Low-voltage circuit breakers have a wide range of applications. They are used not only for the infrequent opening and closing of main lines, branch lines, and circuit ends to protect lines (cables, wires) and electrical equipment from overload, short circuit, and undervoltage faults, but also for the individual use and protection of various loads, such as lighting circuits, electric heating circuits, motor circuits, thyristor rectifier circuits, and capacitor circuits. The rated current and protection characteristics of the circuit breaker vary depending on the nature of the load. 1. Light bulb (incandescent lamp) and electric heater circuits: These circuits are basically resistive loads, and miniature circuit breakers (MCBs) are selected. Theoretically, the rated current of the circuit breaker should be In ≥ IL (In is the rated current of the circuit breaker, and IL is the rated current of the line or electrical equipment). If In is ≤ IL, the MCB may malfunction. Therefore, many countries internationally select In as (1.1~1.15)IL for circuit breakers used in incandescent lamp and electric heating circuits. Incandescent lamps and electric heating circuits may experience flashover (the process of gradually forming hot resistance from cold resistance) at the moment of energization, with a maximum flashover current of up to 10 IL. Therefore, when selecting miniature circuit breakers, type C (instantaneous current setting value 5-10 times In) should be selected. 2. For circuits of high-pressure mercury lamps, sodium lamps, metal halide lamps, etc., the In of the miniature circuit breaker should be ≥ (1.2-1.4) IL. Mercury lamps, etc., are characterized by a current distortion rate (coefficient) of up to 15%. During startup, due to the inductance of the ballast, an inrush current will be generated, but the startup time is also relatively long. Therefore, the instantaneous current of the circuit breaker used for the protection of mercury lamps, etc., should be type C. 3. For motor circuits, circuit breakers that directly protect motor circuits can be motor protection type circuit breakers. Their overload protection is 1.2 In, 1.5 In, 7.2 In (returnable characteristic), and the short-circuit instantaneous current is 12 In. The rated current of this type of circuit breaker is In = IM (IM is the rated current of the motor). If a motor protection circuit breaker cannot be found, a general distribution circuit breaker can be used. However, this type of circuit breaker can only provide short-circuit protection for the motor (overload protection of the circuit uses a thermal relay). Because this type of circuit breaker lacks a return characteristic to avoid the motor starting current, its rated current is set relatively high to prevent the circuit breaker from tripping during motor startup (including transient inrush currents during startup and operation when the motor uses a Y-Δ starter). Japanese standards (including product brochures from some companies) stipulate that when the motor's rated current Im ≤ 50A, the circuit breaker's rated current In ≤ 3IM; when Im > 50A, In ≤ 2.5IM; and the instantaneous current is still 10In (if the rated current is not amplified, the instantaneous current must be greater than 14IM). The circuit breaker's rated short-circuit breaking capacity must be greater than or equal to the motor's short-circuit current. 4. Capacitor Circuits Both the International Electrotechnical Commission (IEC) and the German DIN standard stipulate that capacitor units must operate under current conditions for extended periods, meaning their effective value will not exceed 1.3 times the current flowing at sinusoidal voltage and rated frequency. For most applications, capacitor units do not require overload protection, allowing the upstream power grid to further eliminate harmonics through a filter circuit. For short-circuit protection, fuses are the most common method (in this case, the rated current of the fuse should be 1.6 to 1.7 times the rated current of the capacitor). In many applications where capacitors are used, molded case circuit breakers (MCCBs) are selected for short-circuit protection. The protective switches (fuses, circuit breakers) for capacitors must meet the following three requirements: (1) They should be able to withstand the inrush current generated by the capacitor; (2) The protective switches and electrical appliances should be non-re-ignitable; (3) The short-circuit breaking capacity of the fuse or circuit breaker should be greater than the short-circuit fault current of the capacitor. When a capacitor is switched on and put into operation, the terminal voltage cannot change abruptly (from zero to rated voltage), which is similar to a short circuit. Therefore, there is a high frequency and a large magnitude of surge current. The peak surge current of a single capacitor is calculated according to formula (1): Im = IC (1) Where: Im—peak surge current A; S—short-circuit capacity at the capacitor installation point, kVA; Q—reactive power of the capacitor, kVAR; IC—capacitor current, A. In order to suppress overvoltage and surge current when the capacitor is switched off, the capacitors of centralized compensation are generally equipped with switching resistors. It is required that Im be reduced after the switching resistor is installed. After adding a switching resistor, the inrush current (peak value) can be reduced to 16 Ic, with an effective value of 11.3 Ic. Since the rated current of the selected circuit breaker is In = (1.3~2.0) Ic, if its instantaneous current setting is 10In, it can completely avoid the peak current Im of the capacitor. When Im, Q, and IC are known, the value of S can be calculated, and the short-circuit current of the capacitor can be calculated from the value of S. When using a plastic-cased circuit breaker for capacitor circuit protection, the rated current of the circuit breaker is commonly In = 1.5Ic; the rated operating voltage of the circuit breaker should be ≥ the operating voltage of the capacitor. 