Abstract: 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 benefited a lot. However, I also feel that due to insufficient communication, exchange and publicity between the circuit breaker designers and manufacturers and its users, there are still some biases in the selection of low-voltage circuit breakers by users of electrical products. Based on this, I would like to discuss the selection and application of circuit breakers again, in order to offer some insights and clarify the truth. Keywords: circuit breaker selection 1. Selecting the breaking capacity of the circuit breaker based on the calculation of the expected short-circuit current of the line The calculation of the expected short-circuit current of the line is an extremely tedious task. Therefore, there are some simple calculation methods with small errors that are acceptable in engineering: (1) For transformers with a voltage level of 10/0.4KV, the short-circuit capacity of the high-voltage side can be considered as infinite (the short-circuit capacity of the 10KV side is generally 200~400MVA or even larger, so considering it as infinite, the error is less than 10%). (2) Article 2.1.2 of GB50054-95 "Code for Design of Low-Voltage Power Distribution" stipulates: "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 taken into account." If the short-circuit current is 30KA, taking 1% of it should be 300A. The total power of the motors is about 150KW, and when they are started and used simultaneously, the feedback current to be taken into account should be 6.5∑In. (3) The impedance voltage UK of the transformer represents the transformer secondary side short-circuited (circuited). When the secondary side reaches its rated current, the primary side voltage is a percentage of its rated voltage. Therefore, when the primary side voltage is the rated voltage, the secondary side current is its expected short-circuit current. (4) The rated secondary current of the transformer is Ite = Ste/1.732Ue, where Ste is the transformer capacity (KVA) and Ue is the rated secondary voltage (no-load voltage). At 10/0.4KV, Ue = 0.4KV. Therefore, the rated secondary current of the transformer should be calculated as transformer capacity x 1.44 to 1.50. (5) According to the definition of Uk in (3), the short-circuit current (three-phase short circuit) of the secondary side 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 all current values ​​when the transformer output terminal 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, the SL7 series transformer (with three-core aluminum wire cable) has a capacity of 200KVA. When the transformer output terminal is short-circuited, the three-phase short-circuit current I(3) is 7210A. When the short-circuit point is 100m away from the transformer, the short-circuit current I(3) drops to 4740A; when the transformer capacity is 100KVA, the short-circuit current at its output terminal is 3616A. When the short circuit occurs at a distance of 100m from the transformer, the short-circuit current is 2440A. When the short circuit occurs at a distance of 100m from the transformer, the short-circuit current is 65.74% and 67.47% of that at 0m, respectively. Therefore, when designing, users should calculate the rated current of the installation location (line) and the maximum possible short-circuit current at that location. And select the circuit breaker according to the following principles: the rated current In of the circuit breaker ≥ the rated current IL of the line; the rated short-circuit breaking capacity of the circuit breaker ≥ the expected short-circuit current of the line. Therefore, when selecting a circuit breaker, it is not necessary to make the margin too large, so as not to cause waste. 2. Ultimate Short-Circuit Breaking Capacity and Operating Short-Circuit Breaking Capacity of Circuit Breakers The International Electrotechnical Commission's IEC 947-2 and my country's equivalent standard GB 4048.2, "Low-Voltage Switchgear and Controlgear - Low-Voltage Circuit Breakers," define the ultimate short-circuit breaking capacity and operating short-circuit breaking capacity of circuit breakers as follows: Rated ultimate short-circuit breaking capacity (Icu): The breaking capacity under the conditions specified in the prescribed test procedure, excluding the circuit breaker's ability to continue carrying its rated current. Rated operating short-circuit breaking capacity (Ics): The breaking capacity under the conditions specified in the prescribed test procedure, including the circuit breaker's ability to continue carrying its rated current. The test procedure for ultimate short-circuit breaking capacity Icu is otco. The specific test is as follows: Adjust the line current to the expected short-circuit current value (e.g., 380V, 50KA), with the test button off and the circuit breaker under test in the closed position. Press the test button, and the circuit breaker will pass a 50KA short-circuit current. The circuit breaker should immediately open (OPEN, abbreviated as O) and extinguish the arc. The circuit breaker should be intact and able to be reclosed. t is the interval time (rest time), generally 3 minutes. During this time, the line is in hot standby mode. The circuit breaker will then perform another closing (CLOSE, abbreviated as C) and immediately following it to open (O) (the closing test assesses