1. the applied rated AC or DC voltage
2. single-phase, multi-phase and the number of poles
3. the applicable national electrical standards and the safety regulatory agency standards
4. the short-circuit breaking capacity
5. the circuit breaker's circuit breaker's circuit breaker's short-circuit breaking capacity
6. Short circuit breaking capacity
I. Thermal circuit breaker
The thermal circuit breaker utilizes a bimetal in series with the circuit. The heat generated by the current during an overload deforms the bimetal, which trips the circuit breaker. Thermal protectors have a significant advantage over fuses in their ability to reset after tripping. They can also be used as power on/off switches for the equipment being protected.
The tripping of thermal circuit breakers accelerates as the temperature rises and often occurs at lower current levels. This characteristic is often useful when the circuit breaker and the system are exposed to the same heat source. In this case, the protection circuitry is able to track the equipment's need for enhanced wiring protection at higher temperatures. If a thermal circuit breaker is installed in an environment separate from the protected equipment, the effects of changing ambient temperatures can be corrected by a compensating thermal bimetal. For example, circuit breakers located outside the cockpit of an airplane are temperature-compensated so that their tripping characteristics do not change in response to temperature fluctuations common in flight.
In addition, the inherent latching mechanism within thermal circuit breakers makes them extremely insensitive to shock and vibration. Some high-performance circuit protection devices now offer circuit breakers specifically designed for extremely shock and vibration environments. Applications requiring thermal circuit protection include household appliances, transportation, marine, switchboards, medical equipment, audio-visual equipment, power supplies, and sports equipment.
II. Magnetic Circuit Breakers
Magnetic circuit breakers provide a cost-effective solution to most design problems with high accuracy and reliability.
Magnetic circuit breaker's overcurrent detection mechanism is only in response to changes in the current in the circuit being protected, because of its current-sensing solenoid is not affected by changes in ambient temperature, so the magnetic circuit breaker has a temperature stability, will not be affected by changes in the ambient temperature as obvious as the thermal circuit breaker. Magnetic circuit breakers do not have a warm-up phase and therefore do not slow down the circuit breaker's response to an overload, and there is no cool-down period from the end of the overload until its reset. Source:www.tede.cn
Targeted adjustments to the characteristics of magnetic circuit breakers can be made in four separate areas: the circuit required by the breaker; the trip point (in amps); the delay time (in seconds) and the surge handling capability. The adjustments made to these factors have a minimal effect on the short-circuit breaking capacity of the circuit breaker.
In general, three types of magnetic circuit breakers with varying trip time delay curves are available: slow, medium and fast. These alternative curves provide designers with a high degree of design flexibility when matching circuit breakers in cascade and discrimination circuits.
In addition, magnetic circuit breakers with special inrush configurations are available for equipment that is often subjected to large inrush currents. However, when the equipment is in an unstable position, a thermal circuit breaker may be a better choice because the number of trips of the magnetic circuit breaker can change due to the movement of the solenoid by gravity.
Applications for magnetic circuit breakers cover many markets, such as telecommunications, marine, appliances, industrial automation and control, and medical devices.
Three, through-ground leakage protectors
Through-ground leakage protectors, such as Carling's SmartGuard series, work in the same way as magnetic circuit breakers and are capable of providing user-customizable levels of overload and short-circuit protection. In addition, they use innovative electronics to detect and avoid through-ground leakage. Except for a small amount of leakage, the current returning to the power supply is equal in value to the current flowing from the power supply. If, after passing through the through-ground leakage protector, the difference between the value of the current flowing out of the power supply and the value of the current returning to the power supply exceeds the set value of the leakage sensitivity, the protector trips and the LED indicator lights up to alert the operator, thus providing an "intelligent" feature. The LEDs clearly indicate tripping due to through-ground leakage. This protection helps to avoid serious equipment damage and fires. Applications include resistance and impedance heating systems, telecommunications, theater lighting, marine consoles, office equipment, medical equipment, industrial automation and control, and UPS systems.
Source:Power Transmission and Distribution Equipment Network
Four, the need to consider the secondary factors
In the selection of circuit breakers, we should not only focus on the circuit breaker's delay curve and other primary indicators, should also pay attention to many of its secondary features, which are often easy to overlook the performance of not only for a good design icing on the cake, but also help engineers to design a sophisticated protection circuit for their applications. protection circuitry for their applications. There are many circuit breakers on the market today that are equipped with a variety of optional features that can be helpful in circuit protection design. Listed below are some of the more common features:
1. Auxiliary contacts (auxiliary switches): They are contacts electrically isolated from the main contacts and are suitable for alarm and program switching. Auxiliary contacts can be used to alert the operator or the control system, to sound an alarm, or to turn on backup power in critical applications.
2. Transmission: The choice of actuator type is not just for aesthetic reasons. Circuit breakers with actuated rocker switches that switch twice as fast as on/off switches save cost and board space. Push-pull actuators are the most stable in the event of a surge.
3. Shunt terminals: Traditional circuit breakers are considered "series tripping" because the contacts, current sensing elements, and loads are all connected in series. Shunt terminal from the main circuit branch, so that the secondary load can be connected. If a short or overload occurs in the primary load, the circuit breaker will trip and cut off power to both loads. Unlike the auxiliary contact, the shunt terminals are connected to the circuit breaker carrier circuit located between the switch contact and the current sensing element, which means that the second load is not protected from overload or short circuit. A separate circuit breaker can be used to protect the secondary circuit, which otherwise can only be used in equipment with built-in protection circuitry.
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4. Duplex Control (Remote Trip or Relay Trip): Duplex control circuit breakers combine two electrically isolated sensing elements with each other to perform multiple functions. For example, a compound control circuit breaker can utilize a remote actuator or inductor for traditional overcurrent protection as well as circuit disconnection. Remote tripping is an example of compound control and is often referred to as "relay tripping".
5. Low-voltage trip: This is an independent voltage-sensitive element in a circuit breaker that opens the main contact if the voltage drops below a predetermined value. Switching circuit breakers with low voltage trip are widely used for on/off control of wired connected appliances. Safety authorities require these appliances to be disconnected in the event of a power drop to avoid the danger of the appliance suddenly restarting when power is restored.
6. Auto-trip: An auto-tripping circuit breaker will not remain closed during a fault - because the switching device will not fail by forcing the actuator to stay on. In a fully automatic tripping design, the main contact will always remain open after a fault when the actuator is held in the "on" position. Some circuit breakers known as "cyclic automatic trip" cannot be forced to stay on during a fault, but they will cycle on and off if the actuator remains in the "on" position. If the circuit breaker is installed in an easily accessible location (i.e., unenclosed), an automatic trip circuit breaker should be used.
7. Automatic reset: For applications where circuit breakers are not easily accessible, circuit breakers that automatically reset after a cooling-off period are a good choice. If a device that can automatically restart is specified at this time, the potential for danger is high.