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Abstract: in recent years, I have consulted and discussed with many users of circuit breakers, and read some articles on the selection of circuit breakers in professional journals. I feel that the benefits are great, but I also feel that the communication, exchange and publicity between the design, manufacturer and its users are insufficient, As a result, users of electrical products still have some biases in choosing low-voltage circuit breakers. Therefore, the author intends to discuss the selection and application of circuit breakers again, with a view to attracting jade and removing counterfeits. ASTM (American Society for testing and materials) has completed the preparation and authenticity of the technical specifications for AA (8000) series aluminum alloy conductors

key words: circuit breaker selection

in recent years, I have consulted and discussed with many circuit breaker users, and read some articles on circuit breaker selection in professional journals. I feel that I have gained a lot, but I also feel that due to insufficient communication, exchange and publicity between the circuit breaker design, manufacturer and its users, users of electrical products still have some biases in choosing low-voltage circuit breakers. Accordingly, the author intends to discuss the selection and application of circuit breakers again, with a view to casting a brick to attract jade, eliminating the false and preserving the true

1. Select the breaking capacity of the circuit breaker according to the calculation of the expected short-circuit current of the line. Accurate calculation of the expected short-circuit current of the line is an extremely cumbersome work. Therefore, there are some simple calculation methods that have small errors and can be accepted in Engineering: (1) for transformers with 10/0.4kv voltage level, the short-circuit capacity on the high-voltage side can be considered to be infinite (the short-circuit capacity on the 10kV side is generally 200~400mva or even greater, so considering infinity, the error is less than 10%, and the quality of the experimental box can be guaranteed only after these three factors are guaranteed). (2) Article 2.1.2 of GB code for design of low voltage power distribution stipulates that "when the sum of the rated current of the motor connected near the short-circuit point exceeds 1% of the short-circuit current, the influence of the motor feedback current shall be included". If the short-circuit current is 30ka, take 1% of it, it should be 300A, the total power of the motor is about 150KW, and the feedback current included at this time should be 6.5 Σ in when it is started and used at the same time. (3) The impedance voltage UK of the transformer indicates that the secondary side of the transformer is short circuited (circuit). When the secondary side reaches its rated current, the primary side voltage is the percentage of its rated voltage. Therefore, when the primary voltage is the rated voltage, the secondary current is its expected short-circuit current. (4) The rated current at the secondary side of the transformer ite=ste/1.732u, where ste is the capacity of the transformer (kVA), UE is the rated voltage at the secondary side (no-load voltage), and ue=0.4kv at 10/0.4kv. Therefore, the rated current at the secondary side of the transformer should be simply calculated as the transformer capacity x1.44 ~ 1.50. (5) According to the definition of (3) for UK, the short-circuit current of the secondary side (three-phase short-circuit) is the definition of I (3) for UK, and the short-circuit current of the secondary side (three-phase short-circuit) is I (3) =ite/uk, which is the AC effective value. (6) Under the same transformer capacity, if there is a short circuit between two phases, then I (2) =1.732i (3)/2=0.866i (3) (7) the above calculation is the current value when the transformer outlet is short circuited, which is the most serious short circuit accident. If the short-circuit point is a certain distance from the transformer, the line impedance needs to be considered, so the short-circuit current will be reduced. For example, sl7 series transformer (with three core aluminum cable as the distribution conductor) has a capacity of 200KVA. When the outgoing end of the transformer is short circuited, the three-phase short-circuit current I (3) is 7210a. When the distance between the short-circuit point and the transformer is 100m, the short-circuit current I (3) drops to 4740a; When the transformer capacity is 100kVA, the short-circuit current at its outgoing terminal is 3616a. When the distance from the transformer is 100m, the short-circuit current is 2440a. The short-circuit current at a distance of 100m is 65.74% and 67.47% of 0m respectively. Therefore, when designing, users should calculate the rated current at the installation site (line) and the maximum possible short-circuit current at the site where the crude steel production capacity will be reduced by about 13.27% in five years. The circuit breaker shall be selected according to the following principles: the rated current of the circuit breaker in ≥ the rated current of the line IL, the rated short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the line. Therefore, in selecting the circuit breaker, it is not necessary to put too much margin to avoid waste

2. Ultimate short-circuit breaking capacity and operating short-circuit breaking capacity of circuit breakers IEC of the International Electrotechnical Commission and China adopt the standard of gb4048.2 low voltage circuit breakers for low voltage switchgear and control equipment of IEC equivalently. The ultimate short-circuit breaking capacity and operating short-circuit breaking capacity of circuit breakers are defined as follows: rated ultimate short-circuit breaking capacity (ICU) of circuit breakers: according to the conditions specified in the specified test procedures, It does not include the breaking capacity of the circuit breaker to continue to carry its rated current; Rated operating short-circuit breaking capacity (ICS) of the circuit breaker: the breaking capacity of the circuit breaker to continue to carry its rated current according to the conditions specified in the specified test procedure. The test procedure of limit short-circuit breaking capacity ICU is otco. The specific test is: adjust the current of the line to the expected short-circuit current value (such as 380V, 50kA), and the test button is not closed, the tested circuit breaker is in the closing position, press the test button, the circuit breaker passes the 50kA short-circuit current, the circuit breaker immediately opens (open for short) and extinguishes the arc, and the circuit breaker should be intact and can be reclosed. T is the interval time (rest time), generally 3min. At this time, the line is in the hot standby state, and the circuit breaker is closed again (C for short) and then disconnected (o) (the connection test is to assess the electric and thermal stability of the circuit breaker under peak current and the wear of dynamic and static contacts due to bounce). This procedure is called Co. If the circuit breaker can completely break and extinguish the arc without exceeding the specified damage, it is considered that its limit breaking capacity test is successful; The test procedure of the operating short-circuit breaking capacity (ICU) of the circuit breaker is otco t Co, which is one co more than the test procedure of ICU. After the test, the circuit breaker can completely break and extinguish the arc without exceeding the specified damage, its rated short-circuit breaking capacity test is determined to pass. After the short-circuit breaking test of ICU and ICs, the withstand voltage, protection characteristic re calibration and other tests should be carried out. Since the rated current is also carried after the short-circuit breaking of operation, ICS short 1 is to focus on the development of multifunctional and high-performance materials and additives. After the road test, a retest test of temperature rise needs to be added. The short circuit or actual assessment conditions of ICU and ICs are different, and the latter is more strict and difficult than the former. Therefore, IEC and gb14048.2 determine that ICU has four or three values, which are 25%, 50%, 75% and 100% ICU (for class a circuit breaker, i.e. molded case type) or 50%, 75% and 100% ICU (for class B circuit breaker, i.e. universal or frame type). The ICs value determined by the manufacturer of the circuit breaker and the ICU percentage value conforming to the above standards are valid and qualified products. Universal (frame type) circuit breakers, most (not all specifications) have

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