5 Welding machine (spot welding machine, arc welding machine) circuit: The operating current of welding machines such as AC arc welding machines, steel plate spot welding machines, and energy storage spot welding machines is intermittent and pulsed. Therefore, calculations must be performed to determine the appropriate circuit breaker. Let the maximum operating current of the spot welding machine (arc welding machine) be Ia. Since it is intermittent, Ia = 0 at a certain moment. The thermal equivalent current Ie is: I2at1 + O2t2 = I2e(t1 + t2) (2) Where: t1 - welding time, t2 - rest time (interval time) ∴ Ie = = = Ia = Ia T = t1 + t2 = spot welding cycle, α = is called utilization rate. (Example: Let Ia = 100A, t1 = 0.3s, T = 3s, α = = 0.1, then Ie = Ia = 100 = 32A) The heating caused by the operating current Ia follows a tooth-shaped wave law. Practice tells us that the smaller the utilization rate α, the greater the difference between the thermal average temperature Te and the maximum temperature TM. However, for most of the time, the thermally equivalent continuous current exceeds the current at the thermal average temperature. To prevent the circuit breaker's overload long-delay trip from malfunctioning, the rated current In of the circuit breaker is generally selected as In = (1.2~1.3)Ie. From the above example, we can get In = 32 (1.2~1.3) = 38.4~41.6A. The rated current of the circuit breaker can be taken as In = 40A. For most welding machines that work intermittently and regularly, the utilization rate α is taken as 0.5. For multiple welding machines of the same type, their maximum operating current is Ia, and α is roughly the same. Assuming there are N machines in total, then: Ie = Ia = NIa. When connected in balance to three phases, the rated current of the circuit breaker of the main line is In = NIa. Welding machines, especially arc welding machines, have their arcs short-circuited and not yet ignited at the moment of startup. Therefore, the resistance is very small and the current is very large. The primary side (primary side) of the welding transformer may have 8 to 14 times the rated current. As the electric arc ignites and stabilizes, the resistance and current gradually stabilize. This phenomenon is called transient flashover. Since transient flashover occurs frequently when using welding machines, when using miniature circuit breakers (MCBs) with an instantaneous setting current of (5~10)In or molded case circuit breakers with an instantaneous setting value of 10In for protection, malfunctions often occur under this transient flashover. For the instantaneous current setting value of the circuit breaker, it is recommended to calculate it according to formula (3) abroad. Is = ×B (3) Where: P - welding machine capacity, kVA; E - line voltage, V; B - ratio of transient flashover to rated current (B ranges from 8 to 14); Is - instantaneous tripping current of the circuit breaker, A. Formula (3) can be used as a reference for users when making selection. It is rare for multiple welding machines to generate transient flashover simultaneously, but two welding machines may start at very short intervals. Therefore, Is calculated according to formula (3) should be multiplied by a safety factor of 1.2. It should be noted that the circuit breaker is installed on the primary side (primary side) of the welding transformer. 6 Thyristor Rectifier Circuit The circuit breaker is installed on the AC side of the thyristor rectifier circuit. The rated current of the circuit breaker is ≥1.4IL (IL is the rated current of the rectifier equipment). For three-phase full-wave rectification, the distortion rate of the primary side of the transformer is 30%. Therefore, it is not advisable to select a fully electromagnetic (oil cup type) circuit breaker. Only a thermal-electromagnetic molded case circuit breaker with fast breaking speed and short full breaking time can be selected. 7 Converter Circuit Voltage source inverter and current source inverter are used for constant speed motors designed according to sinusoidal AC voltage. According to the load requirements, the speed of the motor is adjusted to achieve energy saving. For the converter circuit, the rated current In of the circuit breaker is ≥(1.4~2)IL. The input current distortion rate of the converter circuit reaches 90%, and the proportion of high-order harmonics is large. Therefore, a thermal-electromagnetic type molded case circuit breaker should be used. 8. Transformer primary circuit: This refers to low-voltage transformers, i.e., the high-voltage (primary side) voltage is not higher than 400V. The rated current of the circuit breaker In ≥ (1.2~3)Ir (Ir is the rated current of the transformer primary side). The characteristic of this type of circuit is that the transformer excitation impact current is 10~20 times the rated current. A special molded case circuit breaker specifically designed for transformer protection requirements is selected.