A circuit breaker typically consists of a contact system, an arc-extinguishing system, an operating mechanism, a trip unit, and an enclosure.
During a short circuit, the magnetic field generated by the high current (typically 10 to 12 times the rated current) overcomes the force of a counter-spring; the trip unit then actuates the operating mechanism, causing the switch to trip instantaneously. During an overload, the current increases, leading to intensified heating; a bimetallic strip deforms to a specific extent, triggering the mechanism (the higher the current, the shorter the response time).
There are also electronic-type circuit breakers, which utilize current transformers to sample the current magnitude in each phase and compare it against a preset value. When an abnormal current is detected, a microprocessor issues a signal, causing the electronic trip unit to actuate the operating mechanism.
The function of a circuit breaker is to connect and disconnect load circuits, as well as to interrupt fault circuits, thereby preventing the escalation of accidents and ensuring safe operation. High-voltage circuit breakers, in particular, must be capable of interrupting arcs involving voltages of 1500V and currents ranging from 1500 to 2000A-arcs that can stretch up to 2 meters in length yet continue to burn without extinguishing. Consequently, arc extinguishing is a critical challenge that high-voltage circuit breakers must effectively resolve.
The principle behind arc blowing and extinguishing primarily involves cooling the arc to attenuate thermal ionization. Furthermore, by utilizing arc blowing to elongate the arc, the recombination and diffusion of charged particles are enhanced; simultaneously, charged particles within the arc gap are dispersed, thereby rapidly restoring the dielectric strength of the insulating medium.
Low-voltage circuit breakers-also known as automatic air switches-can be used to connect and disconnect load circuits, as well as to control electric motors that undergo infrequent starting. Functionally, they serve as a comprehensive integration of some or all of the capabilities found in separate devices such as knife switches, overcurrent relays, undervoltage relays, thermal relays, and residual current devices (leakage protectors); thus, they constitute a vital protective device within low-voltage power distribution networks.
Low-voltage circuit breakers offer numerous advantages, including multiple protection functions (such as overload, short-circuit, and undervoltage protection), adjustable trip settings, high breaking capacity, convenient operation, and inherent safety. For these reasons, they are widely utilized. In terms of structure and operating principle, a low-voltage circuit breaker comprises an operating mechanism, contacts, protective devices (various types of trip units), and an arc-extinguishing system.
The main contacts of a low-voltage circuit breaker are closed either manually or via an electric closing mechanism. Once the main contacts have closed, the free-trip mechanism locks them in the closed position. The coil of the overcurrent trip unit and the thermal element of the thermal trip unit are connected in series with the main circuit, while the coil of the undervoltage trip unit is connected in parallel with the power supply. In the event of a short circuit or severe overload, the armature of the overcurrent trip unit is attracted, actuating the free-trip mechanism and causing the main contacts to disconnect the main circuit. When the circuit is overloaded, the thermal element of the thermal trip unit heats up, causing the bimetallic strip to bend and trigger the free-trip mechanism. When the circuit experiences an undervoltage condition, the armature of the undervoltage trip unit releases, thereby also actuating the free-trip mechanism. The shunt trip unit serves the purpose of remote control; during normal operation, its coil remains de-energized, but when remote control is required, a start button is pressed to energize the coil.
