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In recent years, I have consulted and discussed with many users of circuit breakers, and read some articles on circuit breaker selection in professional publications. I feel that the benefits are great. However, I also feel that due to insufficient communication, communication and publicity between the designers and manufacturers of circuit breakers and their users, there is still some bias in the selection of low-voltage circuit breakers by users of electrical products. Based on this, the author intends to discuss the selection and application of circuit breakers again, in order to shed some light on the topic and eliminate the false while retaining the true. 1. Select the breaking capacity of the circuit breaker based on 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 tedious task. Therefore, there are some simple calculation methods that are not very error-prone and are acceptable in engineering.: (1) For a transformer with a voltage level of 10/0.4kV, it can be considered that the short-circuit capacity on the high-voltage side is infinite (the short-circuit capacity on the 10kV side is generally 200~400MVA or even greater, so considering it as infinite, the error is less than 10%). (2) Clause 2.1.2 of GB50054-95 "Low Voltage Distribution Design Code": “When the sum of the rated currents of the motors connected near the short-circuit point exceeds 1% of the short-circuit current, the influence of the motor feedback current should be included. If the short-circuit current is 30kA, take 1% of it, which should be 300A. The total power of the motor is about 150kW, and when it is started at the same time, the feedback current included in this time should be 6.5∑In. (3) The impedance voltage UK of the transformer means that the secondary side of the transformer is short-circuited (circuit). 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 the rated voltage, the secondary current is its expected short-circuit current. (4) The secondary rated current of the transformer Ite=Ste/1.732U where Ste is the capacity of the transformer (kVA), Ue is the secondary rated voltage (no-load voltage), Ue=0.4kV at 10/0.4kV, so a simple calculation of the secondary rated current of the transformer should be the transformer capacity x 1.44~1.50. (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 by I(3). The short-circuit current on the secondary side (three-phase short circuit) is I(3)=Ite/Uk. This value 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 calculations are the current values when the outlet end of the transformer is short-circuited, which is the most serious short-circuit accident. If the short-circuit point is some distance from the transformer, the line impedance needs to be taken into account, so the short-circuit current will be reduced. For example, the SL7 series transformer (with three-core aluminum wire cable) has a capacity of 200kVA. When the outlet 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 the outlet end is 3616A. When there is a short circuit at a distance of 100m from the transformer, the short circuit current is 2440A. When 100m away, the short-circuit currents are 65.74% and 67.47% of 0m respectively. Therefore, when designing, users should calculate the rated current of the installation location (line) and the maximum short-circuit current that may occur there. And select the circuit breaker 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, when selecting a circuit breaker, there is no need to put too much margin to avoid waste. 2. The ultimate short-circuit breaking capacity and operating short-circuit breaking capacity of the circuit breaker. The International Electrotechnical Commission's IEC947-2 and my country's equivalent use of IEC's GB4048.2 "Low-voltage switchgear and control equipment low-voltage circuit breakers" standard have the following definitions for the circuit breaker's ultimate short-circuit breaking capacity and operating short-circuit breaking capacity.: Rated ultimate short-circuit breaking capacity of the circuit breaker (Icu): The conditions specified in the prescribed test procedures, excluding the breaking capacity of the circuit breaker to continue to carry its rated current capacity ； Rated operating short-circuit breaking capacity of circuit breaker (Ics): The conditions specified in the prescribed test procedure include the breaking capacity of the circuit breaker to continue carrying its rated current capacity. The test procedure for the ultimate short-circuit breaking capacity Icu is otco. The specific test is: Adjust the current of the line to the expected short-circuit current value (for example, 380V, 50kA), but the test button is not closed, and the circuit breaker under test is in the closing position. Press the test button, the circuit breaker passes 50KA short-circuit current, the circuit breaker immediately breaks (OPEN abbreviated as O) and extinguishes the arc. The circuit breaker should be intact and can be closed again. t is the intermittent time (rest time), generally 3 minutes. At this time, the line is in a hot standby state, and the circuit breaker is connected again (CLOSE for short) and followed by breaking (O) (the connection test is to assess the electric and thermal stability of the circuit breaker under peak current and the wear of the dynamic and static contacts due to bouncing). This procedure is called CO. If the circuit breaker can completely