1, according to the calculation of the expected short-circuit current of the line, it is an extremely tedious work to select the breaking capacity of the circuit breaker and accurately calculate the expected short-circuit current of the line. Therefore, there are some simple calculation methods acceptable in engineering: (1) For the transformer with voltage level of 10.4 kV, the short-circuit capacity on the high-voltage side can be considered infinite (the short-circuit capacity on the 10KV side is generally 200~400MVA or even greater, so if it is considered infinite, the error is less than10. (2) Article 2. 1.2 of 2)GB 50054-95 Code for Design of Low-voltage Distribution stipulates: "When the sum of rated currents of motors connected near short-circuit points exceeds 1% of short-circuit current, the influence of motor feedback current shall be included". If the short-circuit current is 30KA, 1% should be 300A. (3) The transformer impedance voltage UK indicates that the secondary side of the transformer is short-circuited. When the secondary side reaches its rated current, the primary side voltage is a percentage of its rated voltage. Therefore, when the primary voltage is rated voltage, the secondary current is its expected short-circuit current. (4) The rated secondary current of transformer Ite=Ste/ 1.732U, where Ste is the transformer capacity (KVA), Ue is the rated secondary voltage (no-load voltage), and Ue=0.4KV when 10/0.4KV, so simply calculating the rated secondary current of transformer should be the transformer capacity X1. (5) According to the definition of Uk in (3), the short-circuit current on the secondary side (three-phase short circuit) is the definition of Uk in I(3), and the short-circuit current on the secondary side (three-phase short circuit) is I(3)=Ite/Uk, which is the AC effective value. (6) In the case of the same transformer capacity, if two phases are short-circuited, I (2) =1.732i (3)/2 = 0.866i (3) (7) The current values calculated above are all when the transformer outlet is short-circuited, which is the most serious short-circuit accident. If the short-circuit point is at a certain distance from the transformer, the line impedance should be considered, so that the short-circuit current will be reduced. For example, the capacity of SL7 series transformer (with three-core aluminum wire cable) is 200KVA. When the transformer outlet is short-circuited, the three-phase short-circuit current I(3) is 72 10A. When the distance between the short-circuit point and the transformer is 100m, the short-circuit current I(3) decreases to 4740a; When the transformer capacity is 100KVA, the short-circuit current at the outlet is 36 16A. When it is away from the transformer 100m, the short-circuit current is 2440A. When it is far away from 100m, the short-circuit current is 65.74% and 67.47% of 0m respectively. Therefore, users should calculate the rated current of the installation site (line) and the maximum short-circuit current that may occur there when designing. The circuit breaker is 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 is greater than or equal to the expected short-circuit current of the line. Therefore, it is not necessary to leave too much excess when selecting the circuit breaker to avoid waste.
2. Limit short-circuit breaking capacity and operating short-circuit breaking capacity of circuit breakers: China adopts IEC947-2 of the International Electrotechnical Commission and GB4048.2 of IEC "Low-voltage Circuit Breakers for Low-voltage Switchgear and Control Equipment" to define the limit short-circuit breaking capacity and operating short-circuit breaking capacity of circuit breakers as follows: rated limit short-circuit breaking capacity (Icu) of circuit breakers: according to the conditions specified in the specified test procedures, the breaking capacity of circuit breakers to continue carrying its rated current capacity is excluded; Rated short-circuit breaking capacity (Ics) of the circuit breaker: According to the conditions specified in the specified test procedure, including the breaking capacity of the circuit breaker to continue carrying its rated current capacity. The test procedure of Icu limit short circuit breaking ability is otco. The specific test is: adjust the line current to the expected short-circuit current value (such as 380V, 50KA), but the test button is not closed, and the circuit breaker under test is in the closing position. When the test button is pressed, the circuit breaker will pass 50KA short-circuit