known as ground fault interrupters (GFI), which are especially sensitive circuit breakers that are installed in areas likely to experience moisture such as kitchens, bathrooms, and outdoor outlets. Larger breakers are used for commercial and industrial facilities. 

A residential panel box (also called a breaker box) consisting of a series of circuit breakers for each house circuit plus a main breaker that shuts off the whole house if opened.
A ground fault current interrupter
A high-voltage circuit breaker

Circuit breakers are also widely used in the generation, transmission, and distribution sectors of the electric delivery system. A recloser is a circuit breaker that opens when a fault is sensed and then automatically resets itself multiple times before locking open. Reclosers are commonly used on distribution circuits to protect the system while also allowing high levels of reliability by re-energizing circuits quickly and automatically in the event of a transient fault. Larger circuit breakers are used on incoming and/or outgoing lines in substations and generation switchyards to protect equipment in the event of a fault as well as to allow purposeful energizing or de-energizing for maintenance or sectionalizing lines. 

A recloser
Circuit breaker in a substation