Any electrical or electronic product that neglects the design of circuit protection has hidden dangers. In the final analysis, protecting your expensive equipment means protecting the entire electrical system, including control switches, wires and power supplies, to avoid short circuits and excessive currents. At present, there are mainly thermal circuit breakers, magnetic circuit breakers and leakage circuit breakers. When choosing a circuit breaker, the designer should not only consider the following circuit characteristics, but also consider the restrictions including the installation position and shell size of the circuit breaker.
It is not difficult to choose the appropriate circuit protection device for a specific application, but it does require some thinking. If loose circuit protection devices are used in the design of electrical and electronic equipment, the equipment will be easily damaged by power supply impact, which will lead to disastrous consequences of fire. On the contrary, if strict circuit protection devices are used, it will cause annoying frequent trips.
At present, circuit breakers mainly include thermal circuit breakers, magnetic circuit breakers and leakage circuit breakers. When selecting a circuit breaker, the designer should not only consider the following circuit characteristics, but also consider the restrictions including the installation location and shell size of the circuit breaker:
● Apply rated AC or DC voltage.
● Single-phase, multi-phase and pole number
● Applicable national electrical standards and safety management agency standards.
● Short circuit breaking capacity
Thermal circuit breaker
Thermal circuit breakers use bimetals in series with the circuit. When overload occurs, the heat generated by the current will deform the bimetal, thus tripping the circuit breaker. Compared with fuses, thermal protectors have a significant advantage, that is, they can be reset after tripping. They can also be used as power on/off switches for protected equipment.
With the increase of temperature, the tripping speed of thermal circuit breaker is accelerated, and it often trips at a lower current level. This feature is usually useful when the circuit breaker and the system are exposed to the same heat source.
In this case, the protection circuit can track the demand of equipment for enhanced wiring protection at higher temperature. If the thermal circuit breaker is installed in an environment separate from the protected equipment, the influence caused by the change of ambient temperature can be corrected by the compensation type thermal bimetal. For example, the circuit breaker located outside the cockpit of an aircraft is temperature compensated, so its trip characteristics will not change with the common temperature fluctuations in flight.
In addition, due to the inherent locking mechanism inside the thermal circuit breaker, it is extremely insensitive to shock and vibration. At present, some high-performance circuit protection devices provide circuit breakers specially used in extreme shock and vibration environments.
Applications requiring thermal circuit protection include household appliances, transportation, ships, switchboards, medical equipment, audio-visual equipment, power supplies and sports equipment.
Magnetic circuit breaker
Magnetic circuit breakers provide economical and efficient solutions with high accuracy and reliability for most design problems.
The overcurrent detection mechanism of magnetic circuit breaker only responds to the current change in the protected circuit. Because its current induction solenoid is not greatly affected by the change of environmental temperature, the magnetic circuit breaker has temperature stability and will not be obviously affected by the change of environmental temperature like the thermal circuit breaker.
Magnetic circuit breaker has no preheating stage, so it will not slow down the response speed of circuit breaker to overload, and there is no cooling period from the end of overload to reset.
The characteristics of magnetic circuit breaker can be adjusted from four independent aspects: the circuit required by the circuit breaker; Trip point (unit: ampere); Delay time (seconds) and surge handling capacity. The adjustment of these factors has little effect on the short-circuit breaking ability of the circuit breaker.
Generally speaking, there are three kinds of magnetic circuit breakers with different trip delay curves to choose from: slow, medium and fast. These optional curves provide designers with a high degree of design flexibility when matching circuit breakers in cascade circuits and identification circuits.
In addition, for the equipment that often needs to bear huge inrush current, magnetic circuit breakers with special inrush current structure can also be selected. However, when the position of the equipment is unstable, the tripping times of the magnetic circuit breaker will change due to the influence of gravity on the solenoid movement, so the thermal circuit breaker may be a better choice.
The application fields of magnetic circuit breakers cover many markets, such as telecommunications, ships, electrical appliances, industrial automation and control, medical equipment and so on.
Leakage protector
Leakage protectors (such as SmartGuard series of Carling Company) work on the same principle as magnetic circuit breakers and can provide customized overload and short-circuit protection levels. In addition, they use innovative electronic technology to detect and avoid leaks.
Except for a little leakage, the current flowing back to the power supply is equal to the current flowing out of the power supply. If the difference between the current value flowing out and the current value returned by the power supply exceeds the set value of leakage sensitivity after flowing through the leakage protector, the protector will trip and the LED indicator will light up to give the operator a hint, which has the characteristics of "intelligence".
The LED indicator clearly shows the trip caused by leakage. This protection helps to avoid serious equipment damage and fire. Its applications include resistance and impedance heating systems, telecommunications, theater lighting, ship consoles, office equipment, medical equipment, industrial automation and control, and UPS systems.
Some secondary factors to be considered
When choosing a circuit breaker, we should not only pay attention to the main indicators such as the delay curve of the circuit breaker, but also pay attention to its many secondary functions. These often overlooked features can not only add icing on the cake to a good design, but also help engineers design accurate protection circuits for their applications.
At present, many circuit breakers are equipped with various optional functions, which is very helpful for circuit protection design. Some of the more common functions are listed below.
Auxiliary contacts (auxiliary switches): They are electrically isolated from the main contacts and are suitable for alarm and program switches. Auxiliary contacts can be used to alarm the operator or control system, give an alarm, or connect the standby power supply in important applications.
Transmission: the choice of transmission type is not only for beauty. Circuit breakers with rocker switches with switching speed twice that of on/off switches can save cost and circuit board space. Push-pull actuator is the most stable in an emergency.
Shunt terminal: Traditional circuit breakers are considered as "series tripping" because contacts, current sensing elements and loads are all connected in series. The shunt terminal branches from the main circuit so that the secondary load can be connected. If the main load is short-circuited or overloaded, the circuit breaker will trip and cut off the power supply to both loads.
Unlike the auxiliary contact, the shunt terminal is connected to the breaker current path 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, otherwise the circuit can only be used for equipment with built-in protection circuit.
Compound control (remote control tripping or relay tripping): the compound control circuit breaker combines two inductance elements which are electrically isolated from each other to realize multiple functions. For example, multi-control circuit breakers can use remote control actuators or inductors for traditional overcurrent protection and circuit disconnection. Remote control tripping is an example of compound control, which is usually called "relay tripping".
Low voltage trip: This is an independent voltage sensitive element in the circuit breaker. If the voltage drops below a predetermined value, it will open the main contact. Switched circuit breakers with low voltage tripping are widely used for on/off control of wired connection equipment. The safety management department requires these appliances to cut off the power supply in case of power failure to avoid the danger of sudden restart of the appliances when power supply is restored.
Automatic trip: The automatic trip circuit breaker will not remain closed all the time during the fault-because the switching device will not fail because the actuator is forcibly kept open. In the fully automatic tripping design, when the actuator is kept in the "ON" position, the main contact will always remain disconnected after the fault occurs. Some circuit breakers, called "periodic automatic tripping", can't be forced to turn on when there is a fault, but if the actuator is always in the "on" position, it will be turned on and off periodically. If the circuit breaker is installed in an easily accessible place (that is, it is not closed), an automatic trip circuit breaker should be used.
Automatic reset: For applications where circuit breakers are hard to reach, circuit breakers that automatically reset after the cooling-off period are a good choice. At this time, if you specify a device that can automatically restart, there is a great possibility of danger.