Protecting electric circuits against overload and short circuit currents

Circuit breakers are an essential part of the protection system of every electrical network. They automatically disconnect the circuit in the event of an overload or a short circuit, preventing damage to the network and its equipment. They exist in many different sizes, ranging from miniature circuit breakers that protect a single household appliance to large devices that protect high-voltage circuits in industrial and utility networks.

Circuit breakers must be able to carry a high load without excessive heating. They also must withstand the thermal and mechanical effects caused by the maximum short circuit current that can flow through the circuit. Opening the circuit will generate an internal arc between the receding contacts, which the breaker must contain, cool, and extinguish in a controlled manner.

Many different types of circuit breakers exist, differentiated mainly by the following:

  • The medium used to extinguish the arc. This can be air, vacuum, oil or an insulating gas such as SF6.

  • The manner in which the mechanism that releases the latch is triggered. In low voltage breakers, this can be thermal (a bimetallic strip), magnetic (a solenoid) or a combination of both. Most types of medium and high-voltage breakers contain current sensing protective relays operated through a current transformer. Some medium-voltage circuit breakers are operated by an electronic trip unit controlled by a microprocessor. The energy is provided by a solenoid, a motor or compressed air in most cases.

  • They can operate for one phase or for three phases simultaneously.

Circuit breaker contacts utilize complex copper profiles. Likewise, the windings of solenoids and current transformers are made out of copper wire. Many other breaker components also contain copper. These include, among others, bimetallic strips, cable terminals, busbar connections and electronic control units.

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