outdoor shot of breaker box and one circuit breaker with “Main” mark

Figure 1: Breaker box.

In the process of circuit installation, a lot of facilities need to be configured. A circuit breaker is one of them. What is a circuit breaker? Let's learn about it together.

1. Definition

A circuit breaker is generally composed of a contact system, an arc extinguishing system, an operating mechanism, a trip unit, and a casing. A circuit breaker refers to a switching device that can close, carry and break current under normal circuit conditions and the above jobs under abnormal circuit conditions within a specified time.

Circuit breakers can be used to distribute electric energy, start asynchronous motors infrequently, and protect power supply lines and motors. When they meet serious overload or short-circuit and under-voltage faults, they can automatically cut off the circuit. Their function is equivalent to the combination of fuse switch and overheating and underheating relay, etc. Moreover, it is generally not necessary to change the components after breaking the fault current. At present, circuit breakers have been widely used.

Power distribution is an extremely important link in the generation, transmission and use of electricity. The power distribution system includes transformers, various high and low voltage electrical equipment, and the low voltage circuit breaker is a widely used electrical appliance.

Internal structure of a CHINT circuit breaker, including shunt release, auxiliary contact, etc.

Figure 2: Internal structure of a circuit breaker.

2. How Circuit Breakers to Work

When a short circuit occurs, the magnetic field generated by the large current (generally 10 to 12 times) overcomes the reaction force spring, the trip unit pulls the operating mechanism to act, and the switch trips instantaneously. When overloaded, the current becomes larger, the heat generation increases, and the bimetallic strip deforms to a certain extent to promote the action of the mechanism. The higher the current, the shorter the action time.

The circuit breaker has an electronic type, which uses the mutual inductor to collect the current of each phase and compares it with the set value. When the current is abnormal, the microprocessor sends a signal to make the electronic trip unit drive the operating mechanism to act.

3. Classification of Circuit Breaker

A circuit breaker is actually a switch. The main differences between it and other ordinary switches are that it features a high applicable voltage level, arc extinguishing medium and methods including vacuum, less oil, more oil and sulfur hexafluoride, etc., and strong arc extinguishing ability.

electrical breakers inside electric panel in power plant.

Figure 3: Many circuit breakers.

Circuit breakers are divided into high-voltage circuit breakers and low-voltage circuit breakers according to their scope of use. The boundary between high and low voltage is relatively blurred, and generally those suitable for above 3kV are called high-voltage circuit breakers.
According to the operation mode, they are divided into: electric operation, energy storage operation and manual operation.
According to the structure, there are: universal type and plastic shell type.
According to the arc extinguishing medium, they are divided into: oil immersion type, vacuum type and air type.
According to the action speed, there are: fast type and ordinary type.
According to the number of poles, there are: single-pole, two-pole, three-pole and four-pole.
According to the installation method, they are divided into: plug-in type, fixed type and drawer type.

voltage switchboard with circuit breakers are in the ON and OF positions in the big electric box

Figure 4: Circuit breakers.

3.1 Low Voltage Circuit Breaker

Low-voltage circuit breaker, also known as automatic air switches, can be used to switch on and off load circuits, and control motors that start infrequently. Its function is equivalent to the sum of some or all functions of electrical appliances, such as knife switch, overcurrent relay, under-voltage relay, thermal relay and leakage protector. It is an important protective appliance in low-voltage distribution networks.

3.2 High Voltage Circuit Breaker

High-voltage circuit breaker (or high-voltage switches) is the main power control equipment in substations and has arc extinguishing characteristics. When the system is in normal operation, it can cut off and connect the no-load and load current of the line and various electrical equipment. When the system fails, it cooperates with the relay protection to quickly cut off the fault current to prevent the scope of the accident from expanding. Therefore, the working quality of the high-voltage circuit breaker directly affects the safe operation of the power system.

4. The Role of Circuit Breakers

black and white photo of electric arc

Figure 5: Electric arc.

The function of the circuit breaker is to cut off and connect the load circuit, as well as cut off the fault circuit to prevent the accident from expanding, and ensure safe operation. That is, when the power equipment or line fails, the relay protection and automatic devices act on the circuit breaker to quickly remove the faulty part from the power grid to ensure the normal operation of the non-faulty part of the power grid.

The high-voltage circuit breaker needs to break the arc of a voltage of 1500V and a current of 1500-2000A. These arcs can be stretched to 2m and continue to burn without extinguishing. Therefore, arc extinguishing is a problem that high-voltage circuit breakers must solve.

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