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I. Estimation of the motor’s rated current (1) Quick calculation formula: The rated current (in amps) of a motor is determined by doubling the motor’s power; in other words, “1 kilowatt corresponds to 2 amps”. It generally refers to the commonly used three-phase asynchronous motors with a voltage of 380V and a power factor of around 0.8; the rated current of such motors is obtained by doubling the kilowatt value. (2) Empirical formula: Rated current of the motor (A) = Capacity of the motor in kW × 2. The results obtained using both the quick calculation rule and the empirical formula are identical; the calculated rated current is very close to the actual current value indicated on the motor’s nameplate, which meets practical requirements. For example, for a Y132S1-2, 10kW motor, the rated current calculated using either the quick calculation rule or the empirical formula is 10×2=20A. II. Selection of circuit breakers for electric motors: Low-voltage circuit breakers are generally divided into two main categories: plastic enclosure type (also known as assembled type) and frame type (also known as universal type). Molded case circuit breakers are commonly used for motors of 380V 245kW and below. Circuit breakers can be classified by purpose into those used to protect distribution circuits, those used to protect motors, those used to protect lighting circuits, and those for leakage protection. 2.1 Principles for selecting circuit breakers for motor protection (1) The long-time delay current setting value is equal to the motor’s rated current. (2) Instantaneous setting current: For circuit breakers used to protect squirrel-cage motors, the instantaneous setting current is equal to (8–15) times the motor’s rated current, depending on the model, capacity, and starting conditions of the squirrel-cage motor being protected. For circuit breakers used to protect wound-rotor motors, the instantaneous setting current is equal to (3–6) times the motor’s rated current, depending on the model, capacity, and starting conditions of the wound-rotor motor being protected. (3) The return time of the 6-time-long delay current setting value is greater than or equal to the motor’s start-up time. Depending on the severity of the starting load, one of the return times of 1s, 3s, 5s, 8s, or 15s can be selected. 2.2 Quick calculation formula for the setting current of circuit breaker trip devices: “For motor inrush currents, use 20 times the kilowatt value”; “For thermal trip devices, use the rated value.” These formulas apply to circuit breakers used to control and protect 380V three-phase squirrel-cage motors – the setting current for their electromagnetic trip devices can be determined by taking 20 times the kilowatt value. For thermal trip devices, they are selected based on the motor’s rated current. III. Selection of fuses for electric motors: When choosing the type and capacity of fuses, it is necessary to consider the protection characteristics of the load, the magnitude of the short-circuit current, and the operating conditions of the application. Most small and medium-sized motors use full-voltage or reduced-voltage starting under light load conditions, with the starting current typically being 5 to 7 times the rated current ； The power supply capacity is large; the main low-voltage distribution transformers have a capacity of 1000–400 kVA (including the capacity for parallel operation). The system impedance is low, which results in high short-circuit currents in the event of a short-circuit fault ； In workplaces such as kilns and grinding areas, poor ventilation leads to high temperatures in the working environment. Therefore, the breaking capacity of the fuse and the rated current of the fuse element should be selected to be slightly higher than those required for general industrial use. 3.1 Empirical formula for the rated current of the fuse: Rated current of fuse (A) = Rated current of motor (A) × 3. 3.2 Quick calculation rule for the rated current of the fuse: “For fuse protection, multiply kilowatts by 6.” This quick calculation rule applies to 380V cage-type motors that are started at full voltage or with reduced voltage under light load conditions, and have a low operating frequency; it is suitable for cage-type motors of 90kW and below. If the motor actually in use starts frequently or takes a long time to start, the values calculated using the above empirical formulas or quick calculation rules can be increased slightly, but not by too much. In short, it is necessary to ensure that the fuse does not blow when the motor starts ； In the event of a short-circuit fault, the fuse must blow reliably to cut off the power supply, thereby achieving short-circuit protection.