In recent years, I have consulted and discussed with many circuit breaker users and read articles on circuit breaker selection in professional journals, which has been very beneficial. However, I also feel that due to insufficient communication, exchange, and publicity between circuit breaker designers and manufacturers and their users, users of electrical products still have some misconceptions when selecting low-voltage circuit breakers. Therefore, this article intends to reiterate the selection and application of circuit breakers, hoping to stimulate further discussion and clarify misconceptions. I. Selecting the Breaking Capacity of Circuit Breakers Based on the Calculation of the Expected Short-Circuit Current Accurate calculation of the expected short-circuit current is an extremely tedious task. Therefore, there are some simplified calculation methods with minimal error that are acceptable in engineering: 1. For transformers with a voltage level of 10/0.4KV, 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 larger, so considering it as infinite results in an error of 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 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 would be 300A. The total power of the motors would be approximately 150kW, and when they are started and used simultaneously, the feedback current to be taken into account would be 6.5∑In. 3. The impedance voltage UK of a 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 = Se/1.732U, where Se is the capacity of the transformer (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 1.44~.50Se. 5. According to the definition of Uk in (3), the short-circuit current (three-phase short circuit) of the secondary side is I(3) = Ie/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). 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 short-circuit current will decrease considering the line impedance. 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 at the installation location (line) and the maximum possible short-circuit current at that location. And select the circuit breaker according to the following principles: Therefore, when selecting a circuit breaker, it is not necessary to leave too much margin to avoid waste. II. Ultimate Short-Circuit Breaking Capacity and Operating Short-Circuit Breaking Capacity of Circuit Breakers IEC 947-2 and GB 4048.2 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 electrical and thermal stability of the circuit breaker 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 OTCOTCO, which has 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 circuit breakers) or 50%, 75%, and 100% Icu (for Class B circuit breakers, i.e., universal or frame circuit breakers). 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 circuit breakers, in most cases, possess three-stage protection functions: long-time overload protection, short-time short-circuit protection, and instantaneous short-circuit protection, enabling selective protection. Therefore, they are commonly used as main switches on main lines (including transformer outgoing lines). Replacing a circuit breaker after a fault current is interrupted on the main line requires careful consideration, as a power outage on the main line would affect a large number of users. Thus, in the event of a short-circuit fault, two current cutoffs (COs) are required, and the circuit breaker must continue to carry the rated current for a period of time. Therefore, universal circuit breakers prioritize their Icu value. Molded case circuit breakers used on branch lines, after breaking and re-closing the circuit with the ultimate short-circuit current, have completed their mission and no longer carry the rated current, allowing for replacement (with less impact from power outages). Generally, only their Ics value is considered. However, both universal and molded case circuit breakers must possess the two crucial technical indicators of Icu and Ics. The only difference in Ics values ​​between the two types of circuit breakers is that 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% of 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. III. Electrical Clearances and Creepage Distances of Circuit Breakers Determining the electrical clearances of electrical products must be based on the insulation coordination of the low-voltage system. Insulation coordination is established 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 ≥ the rated voltage of the power supply system. 2. The rated impulse withstand voltage of the electrical appliance should ≥ the rated impulse withstand voltage of the power supply system. 3. The transient overvoltage generated by electrical appliances 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 electrical appliances is closely related to the relative ground voltage of the power supply system and the installation category of the electrical appliances: the higher the relative ground voltage and the higher the installation category [divided into I (signal level), II (load level), III (distribution level), IV (power supply level)], the higher the required rated impulse withstand voltage. For example, when the relative ground voltage is 220V and the installation category is III, Uimp is 4.0KV; if the installation category is IV, Uimp is 6.0KV. Generally, the Uimp of molded case circuit breakers is 6.0KV, pollution level 3 or 4, and its minimum electrical clearance is 5.5mm. However, the actual electrical clearance of the products is 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 place where the electrical product is used, and the nature (insulation group) of the insulation material used in the product itself. For example, if the rated insulation voltage is 660 (690) V, the pollution level is 3, and the insulation material used in the product is group Ⅲa (175≤cti＜400, CTI is the tracking index of the insulation material), the minimum creepage distance is 10 mm. Generally, the creepage distance of molded case circuit breakers greatly exceeds the specified value. In summary, if the electrical clearances and creepage distances of electrical products meet the insulation coordination requirements, dielectric breakdown of the equipment will not occur due to external overvoltages or operational overvoltages 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 propose that the phase-to-phase (air) distance of the copper busbars used for circuit breaker wiring should be greater than 12mm, and some even propose that the electrical clearance of the circuit breaker should be greater than 20mm. This requirement is unreasonable; it exceeds the insulation coordination requirements. The requirements are met. 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 minimum distance of 100mm. IV. Application of Four-Pole Circuit Breakers Regarding the application of four-pole circuit breakers, current national standards or regulations do not make strict provisions on their use. Although regional design specifications for four-pole circuit breakers have been issued, the debate over whether or not to install them continues. In some areas, there has been a rush to adopt them in recent years, with various circuit breaker manufacturers designing and manufacturing various models of four-pole circuit breakers for the market. The author agrees with the opinion that the use or non-use should be based on whether the reliability and safety of the power supply can be ensured. Therefore, generally speaking: 1. TN-C system. In a 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. TT, TN-CS, and TN-S systems can use four-pole circuit breakers to ensure the safety of maintenance personnel during repairs. However, in TN-CS and TN-S systems, the N pole of the circuit breaker can only be connected to the N line, and not to the PEN or PE line. 3. In locations where dual power supply switching is installed, since all neutral lines (N lines) in the system are connected, a four-pole circuit breaker must be used to ensure the safety of maintenance of the power switch (circuit breaker) being switched; 4. For 380V systems, a four-pole residual current device (leakage circuit breaker) should be selected.