current, and the circuit breaker will immediately open (O for short) and extinguish the arc. The circuit breaker shall be intact and can be closed again. T is the intermittent time (rest time), usually 3 minutes. At this time, the line is in hot standby state, and the circuit breaker is turned on again (referred to as closed for short, C) and then turned off (O) (the turn-on test is to assess the electrical and thermal stability of the circuit breaker under peak current and the wear of the dynamic and static contacts caused by bouncing). This scheme is co, the circuit breaker can completely break and extinguish arc, and there is no damage beyond the specified limit, so it is judged that its limit breaking ability test is successful; The short-circuit breaking ability (Icu) test program of circuit breaker is otco t co, which is one more co than Icu. After the test, if the circuit breaker can completely break and extinguish the arc without exceeding the specified damage, it is considered that its rated short-circuit breaking capacity test has passed. After the short-circuit breaking test of Icu and Ics, it is necessary to conduct tests such as recalibration of withstand voltage and protection characteristics. Because it still carries rated current after short circuit disconnection, it is necessary to carry out retest test of temperature rise after Ics short circuit test. Icu and Ics have different conditions for short circuit or actual assessment, and the latter is stricter and more difficult than the former. Therefore, IEC947-2 and GB 14048.2 determine that Icu has four or three values, which are 25%, 50%, 75% and 100%Icu (for Class A circuit breakers) or 50% and 75% respectively. The Ics value determined by the circuit breaker manufacturer meets the Icu percentage specified in the above standards and is an effective and qualified product. Most general-purpose (frame-type) circuit breakers (not all specifications) have three-level protection functions of overload, long delay, short delay and short-circuit instantaneous action, which can realize selective protection. Therefore, most trunk lines (including the outlet terminals of transformers) use it as the main (protection) switch, while molded case circuit breakers generally do not have the function of short circuit and short delay (only two levels of protection are overload and long delay and short circuit instantaneous action), so they can only do selective protection. Due to the different usage (applicability), IEC92 "Marine Electricity" suggests that the universal circuit breaker with three-level protection should focus on its short-circuit breaking ability and be widely used in branch-line plastic case circuit breakers to ensure its sufficient limit short-circuit ability. Our understanding is that it is prudent to replace the circuit breaker after the main line cuts off the fault current, and the power failure of the main line will affect a large number of users, so two COs are needed when a short-circuit fault occurs, and it is required to continue to carry the rated current for a period of time. However, in the branch line, after the limit short-circuit current is cut off, it has completed its mission, can no longer carry the rated current, and can be replaced with a new one (with little impact of power failure). However, whether it is universal or molded case circuit breaker, it must have three important technical indicators: Icu and Ics. Only the Ics value is slightly different between the two types of circuit breakers. The minimum allowable Ics of plastic shell can be 25%Icu, and the minimum allowable Ics of universal shell is 50%. Ics is rare =Icu, even universal box is rare =100% [there are plastic case circuit breakers with rotating double-break (point) technology abroad, which have excellent current limiting performance and large breaking capacity margin. In China, the IC of DW45 intelligent universal circuit breaker is 62.5% ~ 65% ICU. Only about 70% of ABB's F series and Schneider's M series in the world, while the Ics of various new models of molded case circuit breakers in China is generally between 50% and 75% ICU. Some designers who use circuit breakers choose circuit breakers according to their own calculated expected short-circuit current, and judge a circuit breaker (the limit short-circuit capacity of the circuit breaker is greater than the expected short-circuit current of the line, but the running short-circuit breaking capacity is lower than the calculated current) as unqualified. This is a misunderstanding.