break, extinguish the arc, and there is no damage beyond the prescribed limits, it is deemed that its ultimate breaking capacity test has been successful. ； The test procedure for the operating short-circuit breaking capacity (Icu) of the circuit breaker is otcotco, which has one more co than the Icu test procedure. After testing, if the circuit breaker can completely break and extinguish the arc without causing any damage beyond the regulations, it is deemed to have passed the rated short-circuit breaking capacity test. After the Icu and Ics short-circuit breaking test, tests such as withstand voltage and protection characteristics recalibration must be carried out. Since the rated current must be carried after the operating short circuit is broken, a temperature rise retest needs to be added after the Ics short circuit test. Icu and Ics have different conditions for short circuit or actual assessment. The latter is more stringent and difficult than the former. Therefore, IEC947-2 and GB14048.2 determine that Icu has four or three values, which are 25%, 50%, 75% and 100%Icu (for Class A circuit breakers, that is, molded case type) or 50%, 75%, 100%Icu (for Class B circuit breakers, that is, universal type or frame type). The Ics value determined by the manufacturer of the circuit breaker and the Icu percentage that meets the above standards are valid and qualified products. Universal (frame type) circuit breakers, most (but not all specifications) have three-stage protection functions of overload long delay, short-circuit short delay and short-circuit instantaneous protection, which can achieve selective protection. Therefore, most main lines (including the outlet end of the transformer) use it as the main (protection) switch, while molded case circuit breakers generally do not have short-circuit short-delay functions (only overload long-delay and short-circuit instantaneous two-stage protection) and cannot provide selective protection. They can only be used in branch circuits. Due to different usage (applicability) situations, IEC92 "Ship Electrical" recommends: The universal circuit breaker with three-stage protection focuses on its operating short-circuit breaking capacity value, while the large number of branch line molded case circuit breakers ensures that it has sufficient ultimate short-circuit capacity value. Our understanding of this is: Be careful when replacing the circuit breaker after removing the fault current from the main line. A power outage on the main line will affect a large number of users. Therefore, two COs are required when a short-circuit fault occurs, and they are required to continue to carry the rated current for a period of time. In the branch circuit, after breaking the limit short-circuit current and closing and opening again, it has completed its mission. It no longer carries the rated current and can be replaced with a new one (the impact of the power outage is smaller). However, whether it is a universal or molded case circuit breaker, it must have two important technical indicators: Icu and Ics. Only the Ics value behaves slightly differently between the two types of circuit breakers. The minimum allowable Ics of the plastic case type can be 25%Icu, and the minimum allowable Ics of the universal type is 50%. There are very few circuit breakers with Ics=Icu, and even the universal type rarely has Ics=100%. The Ics of my country's DW45 intelligent universal circuit breaker is 62.5% to 65% Icu. Internationally, ABB's F series and Schneider's M series are only about 70%. As for plastic case circuit breakers, the Ics of various new domestic models is generally between 50% and 75% Icu. Some circuit breaker application designers, when selecting a circuit breaker based on the calculated expected short-circuit current of the line, use the rated operating short-circuit breaking capacity of the circuit breaker to determine that a certain circuit breaker (the ultimate short-circuit capacity of this circuit breaker is greater than the expected short-circuit current of the line, but the operating short-circuit breaking capacity is lower than the calculated current) is unqualified. This is a misunderstanding. 3. The electrical clearance and creepage distance of the circuit breaker determine the electrical clearance of electrical products, which must be based on the insulation coordination of the low-voltage system. The insulation coordination 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 impulse voltage specified by the power system. therefore: (1) The rated insulation voltage of the electrical appliance should be ≥ the rated voltage of the power supply system (2) The rated impulse withstand voltage of the electrical appliance should be ≥ the rated impulse withstand voltage of the power supply system (3) The transient overvoltage generated by the electrical appliance should be ≤ 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 an electrical appliance is closely related to the maximum value of the phase-to-ground voltage determined by the rated voltage of the power supply system and the installation category (overvoltage category) of the electrical appliance.: The greater the phase-to-ground voltage value and the higher the installation category, the greater the rated impulse voltage. For example, when the phase-to-ground voltage is 220V and the installation category is III, Uimp is 4.0KV. If the installation category is IV, 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 