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Selection principles of low-voltage circuit breakers

Jun 26,2023 | simplybuy industrial

Low-voltage circuit breakers (air switches) The main classification method of low-voltage circuit breakers, a typical product, is based on the structural form, namely open type and device type.
The open type is also called frame type or universal type, and the device type is also called plastic shell type.

General selection principles of circuit breakers

(1) The rated working voltage of the circuit breaker ≥ the rated voltage of the line.
(2) The rated current of the circuit breaker ≥ the line load current.
(3) The rated short-circuit breaking capacity of the circuit breaker ≥ the maximum short-circuit current that may appear in the line (calculated according to the effective value).
(4) The single-phase-to-ground short-circuit current at the end of the line is ≥1.25 times the setting current of the instantaneous release of the circuit breaker.
(5) The rated voltage of the undervoltage release of the circuit breaker = the rated voltage of the line.
(6) Rated voltage of circuit breaker shunt release = control power supply voltage.
(7) Rated operating voltage of the electric drive = control power supply voltage.
(8) Check the wiring direction allowed by the circuit breaker. Some models of circuit breakers only allow incoming lines from above, and some models allow incoming lines from above or below.

Selection principles of low-voltage circuit breakers

(1) Determine the type and protection form of the circuit breaker according to the protection requirements of the line - determine the choice of frame type, device type or current limiting type, etc.

1. Device-type circuit breaker: Device-type circuit breaker 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 high breaking capacity and dynamic stability, has a relatively complete selective protection function, and is widely used in power distribution lines. Currently commonly used are DZl5, DZ20, DZXl9, and C45N (now upgraded to C65N) and other series of products. Among them, the C45N (C65N) circuit breaker has the advantages of small size, high breaking capacity, good current limiting performance, easy operation, complete models and specifications, and can be easily combined into two-pole, three-pole, and four-pole circuit breakers on the basis of single-pole structure. , widely used in 60A and below civil lighting branch lines and branches (mostly used for incoming line switches of residential users and lighting branch switches of shopping malls).
2. Frame-type low-voltage circuit breaker: Frame-type circuit breakers generally have a large capacity, high short-circuit breaking capacity, and high dynamic stability. It is suitable for the distribution network with AC 50Hz and rated current 380V as the main protection of distribution trunk lines. A frame-type circuit breaker is mainly composed of a contact system, operating mechanism, overcurrent release, shunt release, Undervoltage release, accessories, and frame, etc. All components are insulated and installed in the frame structure base. At present, there are DWl5, ME, AE, AH, and other series of frame-type low-voltage circuit breakers commonly used in our country. DWl5 series circuit breakers are developed and produced by our country. The whole series has several models such as 1000, 1500, 2500, and 4000A. ME, AE, and AH series circuit breakers are produced with imported technology. They have relatively complete specifications and models (I switch current levels range from 630A to 5000A, a total of 13 levels), and their rated breaking capacity is stronger than DWl5. They are often used for the main protection of low-voltage power distribution trunk lines.
3. Intelligent circuit breakers: At present, there are two types of intelligent circuit breakers produced in China: frame type and plastic shell type. The frame-type intelligent circuit breaker is mainly used as the main circuit breaker in the intelligent automatic power distribution system, and the plastic shell-type intelligent circuit breaker is mainly used for the distribution of electric energy in the power distribution network and as the control and protection of the line and power supply equipment. Used as the control of a three-phase cage asynchronous motor. The feature of the intelligent circuit breaker is that it adopts an intelligent controller (intelligent release) with a microprocessor or a single-chip microcomputer as the core. Electrical parameters (current, voltage, power, power factor, etc.), online monitoring, self-adjustment, measurement, test, self-diagnosis, communication, and other functions for the circuit, can display, set and monitor the action parameters of various protection functions Modify, the fault parameters when the protection circuit operates can be stored in the non-volatile memory for query, domestic DW45, DW40, DW914(AH), DW18(AE-S), DW48, DW19(3WE), DW17(ME) and other intelligent Both the frame circuit breaker and the intelligent molded case circuit breaker are equipped with ST series intelligent controllers and supporting accessories. ST series intelligent controllers are the key projects of the Ministry of Machinery during the "Eighth Five-Year Plan" to "Ninth Five-Year Plan". Product performance indicators reached the international advanced level in the 1990s. It adopts a building block matching scheme, which can be directly installed in the circuit breaker body, without repeated secondary wiring, and can be combined with various schemes.

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

Selection principles of circuit breakers for power distribution

(1) The setting value of the circuit breaker's long-term operating current ≤ the allowable carrying capacity of the wire. For the case of using wire and cable, it is desirable to take 80% of the allowable current carrying capacity of the wire and cable.
(2) The returnable time of the 3 times long time-delay operating current setting value is greater than or equal to the starting time of the motor with the maximum starting current in the line.
(3) Instantaneous current setting value ≥ 1.1X (Ijx+k1kIedm)

Ijx————line calculation load current;
k1————the impact coefficient of the starting current of the motor, generally take k1=1.7-2;
k————Motor starting current multiple;
Icdm————the rated current of the largest motor

Selection principle of motor protection circuit breaker

(1) Long-time delay current setting value = motor-rated current
(2) Instantaneous setting current:
For circuit breakers protecting cage motors, the instantaneous setting current = (8-15) times the rated current of the motor;
For circuit breakers protecting wound rotor motors, the instantaneous setting current = (3-6) times the rated current of the motor.
(3) The returnable time of the 6 times long delay current setting value ≥ the actual starting time of the motor, according to the weight of the load at the time of starting, the optional returnable time is one of 1S, 3S, 5S, 8S, 12S, 15S.

Coordination principle of circuit breaker and fuse

(1) If the expected short-circuit current at the installation point is less than the rated breaking capacity of the circuit breaker, the fuse can be used as backup protection, because the rated short-circuit analysis ability of the fuse is strong. As shown in Figure 1, characteristic 1 of the back-up fuse intersects characteristic 2 of the circuit breaker. When the line is short-circuited, the breaking time of the fuse is shorter than that of the circuit breaker, which can ensure the safety of the circuit breaker. The transfer point on the characteristics can be selected at 80% of the rated short-circuit breaking capacity of the circuit breaker.
(2) The fuse should be installed on the power side of the circuit breaker to ensure safe use.