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Determination of rated current
Calculate the total load current: First, the rated current of all devices in the PLC control cabinet (including the PLC module itself, power module, input/output devices such as sensors and actuators, etc.) must be counted in detail. Add these current values to get the total load current. For some devices that may have current fluctuations, such as inductive loads such as motors, the starting current must also be considered. Generally, the starting current of a motor is 4-7 times the rated current, so make sure that the rated current of the circuit breaker can withstand this instantaneous high current shock.
Reserve a certain margin: After calculating the total load current, you cannot directly select a circuit breaker with the same rated current. In order to ensure that the circuit breaker does not malfunction during normal operation and some small current fluctuations, you should select a circuit breaker with a rated current greater than the total load current. It is usually recommended to reserve a margin of 20%-30%. For example, if the calculated total load current is 10A, you can choose a circuit breaker with a rated current of about 12-13A.
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Matching of rated voltage
Consider power supply voltage: The power supply voltage of the PLC control cabinet is a key factor in selecting the rated voltage of the circuit breaker. If it is a single-phase 220V power supply system, you should choose a circuit breaker with a rated voltage of 220V or higher (such as 230V, 250V, etc.); if it is a three-phase 380V power supply system, you need to choose a circuit breaker with a rated voltage of 380V or higher (such as 400V, 415V, etc.). Make sure that the rated voltage of the circuit breaker matches the power supply voltage, so that the circuit breaker can work reliably under normal voltage and effectively protect the circuit in the event of overvoltage.
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Consideration of short-circuit breaking capacity
Assess short-circuit risk: It is necessary to conduct a short-circuit risk assessment on the electrical system environment where the PLC control cabinet is located. If the electrical system where the control cabinet is located may have a large short-circuit current, such as when it is connected to the power line of a large motor or other high-power equipment, it is necessary to select a circuit breaker with a higher short-circuit breaking capacity. Short-circuit breaking capacity refers to the maximum short-circuit current value that a circuit breaker can cut off under specified conditions.
Matching system short-circuit current: To understand the maximum short-circuit current of the electrical system (which can be obtained through short-circuit calculation or consulting the power supply department), then select a circuit breaker with a short-circuit breaking capacity greater than or equal to the short-circuit current. For example, if the maximum short-circuit current of the system is expected to be 10kA, then a circuit breaker with a short-circuit breaking capacity of not less than 10kA should be selected to ensure that the circuit can be quickly cut off in the event of a short circuit to protect the safety of equipment and personnel.
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Selection of circuit breaker type
Select according to load characteristics: For PLC control cabinets that are mainly resistive loads (such as heating equipment, etc.), ordinary small circuit breakers can be selected; if there are a large number of inductive loads (such as motors, contactors, etc.), it is recommended to select a circuit breaker with magnetic tripping characteristics, which can better cope with current changes during the startup and operation of inductive loads. In addition, for some occasions with high requirements for power supply continuity, such as uninterruptible power supply (UPS) systems, you can consider choosing a circuit breaker with selective protection function, which can selectively cut off the faulty branch when a fault occurs, while trying not to affect the power supply of other normal branches.
Consider the protection function requirements: In addition to the basic overload and short-circuit protection functions, some circuit breakers also have leakage protection, undervoltage protection and other functions. If the environment where the PLC control cabinet is located is relatively humid or there is a possibility of people contacting the equipment, choosing a circuit breaker with leakage protection function can effectively prevent electric shock accidents; in areas with large voltage fluctuations, a circuit breaker with undervoltage protection function can automatically cut off the circuit when the voltage is too low to protect the equipment from low voltage damage.
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Brand and quality considerations
Choose a well-known brand: Circuit breakers of well-known brands usually undergo strict quality testing and certification, and their product quality and performance are more guaranteed. For example, international brands such as Schneider, Siemens, ABB, and domestic well-known brands such as Chint and Delixi have rich experience and a good reputation in the field of circuit breaker manufacturing.
Check the quality certification: Make sure that the selected circuit breaker meets the relevant international (such as IEC standards) and domestic (such as GB standards) quality certification requirements. These certifications are the basic guarantee for the quality and safety of circuit breakers. Products with certification marks are more trustworthy in terms of performance, reliability, etc.