3. The electrical clearance and creepage distance of the circuit breaker must be determined according to the insulation coordination of the low-voltage system, which is based on the fact that the instantaneous overvoltage is limited to the specified impulse withstand voltage, and the instantaneous overvoltage generated by the electrical appliances or equipment in the system must also be lower than the specified impulse voltage of the power supply system. Therefore: (1) The rated isolation voltage of electrical appliances should be ≥ the rated voltage of power supply system; (2) The rated impulse withstand voltage of electrical appliances shall be equal to or higher than that of power supply system; (3) The transient overvoltage generated by electrical appliances shall be less than or equal to the rated impulse withstand voltage of the power supply system. Based on the above three principles, the rated impulse withstand voltage (priority value) Uimp of electrical appliances is closely related to the rated voltage of power supply system and the maximum relative ground voltage determined by the installation category (overvoltage category) of electrical appliances: the greater the relative ground voltage value, the higher the installation category [divided into I (signal level), II (load level), III (distribution level) and IV (power level)]. For example, when the relative ground voltage is 220V and the installation category is Class III, the Uimp is 4.0KV; if the installation category is Class IV, the Uimp is 6.0KV. The Uimp of electrical products (such as circuit breakers) is 6.0KV pollution level 3 or 4, and its minimum electrical gap is 5.5 mm The electrical gaps of series DZ20, CM 1, HSM 1 molded case circuit breakers in our factory are all 5.5≥8mm (installation category III), which is only used for power level installation. For example, the specification of DZ20 series is above 800, Uimp is 8.0KV, and the electrical gap will increase to ≥ 8 mm And the actual electrical gap of the product, such as HSM 1 series, when Inm (Frame Level Current) = 125A, the electrical gap is 1 1mm, 160A is 16mm, and 250A is 65438+5.5mm As for the creepage distance, GB/T 14048. 1 General Rules for Low-voltage Switchgear and Electrical Equipment, for example, the rated isolation voltage is 660(690)V, and the pollution level is high. The insulation material used in the product is Ⅲ a (175 ≤ CTI < 400, CTI is the creepage index of insulation material), and the minimum creepage distance is 10 mm, which greatly exceeds the specified value. To sum up, if the electrical gap and leakage distance of electrical products meet the requirements of insulation coordination, the dielectric breakdown of equipment will not be caused by external overvoltage or operating overvoltage of line equipment itself. Gb7251.1-1997 "Low-voltage switchgear and controlgear Part I: Type test and partial type test equipment" (equivalent to IEC 439- 1: 1992), insulation coordination requirements and GB/T 652. Some manufacturers of complete sets of electrical appliances suggest that the wiring of circuit breakers should use copper bars, and the distance between phases (air) should be greater than 12mm, and some even suggest that the electrical gap of circuit breakers should be greater than 20 mm ... This requirement is unreasonable, and it has exceeded the requirements of insulation coordination. For large current specifications, the space distance between phases can also be appropriately widened to avoid generating electric repulsion when short-circuit current occurs, or to increase the heat dissipation space when the conductor heats up under large current. At this time, whether it reaches 12mm or 20mm, it can be solved by the complete set of electrical appliances manufacturers themselves, or the electrical components factory can be asked to provide terminals or connecting plates (sheets) with elbows. Generally, when the circuit breaker leaves the factory, it is equipped with a spare arc extinguishing plate at the power end to prevent short circuit between phases when the arc is ejected. In order to prevent ionized molecules from escaping when breaking short-circuit current, the circuit breaker with zero arc extinguishing is also equipped with this arc extinguishing plate. If there is no arc partition, the bare copper bar can be wrapped with insulating tape, and the distance should be not less than100 mm.
4. Application of Quadrupole Circuit Breaker As for the application of Quadrupole Circuit Breaker, at present, there is no rigid use requirement of national standards or regulations in China. Although the design specification of regional four-pole electrical appliances (circuit breakers) has been issued, the debate about whether to install four-pole electrical appliances continues. In recent years, the use of quadrupole electrical appliances in some fields is very popular, and various circuit breaker manufacturers have also designed and manufactured various types of quadrupole circuit breakers on the market. The author agrees that whether it should be used or not should be based on whether the reliability and safety of power supply can be guaranteed, so it is generally: (1)TN-C system. In TN-C system, N line and protection line PE are combined into one (PEN line). Considering the safety, it is not allowed to disconnect the pen line at any time, so the four-pole circuit breaker is absolutely prohibited. (2)TT system, TN-C-S system and TN-S system can use four-pole circuit breakers to ensure the safety of maintenance personnel during maintenance. However, in TN-C-S and TN-S systems, the N pole of the circuit breaker can only be connected with N line, but not with PEN or PE line. (3) Where the dual power switch is installed, because all neutral wires (N wires) in the system are connected, in order to ensure the maintenance safety of the switched power switch (circuit breaker), a four-pole circuit breaker must be adopted; (4) For the single-phase master switch entering the residence, N-pole bipolar circuit breaker (used as isolating switch during maintenance) should be selected. (5) The residual current protector (leakage circuit breaker) of 380/220V system should be used, and the neutral wire must pass through the zero-sequence current transformer (core) of the protector to prevent the neutral wire from passing through, so that the 220V load will generate leakage current and misoperation. At this time, the four-pole or two-pole residual current protector of neutral wire should be selected.