Molded case circuit breakers and frame circuit breakers are both very common products in circuit breaker products, but many people do not fully understand the difference between the two, and often indecision when purchasing, here is the first to understand The following two concepts, followed by understanding the difference between the two, and finally understanding the choice of the two.


(1) Concept of frame and molded case circuit breaker

1) Frame circuit breaker

Frame circuit breakers are generally also called air circuit breakers or universal circuit breakers, which are mainly used for the connection of incoming lines, bus ties and other high-current circuits of low-voltage power distribution systems. It consists of contacts, arc extinguishing device, operating mechanism and tripping system, housing and control unit.

According to the installation method, it can be divided into two types: fixed type and drawer type. The fixed type shell is made of metal material, which has a larger size and a lower protection level. The drawer type adopts engineering plastic shell, which is compact in structure, high in protection level and convenient in maintenance. According to the protection characteristics, frame circuit breakers can be divided into non-selective (Type A) and selective (Type B). Type B circuit breakers mostly use electronic overcurrent releases, which have long-delay, short-delay and instantaneous Action protection Three-stage protection feature.


The intelligent frame circuit breaker has a built-in single-chip microcomputer, which has the functions of display, alarm, self-check and communication, and can form a monitoring system with the industrial computer. The biggest feature of the frame circuit breaker is its large capacity, high ultimate breaking capacity and sufficient short-time withstand current. For example, the rated current of Delixi CDW3 circuit breaker is as high as 6300A, and the rated short-time withstand current is as high as 85KA, which makes the frame circuit breaker. The circuit breaker has good selectivity and stability, and it is precisely because of these excellent features that the price of the frame circuit breaker is higher.

2) Molded case circuit breaker

Molded case circuit breakers, also known as device circuit breakers, are mainly used for incoming and outgoing lines and motor protection of low-voltage power distribution systems. All parts are sealed in the shell, auxiliary contacts, undervoltage releases and shunt tripping. Most of the devices are modularized. Due to its very compact structure, molded case circuit breakers are basically impossible to overhaul, most of them are manually operated, and electric switching can be selected for large capacity. Due to the application of electronic overcurrent release, molded case circuit breakers can also be divided into two types: class A and class B. The use category of molded case circuit breakers is usually class A. In the case of short circuit, there is no artificial short delay for selective protection. Therefore, there is no short-time withstand current (Icw) value; the characteristics of the molded case circuit breaker are small size, good contact protection, convenient installation and application and low price.

(2) The difference between frame and molded case circuit breaker

1) The appearance and installation dimensions of the two are different

First of all, in terms of appearance, the sizes of the two are different, the size and volume of the frame circuit breaker are larger, and the size and volume of the molded case circuit breaker are relatively small.


2) The protection functions of the two releases are different

The intelligent release of the frame circuit breaker has protection functions, measurement functions, auxiliary functions, special functions and communication functions, among which the protection functions include long delay protection, short delay protection, instantaneous protection, MCR protection, grounding protection, low voltage Protection/alarm, overvoltage protection/alarm, voltage unbalance and phase loss protection/alarm, phase sequence protection, low frequency protection, high frequency protection, reverse power protection, voltage harmonic alarm, current harmonic alarm;

Measurement functions: current measurement, voltage measurement, power measurement, frequency measurement, energy measurement, harmonic measurement;

Auxiliary functions: pre-alarm, self-diagnosis function, fault history record function, test function;

Special functions: load monitoring, regional joint control, high and low temperature range;

Communication function: communication protocol function, etc.

Molded case circuit breakers mainly have long delay protection, short delay protection (ie overload protection) and instantaneous protection (ie short circuit protection).

It can be seen that the protection function of the frame circuit breaker is stronger than that of the plastic case.

3) The scope of application of the two is different

The rated current of the frame circuit breaker ranges from 400A to 6300A. It is mainly used in the power distribution network to distribute electric energy, protect lines and power equipment from overload, undervoltage, short circuit, single-phase grounding and other faults;

The rated current of the molded case circuit breaker is from 10 to 800A. It is used to protect the line and equipment when overload, short circuit, and undervoltage occur in the distribution line. It can also provide overload and short circuit for the infrequent starting of the motor. , Undervoltage protection.


The first-level power distribution equipment, collectively referred to as the power distribution center, is centrally installed in the substation of the enterprise and distributes electrical energy to the lower-level power distribution equipment in different locations. This level of equipment is close to the transformer side, so the electrical parameters are required to be high, and the output circuit capacity is also large. Generally, in the main power distribution cabinet, the main components are framed circuit breakers.

Secondary power distribution equipment is a general term for power distribution cabinets and motor control centers. Power distribution cabinets are used in situations where the load is relatively scattered and there are few circuits; the motor control center is used in situations where the load is concentrated and there are many circuits. They distribute the electrical energy of a certain circuit of the upper-level power distribution equipment to the nearest load. This level of equipment should provide protection, monitoring and control of the load. Generally, the circuit breakers used are selected: molded case circuit breakers, micro-break switches, etc.

4) Use different categories

The use category of molded case circuit breakers is usually Class A. Under short-circuit conditions, there is no artificial short-time delay for selective protection, so there is no short-time withstand current (Icw) value;

The use category of the frame circuit breaker is usually Class B. In the case of short circuit, the selective protection is artificially short-delayed (adjustable), so it has a short-time withstand current (Icw) value;

For specific use, it is necessary to select different circuit breaker products according to the load conditions.

(3) Selection of frame and plastic shell

Compared with frame circuit breakers, molded case circuit breakers have lower parameters and simpler structures. Usually its rated current does not exceed 1600A. Since the molded case circuit breaker is at the next level of the frame circuit breaker, the short-circuit current will decrease after a section of the line. The parameters and performance of the molded case circuit breaker can meet the requirements of operation and use, and the size is small, the installation is flexible, and the price is low. For frame circuit breakers, it is suitable for a large number and wide range of installations.

Usually in use, due to the high parameters of the frame circuit breaker (rated voltage and rated current, as well as the corresponding breaking capacity and other performance are higher than plastic case and micro-break products), it is usually installed in the load current and fault current is relatively large, need A place of relatively high security and reliability. Frame circuit breakers are mostly used for incoming lines, ties and feeders with large current loads. Its next level can use molded case circuit breakers.

As shown in the figure above, the low-voltage switches in the line should be determined in the following order: low-voltage power supply - frame circuit breaker - molded case circuit breaker - miniature circuit breaker. In some special environments, you can also choose to meet the use requirements according to the requirements of use The product. If the parameters meet the application conditions, the molded case circuit breaker can also be used as the incoming switch of the low-voltage side connected transformer.