the circuit breaker's electrical and thermal stability under peak current and the wear of the moving and stationary contacts due to bounce). This procedure is called CO. If the circuit breaker can completely disconnect and extinguish the arc without exceeding the specified damage, its ultimate breaking capacity test is considered successful. The test procedure for the circuit breaker's operational short-circuit breaking capacity (Icu) is otco t co, which is one more CO step than the Icu test procedure. If, after testing, the circuit breaker can completely interrupt and extinguish the arc without exceeding the specified damage, it is deemed to have passed the rated short-circuit breaking capacity test. After the Icu and Ics short-circuit breaking tests, withstand voltage and protection characteristic recalibration tests are also required. Since it must carry the rated current after the operational short-circuit breaking test, a temperature rise retest is added after the Ics short-circuit test. The conditions for Icu and Ics short-circuit or actual testing differ; the latter is more stringent and difficult than the former. Therefore, IEC 947-2 and GB 14048.2 define Icu as having four or three values: 25%, 50%, 75%, and 100% Icu (for Class A circuit breakers, i.e., molded case type) or 50%, 75%, and 100% Icu (for Class B circuit breakers, i.e., universal type or frame type). Any Ics value determined by the circuit breaker manufacturer that meets the Icu percentage specified in the above standards is considered a valid and qualified product. Universal (frame-type) circuit breakers, in most cases (but not all specifications), have three-stage protection functions: overload long-time delay, short-circuit short-time delay, and short-circuit instantaneous trip. They can achieve selective protection, therefore, they are used as the main (protective) switch in most main lines (including transformer outgoing lines). Molded case circuit breakers, on the other hand, generally do not have short-circuit short-time delay functionality (only overload long-time delay and short-circuit instantaneous trip protection), and cannot provide selective protection; they can only be used in branch lines. Due to the different application scenarios, IEC 92 "Marine Electrical Systems" recommends that universal circuit breakers with three-stage protection should prioritize their operational short-circuit breaking capacity, while molded case circuit breakers used extensively in branch lines should ensure they have sufficient ultimate short-circuit capacity. Our understanding is that replacing circuit breakers after the main line has cut off the fault current requires caution, as a power outage on the main line will affect a large number of users. Therefore, when a short-circuit fault occurs, two circuit breakers (COs) are required, and the circuit breaker must continue to carry the rated current for a period of time. However, in the branch circuit, after the ultimate short-circuit current is interrupted and then reconnected, the circuit breaker has completed its mission and no longer carries the rated current, so it can be replaced with a new one (the impact of the power outage is relatively small). However, both universal and molded case circuit breakers must possess the two important technical indicators of Icu and Ics. Only the Ics value differs slightly between the two types of circuit breakers. The minimum allowable Ics for molded case circuit breakers can be 25% of Icu, while the minimum allowable Ics for universal circuit breakers is 50%. Circuit breakers with Ics=Icu are rare, and even universal circuit breakers rarely have Ics=100%. [There is a type of molded case circuit breaker abroad that uses rotating double breaking (point) technology. It has excellent current limiting performance and a large breaking capacity margin, and can achieve Ics=Icu, but it is very expensive.] In my country, the Ics (Inductance Constituent) of the DW45 intelligent universal circuit breaker is 62.5%–65% Icu. Internationally, ABB's F series and Schneider's M series are only around 70%. For molded case circuit breakers, the Ics of various new domestic models are generally between 50% and 75% Icu. Some circuit breaker designers, when selecting circuit breakers based on their calculated expected short-circuit current, use the rated operating short-circuit breaking capacity of the circuit breaker as a criterion, thus deeming a certain circuit breaker (whose ultimate short-circuit capacity is greater than the expected short-circuit current, while its operating short-circuit breaking capacity is lower than the calculated current) unqualified. This is a misunderstanding. 