Its basic function is to interrupt current flow after a fault is detected. Its purpose was miniature circuit breaker pdf protect lighting circuit wiring from accidental short circuits and overloads. Stotz’s invention was the forerunner of the modern thermal-magnetic breaker commonly used in household load centers to this day. Interconnection of multiple generator sources into an electrical grid required development of circuit breakers with increasing voltage ratings and increased ability to safely interrupt the increasing short-circuit currents produced by networks.
2,500 MVA, in three cycles of the AC power frequency. All circuit breaker systems have common features in their operation, but details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must firstly detect a fault condition. Typically, the heating or magnetic effects of electric current are employed. This is commonly done using mechanically stored energy contained within the breaker, such as a spring or compressed air to separate the contacts. Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. Contacts are made of copper or copper alloys, silver alloys and other highly conductive materials.
Service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded-case circuit breakers are usually discarded when the contacts have worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts. This arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. 100 times per second for 50 Hz and 120 times per second for 60 Hz AC.
Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit. These circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal plates which divide and cool the arc. By splitting the arc into smaller arcs the arc is cooled down while the arc voltage is increased and serves as an additional impedance which limits the current through the circuit breaker. So The number of plates in the arc chute is dependent on the short-circuit rating and nominal voltage of the circuit breaker. In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc.
Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 38,000 volts. Circuit breakers are usually able to terminate all current very quickly: typically the arc is extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending upon age and construction of the device. The maximum current value and let-through energy determine the quality of the circuit breakers. Circuit breakers are rated both by the normal current that they are expected to carry, and the maximum short-circuit current that they can safely interrupt.