Selection of miniature circuit breakers

28/04/2022 WITPROTON

There are four general tripping characteristics of circuit breakers: A, B, C, and D. So how do we choose?

I Choice of A, B, C, D types of miniature circuit breakers!

D-type circuit breaker: 10-20 times the rated current, mainly used in the environment where the instantaneous current of electrical appliances is large, and it is rarely used in general households. It is suitable for systems with high inductive loads and large inrush currents, and is often used to protect systems with high impact electrical current devices, such as electric motors.

C-type circuit breaker: 5-10 times the rated current, it needs to be tripped within 0.1 seconds. The circuit breaker with this characteristic is the most commonly used, and is often used to protect distribution lines and lighting lines with high on-current.

Type B circuit breaker: 2-3 times the rated current, generally used for pure resistive loads and low-voltage lighting circuits, often used in household distribution boxes to protect household appliances and personal safety, currently used less.

A-type circuit breaker: 2 times the rated current, rarely used, generally used for semiconductor protection (generally, fuses are used);

The so-called multiple current is the anti-impulse current. The switch will not trip for a certain duration. Its characteristic is to avoid the inrush current.

Selection of the release of low-voltage circuit breakers: the types of release of circuit breakers include overcurrent release, undervoltage release, shunt release, etc.

Overcurrent release can also be divided into overload release and short circuit current release, and there are long delay, short delay, instantaneous points, overcurrent release is the most commonly used. The setting value of the operating current of the overcurrent release can be fixed or adjustable, and it is usually adjusted by rotating or adjusting the lever. The electromagnetic overcurrent release also has two kinds of fixed and adjustable. The electronic overcurrent release is generally adjustable. The breaking capacity of the circuit breaker refers to the ability to withstand the maximum short-circuit current, so the breaking capacity of the rotating circuit breaker must be greater than the short-circuit current of its protective equipment.

The overcurrent release can be divided into fixed installation or module installation according to the installation method. The fixed installation means that the release and the circuit breaker are processed into one body when leaving the factory. As an installation module of the circuit breaker, the chemically installed release can be adjusted at any time and has strong flexibility.

Instantaneous type: 0.02S, for short circuit protection

Short delay type: 0.1-0.4S, used for short circuit and overload protection

Long delay type: less than 10S, used for overload protection

Circuit breakers are used to protect wires and prevent fires, so they should be selected according to the size of the wire rather than the electrical power.

If the circuit breaker is too large, it will not protect the wire. When the wire is overloaded, the circuit breaker will not trip, which will bring hidden dangers to the safety of the family.


1.5 square wire with C10 switch

2.5 square wire with C16 or 20 switch

4 square wire with C25 switch

6 square wire with C32 switch


For the air switch used for the load of the motor, the D-type characteristic should be selected to avoid the high starting current of 5-8 times that of the motor starting.


II How should DC circuit breakers be selected?

In power plants and substations (booster stations), the power supply systems that supply power to control, signal, protection, automation devices and some actuators are collectively referred to as control power supplies. If they are DC power supplies, they are called DC power supply systems; such as AC power supplies , it is called an AC power system. In the DC system, DC circuit breakers are often used, so how should we choose the DC system in the DC system?

The DC circuit breaker should have instantaneous current quick-break and inverse-time overcurrent protection. When the cooperative relationship of selective protection is not satisfied, short-delay current quick-break protection can be added.


The selection of DC circuit breakers should meet the following requirements:

 1. The rated voltage should be greater than or equal to the maximum working voltage of the circuit.

 2. The rated current should be greater than the maximum working current of the loop.

 3. The interrupting capacity should meet the requirements of the maximum expected short-circuit current of the DC power system at the installation site.

 4. The rated current of the emergency tie breaker of the DC power system should not be greater than 50% of the rated current of the battery outlet fuse.

 5. When short-circuit short-time delay protection is used, the rated short-circuit breaking current and short-time withstand current of the DC circuit breaker should be greater than the maximum short-circuit current at the installation site (flowing through the circuit breaker).

 6. The protective operating current and operating time of circuit breakers at all levels should meet the requirements of the selective cooperation between the upper and lower levels, and should have sufficient sensitivity coefficients. The selective coordination of upper and lower circuit breakers shall meet the following requirements.

 7. The rated current of the power circuit breaker of the DC distribution cabinet should be selected according to the sum of the calculated currents of all the power circuits on the DC distribution cabinet.

 8. Protection setting of DC circuit breaker.

The selection principle of DC circuit breaker should be considered according to the above eight points


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