Molded case circuit breakers and frame circuit breakers are both very common products used in circuit breaker products. However, many people do not fully understand the difference between the two, and there are often uncertainties when purchasing. Here is the first to understand Next, understand the concept of the two, secondly understand the difference between the two, and finally understand the choice of the two.


(1) The concept of frame and molded case circuit breaker

1) Frame circuit breaker

Frame circuit breakers are generally called air circuit breakers or universal circuit breakers, which are mainly used for the closing of incoming lines, bus couplers 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 shell adopts metal materials, which has a larger appearance and a lower protection level. The drawer type adopts engineering plastic shell, which has a relatively compact structure, high protection level and convenient maintenance. According to the protection characteristics, frame circuit breakers can be divided into non-selective (A type) and selective (B type). Type B circuit breakers mostly use electronic overcurrent releases, which have long time delay, short time delay and instantaneous Action protection features three-stage protection.


The intelligent frame circuit breaker has a built-in single-chip microcomputer, which has display, alarm, self-check and communication functions, and can form a monitoring system with an industrial computer. The biggest feature of the air circuit breaker is 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. 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 are also called device-type circuit breakers. They are mainly used for the protection of incoming and outgoing lines of low-voltage power distribution systems and motor protection. All parts are sealed in the casing, auxiliary contacts, undervoltage trips, and shunt trips. Most of the devices are modularized. Due to its very compact structure, plastic case circuit breakers are basically impossible to overhaul, and most of them are manually operated, and electric switching can be selected for large capacity. Due to the application of electronic overcurrent releases, molded case circuit breakers can also be divided into two types: A and B. The use of molded case circuit breakers is usually A. In the case of short circuit, the selective protection is short-time delay. Therefore, there is no short-time withstand current (Icw) value; molded case circuit breakers are characterized by small size, good contact protection, convenient installation 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 frame circuit breaker is larger in size and volume, and the molded case circuit breaker is relatively small in size and volume.


2) The protection functions of the two releases are different

The intelligent trip unit of the frame circuit breaker has protection functions, measurement functions, auxiliary functions, special functions and communication functions. The protection functions include long-time protection, short-time protection, instantaneous protection, MCR protection, grounding protection, and low voltage Protection/alarm, overvoltage protection/alarm, voltage unbalanced 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, electric 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-time protection, short-time protection (that is, overload protection), and instantaneous protection (that is, short-circuit protection).

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

3) The scope of application of the two is different

The rated current of the frame circuit breaker is from 400A to 6300A. It is mainly used in the distribution network to distribute electric energy, protect lines and power equipment, and avoid the hazards of 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 the line and equipment in the distribution line are overloaded, short-circuited, or under-voltage. 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 concentrated in the substations of the enterprise and distributes electric energy to lower-level power distribution equipment in different locations. This level of equipment is close to the transformer side, so the electrical parameter requirements are higher, and the output circuit capacity is also larger. Generally, the main component frame circuit breaker is in the main distribution cabinet.

Secondary power distribution equipment is the collective term for power distribution cabinets and motor control centers. The power distribution cabinet is used in the occasions where the load is relatively dispersed and there are few circuits; the motor control center is used in the occasions where the load is concentrated and there are many circuits. They distribute the electric energy of a certain circuit of the upper-level power distribution equipment to the nearby load. This level of equipment should provide protection, monitoring and control of the load. Generally, the circuit breakers used are plastic case circuit breakers, micro-break switches, etc.

4) Different usage categories

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

The use category of air circuit breakers is usually category B. In the case of a short circuit, the selective protection of man-made short time delay (adjustable), so there is a short-time withstand current (Icw) value;

For specific use, different circuit breaker products should be selected according to the load situation.

(3) Selection of frame and plastic case

Compared with frame circuit breakers, plastic case circuit breakers have lower parameters and relatively simple structure. 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 air circuit breakers, it is suitable for installation and use in a large number and a wide range.

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

As shown in the figure above, the low-voltage switch in the circuit 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 requirements of use according to the requirements of use. The product. If the various parameters meet the application conditions, the molded case circuit breaker can also be used as the incoming switch of the low-voltage side connecting transformer.