Main classification and product features of low-voltage circuit breakers (air switches)

Main classification and product features of low-voltage circuit breakers (air switches)

The main classification method of low-voltage circuit breakers, based on their structural form, is open and equipment type, which is a typical product classification. Open is also called frame type or universal type, while equipment type is called plastic shell type.

General selection principle of circuit breakers.

The rated working voltage of the circuit breaker should be greater than or equal to the rated voltage of the line.The rated current of the circuit breaker should be greater than or equal to the load current of the line.The rated short-circuit breaking capacity of the circuit breaker should be greater than or equal to the maximum short-circuit current that may occur on the line (calculated as effective value).The single-phase ground short-circuit current at the end of the line should be greater than or equal to 1.25 times the set current of the instantaneous release of the circuit breaker.The rated voltage of the undervoltage release of the circuit breaker should be equal to the rated voltage of the line.The rated voltage of the shunt release of the circuit breaker should be equal to the control power supply voltage.The rated operating voltage of the electric drive should be equal to the control power supply voltage.Check the wiring direction allowed by the circuit breaker. Some models of circuit breakers only allow the line to enter from the top, while some models allow the line to enter from the top or bottom.

Principles for selecting low-voltage circuit breakers

1.Determine the type and protection form of the circuit breaker according to the protection requirements of the line: select frame type, equipment type or current limiting type, etc.
Equipment type circuit breaker: It has an insulating plastic shell, built-in contact system, arc extinguishing chamber and release, etc., and can be closed manually or electrically (for large-capacity circuit breakers). It has strong breaking capacity, high dynamic stability, and relatively complete selective protection functions. It is widely used in distribution lines. Currently, the commonly used products include DZ15, DZ20, DZX19, C45N (now upgraded to C65N) and other series. Among them, C45N (C65N) circuit breakers have the advantages of small size, strong breaking capacity, good current limiting performance, simple operation, and complete models and specifications. They can be easily combined into two-pole, three-pole and four-pole circuit breakers based on the single-pole structure, and are widely used in civil lighting branches and branch lines of 60A and below (mostly used for incoming line switches for residential users and lighting branch switches in shopping malls).
2.Frame-type low-voltage circuit breakers: large capacity, strong short-circuit breaking capacity, high dynamic stability, suitable for AC 50Hz, rated voltage 380V distribution network, as the main protection of distribution trunk lines. Frame-type circuit breakers are mainly composed of contact system, operating mechanism, overcurrent release, shunt release, undervoltage release, accessories and frame, etc. All components are insulated and installed on the frame structure base. At present, the commonly used frame-type low-voltage circuit breakers in my country are DW15, ME, AE, AH and other series. The DW15 series circuit breakers are developed and produced in my country. The full series includes several models such as 1000, 1500, 2500, and 4000A. The ME, AE, and AH series circuit breakers are produced using imported technology, with relatively complete specifications and models (I switch current levels range from 630A to 5000A, a total of 13 levels), and the rated breaking capacity is stronger than that of DW15. They are often used for the main protection of low-voltage distribution trunks.
3.Intelligent circuit breakers: At present, there are two types of intelligent circuit breakers produced in my country: frame type and plastic shell type. Frame type intelligent circuit breakers are mainly used as main circuit breakers for intelligent automatic distribution systems, while plastic shell type intelligent circuit breakers are mainly used for power distribution in distribution networks and control and protection of lines and power supply equipment. Used as the control of three-phase cage asynchronous motors. The characteristics of intelligent circuit breakers are that they use intelligent controllers (intelligent trip devices) with microprocessors or single-chip microcomputers as the core, and have the functions of online monitoring, automatic adjustment, measurement, testing, self-diagnosis, communication, etc. of the electrical parameters of the circuit (current, voltage, power, power factor, etc.), and can display, set and monitor the action parameters of various protection functions, modify the fault parameters when the protection circuit is in action, and store these parameters in non-volatile memory for query. Common domestic intelligent circuit breaker models include DW45, DW40, DW914 (AH), DW18 (AE-S), DW48, DW19 (3WE), DW17 (ME), etc.

When selecting a circuit breaker, the following principles should be considered:

1. The rated voltage (UN) of the circuit breaker should be equal to or greater than the rated voltage of the protected line.
2. The rated voltage of the circuit breaker's undervoltage release should match the rated voltage of the protected line.
3. The rated current of the circuit breaker and its overcurrent release should be greater than or equal to the calculated current of the protected circuit.
4. The ultimate breaking capacity of the circuit breaker should exceed the effective value of the maximum short-circuit current in the line.
5. The protection characteristics of the upper and lower circuit breakers in the distribution line should be coordinated, with the lower-level protection characteristics positioned below and not intersecting with the upper-level protection characteristics.
6. The long-time delay tripping current of the circuit breaker should be less than the allowable continuous current of the wire.

For power distribution circuit breakers, the following selection principles apply:

1. The setting value of the circuit breaker's long-term operating current should not exceed the allowable carrying capacity of the wire. For wire and cable applications, it is advisable to use 80% of the wire and cable's allowable current carrying capacity.
2. The returnable time for the three-times long-time delay operating current setting value should be greater than or equal to the starting time of the motor with the maximum starting current in the line.
3. The instantaneous current setting value should be at least 1.1 times the sum of the line calculation load current (Ijx) and the product of the motor starting current impact coefficient (k1, typically 1.7-2), the motor starting current multiple (k), and the rated current of the largest motor (Icdm).

For motor protection circuit breakers, the following selection principles apply:

1. The long-time delay current setting value should equal the motor's rated current.
2. The instantaneous setting current for circuit breakers protecting cage motors should be 8-15 times the motor's rated current, while for wound rotor motors, it should be 3-6 times the motor's rated current.
3. The returnable time for the six-times long delay current setting value should be greater than or equal to the motor's actual starting time, with options of 1S, 3S, 5S, 8S, 12S, or 15S based on the load's weight at startup.

When coordinating circuit breakers and fuses, the following principles should be considered:

1. If the expected short-circuit current at the installation point is less than the circuit breaker's rated breaking capacity, a fuse can be used as backup protection due to its strong rated short-circuit breaking ability. As shown in Figure 1, the backup fuse's characteristic (1) intersects with the circuit breaker's characteristic (2). In the event of a line short-circuit, the fuse's breaking time is shorter than the circuit breaker's, ensuring the circuit breaker's safety. The transfer point on the characteristics can be selected at 80% of the circuit breaker's rated short-circuit breaking capacity.
2. The fuse should be installed on the power side of the circuit breaker to ensure safe use.