CIRCUIT BREAKERS

A simple circuit breaker was developed by Edison as early as 1878 when he worked on his electric light circuit. Even though he did not use his mechanical breaker in his commercial circuit, the function was based on the same principle that is used in a modern circuit breaker.

The circuit breaker is, in contrast to the melting fuse, resettable. This basically means that a circuit breaker can be seen as a switch with high current handling capabilities. Depending on the area of use, there are many different types of circuit breakers but the function in general is the same. When an over current occurs, a thermal or a solenoid sensor detects the fault current and presses the breaker apart with mechanically stored energy, e.g. a spring or compressed air.

As for a melting fuse, an arc is created in the break. Since the circuit breaker must be able to handle the high current and the high temperature created from the arc without being destroyed, the arc has to be extinguished. Some ways to handle this is to use intensive cooling in a jet chamber, divide the arc in to several smaller arcs or to let the arc grow in length, as in a melting fuse. Depending on the type of the circuit breaker, the arc can pass in different media, e.g. insulating gas, vacuum, air or oil. The lifetime of a breaker often depends on the eroding of the contacts exposed to the arc. The internal resistance of the circuit breaker will be affected from this eroding and depending on the value of the fault current a certain number of over currents will wear out the circuit breaker. When worn out, low power circuit breakers are often replaced but for high voltage circuit breakers wear parts can be exchanged. The biggest advantage in comparison to a melting fuse is of course the feasibility to restore the breaker.