A circuit breaker is protective device which is design to automatically open an electrical circuit thus preventing harm and damages to equipment and personal. The damages are due to overload, short circuits and sometimes earth faults. Circuit breakers are made in varying sizes, from small device which carry fractional amperes (e.g. a 10 milli ampere residual circuit breaker) like that in a typical house up to extremely large ones like a generator circuit breaker which continuously few tens of thousands of amperes. An LV/MV system circuit breaker is similar to its fuse counterpart in its main function of fault interruptions. However, a circuit breaker differs from a fuse in its other functions which are breaking loads as well as normal opening with or without current in the circuit.
Further, it performs closing function and is non-destructive (except requiring periodic maintenance, replacement of current interrupting contact and in some forms becomes device of that needs a part of it to be replaced after specific numbers of operations) a fuse however, operated once and then has to be replaced, a circuit breaker can be reset (either manual or automatically) to resume normal operation.
There are two attributes to electricity which is quite apparent due to its obviousness to the real world which are the Current and the Voltage as well as some functional derivative of these two like Real power (Watt), Reactive power (VAR), Energy (kWH) etc. Performance of a circuit breaker in its main function relates with making, carrying and breaking currents in an Electrical circuit. To make or break a circuit there must be some component which connects and disconnects which is called Contacts/interrupters. While making a circuit the components (one or both or many has to move in an Electric field) and since the movement is to make a circuit, the field gets progressively increased as the components move and we can imagine a pre-arc (prestrike). While breaking, it is opposite thing and we imagine a restrike. Voltage and currents are related same way as an Electric field (due to available static electric charge) with a Magnetic field (due to moving Electric charge). Some more parameters gets added since circuit breakers are often linked in some way with neighboring circuit components, or magnetic devices (like motors, welding etc) which introduces switching transients and atmosphere (lightning transients). These are actually are called impulses (surges) and have a characteristic front, peak and tail of a definite polarity (+ve or negative). Some others may be oscillatory (decaying, steady, increasing) which can be for the sake of study and computation addressed as a function of the fundamental part (frequency 50Hz. 60Hz etc). Thus we added Impulse withstand requirement for a circuit breaker, reactive (capacitive/inductive) duty and harmonics performance.
Operation of a circuit breaker
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. To breaking a fault, firstly a fault condition needs to be detected. For higher capacity CBs this function is delegated to external Protective devices due to size, proximity to High volatges, sophistication in detection of all types of faults accurately to specific requirement of time. In low-voltage circuit breakers this is usually done within the breaker enclosure. To perform in a small space with cost effective and efficient way against demanding forces and heat we can imagine special design is required to confine it to limited area within a CB. So we identified some interrupting means. Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit; some mechanically-stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. Small circuit breakers may be manually operated; larger units have sophisticated to trip the mechanism, and electric motors to restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys, and other materials.
Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature and molded case circuit breakers (MCB and MCCB) are usually discarded when the contacts are worn, but power circuit breakers and high voltage circuit breakers (HVCB) have replaceable contacts. When a current is interrupted, an arc is generated. 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. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium in which the arc forms.
Finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit.
Resource: Circuit Breakers in Power Systems
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