3. Determining the electrical clearance and creepage distance of circuit breakers: The electrical clearance of electrical products must be based on the insulation coordination of the low-voltage system. Insulation coordination is based on the premise that transient overvoltages are limited to the specified impulse withstand voltage, and the transient overvoltages generated by electrical appliances or equipment in the system must also be lower than the impulse voltage specified by the power supply 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 (preferred value) Uimp of the electrical appliance is closely related to the maximum value of the relative voltage to ground determined by the rated voltage of the power supply system and the installation category (overvoltage category) of the electrical appliance. The larger the relative voltage to ground value and the higher the installation category [divided into I (signal level), II (load level), III (distribution level), IV (power supply level)], the larger the rated impulse voltage. For example, when the relative voltage to ground 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 clearance of DZ20, CM1, and our factory's HSM1 series molded case circuit breakers is 5.5mm (installation category III). This is only for power supply level installations. For example, in the DZ20 series, specifications above 800V, with Uimp of 8.0KV, the electrical clearance is increased to ≥8mm. The actual electrical clearance of the products, such as the HSM1 series, is 11mm for Inm (frame current rating) = 125A, 16mm for 160A, 15mm for 250A, 18.75mm for 400A, and 300mm for both 630A and 800A, all greater than 5.5mm. Regarding creepage distance, GB/T14048.1 "General Rules for Low-Voltage Switchgear and Controlgear" stipulates that the minimum creepage distance of electrical appliances (products) is related to the rated insulation voltage (or actual operating voltage), the pollution level of the location where the electrical product is used, and the nature of the insulation material used in the product (insulation group). For example: the rated insulation voltage is 660 (690) V, the pollution degree 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 10 mm. The creepage distances of the molded case circuit breakers mentioned above all greatly exceed the specified values. In summary, if the electrical clearance and leakage distance of electrical products meet the insulation coordination requirements, the dielectric breakdown of the equipment will not be caused by external overvoltage or the operating overvoltage of the line equipment itself. GB7251.1-1997 "Low-voltage switchgear and controlgear assemblies - Part 1: Type testing and partial type testing assemblies" (equivalent to IEC439-1:1992) has the same insulation coordination requirements as GB/T14048.1. Some complete electrical equipment manufacturers have proposed that the phase-to-phase (air) distance of the copper busbars used for circuit breaker wiring should be greater than 12 mm, and some have even proposed that the electrical clearance of the circuit breaker should be greater than 20 mm. This requirement is unreasonable and exceeds the requirements for insulation coordination. For high-current specifications, to avoid electrodynamic repulsion during short-circuit currents or conductor heating during high currents, and to increase heat dissipation space, it is acceptable to appropriately widen the phase-to-phase spacing. Whether it reaches 12mm or 20mm, this can be achieved by the complete electrical equipment manufacturer, or by requesting the electrical component manufacturer to provide bent terminals or connecting plates (plates). Generally, circuit breakers are shipped with arc-shielding plates between the power supply phases to prevent phase-to-phase short circuits caused by arcing. Zero-arc circuit breakers also install such arc-shielding plates to prevent the escape of ionized molecules when interrupting short-circuit currents. If there are no arc-shielding plates, the bare copper busbars can be wrapped with insulating tape, with a distance of not less than 100mm. 4. Application of Four-Pole Circuit Breakers Regarding the application of four-pole circuit breakers, there are currently no national standards or regulations in China that make strict requirements for their use. Although regional design specifications for four-pole electrical appliances (circuit breakers) have been issued, the debate over whether or not to install four-pole electrical appliances is still ongoing. In recent years, there has been a rush to use them in some areas, and various circuit breaker manufacturers have designed and manufactured various models of four-pole circuit breakers for the market. I agree with the opinion that whether or not to use them should be based on whether the reliability and safety of power supply can be ensured. Therefore, in general, it is: (1) TN-C system. In the TN-C system, the N line and the protective earth (PE) line are combined into one (PEN line). For safety reasons, the PEN line must not be disconnected at any time. Therefore, four-pole circuit breakers are absolutely prohibited. (2) Four-pole circuit breakers can be used in TT, TN-CS and TN-S systems 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 and cannot be connected to 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, four-pole circuit breakers must be used to ensure the maintenance safety of the switched power switch (circuit breaker). (4) For single-phase main switches entering residential buildings, two-pole circuit breakers with N poles should be selected (used as isolators during maintenance). (5) For residual current circuit breakers (RCCBs) used in 380/220V systems, the neutral wire must pass through the zero-sequence current transformer (iron core) of the RCCB to prevent the 220V load from malfunctioning due to leakage current if there is no neutral wire passing through. In this case, a four-pole or two-pole residual current circuit breaker with a neutral wire should be selected.