clearance is 5.5mm. The electrical clearances of DZ20, CM1 and our factory's HSM1 series molded case circuit breakers are all 5.5mm (installation category III). They are only used for power-level installation. For example, the DZ20 series of specifications above 800 have a Uimp of 8.0KV and the electrical clearance is increased to ≥8mm. The actual electrical clearance of the product, such as HSM1 series, when Inm (frame level current) = 125A, the electrical clearance is 11mm, 160A is 16mm, 250A is 15mm, 400A is 18.75mm, 630 and 800A are both 300mm, all larger than 5.5mm. Regarding creepage distance, GB/T14048.1 "General Principles of Low-Voltage Switchgear and Control Equipment" stipulates: The minimum creepage distance of an electrical appliance (product) is related to the rated insulation voltage (or actual working voltage), the pollution level of the place where the electrical product is used, and the nature (insulation group) of the insulating material used in the product itself. For example: The rated insulation voltage is 660 (690) V, the pollution level is 3, the insulation material group used in the product is IIIa (175≤cti<400, CTI is the tracking index of the insulation material), and the minimum creepage distance is 10mm. The creepage distances of the molded case circuit breakers mentioned above are all * * exceeds the specified value. To sum up, if the electrical clearance and leakage distance of electrical products meet the insulation coordination requirements, there will be no dielectric breakdown of the equipment due to external overvoltage or operating overvoltage of the line equipment itself. GB7251.1-1997 "Low voltage complete sets of switchgear and control equipment Part 1: Type test and partial type test complete equipment》(equivalent to IEC439-1: 1992), the requirements for insulation coordination are exactly the same as GB/T14048.1. Some complete sets of electrical appliance manufacturers have proposed that copper bars be used for circuit breaker wiring, and the (air) distance between phases should be greater than 12mm, and some even propose that the electrical gap of the circuit breaker should be greater than 20mm. This requirement is unreasonable and exceeds the insulation coordination requirements. For large current specifications, in order to avoid electromechanical repulsion when a short-circuit current occurs, or the conductor heats up when the current is large, and in order to increase the heat dissipation space, it is also possible to appropriately widen the space distance between phases. At this time, whether it reaches 12mm or 20mm, it can be solved by the complete set of electrical appliance manufacturers, or the electrical component factory can be asked to provide elbow terminal blocks or connecting plates (pieces). Generally, when circuit breakers leave the factory, arc isolation plates are provided between the power supply terminal phases to prevent phase short circuits caused by arc eruption. Zero-flashover circuit breakers are also equipped with this kind of arc isolation plate to prevent ionized molecules from escaping when breaking short-circuit current. If there is no arc isolation plate, the bare copper bar can be wrapped with insulating tape, and the distance should be no less than 100mm. 4. Application of four-pole circuit breakers Regarding the application of four-pole circuit breakers, there is currently no way to understand the application of four-pole circuit breakers in China. * * Standards or regulations provide rigid usage requirements. Although the design specifications for regional four-pole electrical appliances (circuit breakers) have been promulgated, the debate over whether to install or not install four-pole electrical appliances is still ongoing. In recent years, the use of four-pole electrical appliances in some areas has shown a swarm trend. Various circuit breaker manufacturers have also designed and manufactured various types of four-pole circuit breakers for the market. The author agrees with an opinion that whether to use or not should be based on whether the reliability and safety of power supply can be ensured, so generally speaking: (1)TN-C system. In the TN-C system, the N line and the protective line PE are combined into one (PEN line). Considering safety, the PEN line is not allowed to be disconnected at any time, so the four-pole circuit breaker is absolutely disabled. ； (2) TT system, TN-CS system and TN-S system can use four-pole circuit breakers to ensure the safety of maintenance personnel during maintenance. However, in TN-CS and TN-S systems, the N pole of the circuit breaker can only be connected to the N line, not the PEN or PE line. ； (3) In places where dual power supply switching is installed, since all neutral lines (N lines) in the system are connected, in order to ensure the safety of maintenance of the switched power switch (circuit breaker), a four-pole circuit breaker must be used ； (4) For the single-phase main switch entering the residence, a two-pole circuit breaker with N pole should be used (used as an isolator during maintenance) (5) Residual current protector (leakage circuit breaker) for 380/220V system, neutral line It must pass through the zero-sequence current transformer (core) of the protector to prevent the 220V load from leaking current and malfunctioning due to the passage of no neutral wire. In this case, a four-pole or two-pole residual current protector with a neutral wire should be selected.