Selection Criteria for Fuses and Circuit Breakers

Fuses and circuit breakers are common protective devices in electrical systems, playing crucial roles in preventing damage caused by overload and short circuits in circuits. During normal operation of electrical equipment, fuses and circuit breakers effectively isolate faulty parts, ensuring the stability and safety of the entire system. So, what factors should be considered when choosing circuit breakers? This article will discuss in detail the key points for selecting fuses and circuit breakers, helping electrical professionals make wise choices in practical applications.

01 Criteria for Selecting Fuses

(1) Functional Role of Fuses

Fuses are primarily used to protect electrical systems from short circuits. They are suitable for overload protection when the system's transient load is small or zero, or when the electrical equipment has a small capacity or low protection requirements.

When a protected circuit or device experiences a short circuit fault, the fuse immediately melts, cutting off the short-circuit current and protecting the circuit and equipment. In the event of overload, the fuse melts with a delay, cutting off the overload current. However, due to the inconsistent delay characteristics of various fuse models and the varying overload capacities of equipment and circuits, it's challenging to match the delay characteristics and overload capacities. Therefore, fuses are generally only used for short-circuit protection, not overload protection.

(2) Considerations for Fuse Selection (Including Fuse Element Selection)

1.The voltage rating of the fuse must be greater than or equal to the voltage rating of the protected circuit or equipment.

2.The rated current of the fuse should be greater than or equal to the rated current of the installed fuse element.

3.The type of fuse should meet the installation conditions (indoor or outdoor) and the technical requirements of the protected equipment and circuits.

4.The breaking current of the fuse must meet the needs of the short-circuit current calculation at the installation location.

5.There should be selective coordination between fuses at different levels before and after the circuit.

6.The selection of fuse current varies depending on the protected device. The ratio of the rated current of the fuse to the rated current of the electrical equipment differs for different types of fuse wires.

7.There are many types of fuses with significant differences in protective characteristics. When selecting, consider the above conditions. High-voltage fuses, transformer low-voltage outputs, and total fuses for subcircuits must be selected according to the above conditions. For various low-voltage electrical devices and protective fuses for electrical appliances, only rated current and peak current are generally considered

02 Criteria for Selecting Circuit Breakers

(1) Role of Circuit Breakers

Circuit breakers, commonly known as MCCBs (Molded Case Circuit Breakers), are mainly used to connect and disconnect electrical circuits and equipment. They also provide short-circuit protection and overload protection, and can be used for undervoltage and overvoltage protection. Low-voltage circuit breakers are structurally complex, with excellent performance as switching components. They can instantly cut off short-circuit currents, delay cutting off overload currents, operate remotely, and replace combination switch circuits composed of AC contactors, thermal relays, and fuses. They are safe to operate and convenient to use.

(2) Criteria for Selecting Low-Voltage Circuit Breakers

1.The rated voltage of the circuit breaker should be greater than or equal to the rated voltage of the installation point.

2.The rated current of the low-voltage circuit breaker should be greater than or equal to the calculated current of the installation point.

3.The rated current of the undervoltage trip device of the low-voltage circuit breaker should be greater than or equal to the calculated current of the installation point.

4.The ultimate breaking capacity of the low-voltage circuit breaker should be greater than or equal to the maximum short-circuit current at the installation point.

5.The ratio of the single-phase-to-ground short-circuit current at the end of the line to the instantaneous or short-delay trip setting current of the circuit breaker should be greater than or equal to 1.25.

6.The rated voltage of the undervoltage trip device of the low-voltage circuit breaker should be equal to the rated voltage of the installation point.

(3) The selection method for low-voltage circuit breakers used for distribution devices, electric motors, lighting, and their matching with adjacent electrical appliances is different, and attention should be paid when selecting them.

(4) Used in conjunction with circuit breakers: For low-voltage circuit breakers with smaller capacity or when the breaking capacity of low-voltage circuit breakers is less than the calculated short-circuit current, it is advisable to use fuses connected in series with an air switch power supply to compensate for the small breaking capacity of low-voltage circuit breakers. This also avoids the cost of selecting low-voltage circuit breakers with larger breaking capacity and has a backup protection function. Short delay (0.1 seconds) circuit breakers can be selected