Can molded case circuit breakers replace frames when the rated current, breaking capacity, and protection are all met? For example, can the ABB1600 T7 molded case circuit breaker be used on the main incoming line? If possible, why is the common practice still to use frames instead of much cheaper plastic shells?

Molded case circuit breakers and frame circuit breakers are both commonly used products in circuit breaker products. However, many people are not very familiar with the differences between the two, and there are often indecisive situations when choosing between them. Here, first understand the concepts of the two, then understand their differences, and finally understand their selection.

The concept of frame and molded case circuit breaker

1) Frame circuit breaker

Frame circuit breakers, also known as air circuit breakers or universal circuit breakers, are mainly used for closing incoming lines, busbars, and other high current circuits in low-voltage distribution systems. Composed of contacts, arc extinguishing devices, operating mechanisms and trip systems, housing, and control units.

According to the installation method, it can be divided into fixed and drawer types. The fixed shell is made of metal material, with larger external dimensions and lower protection level. The drawer style adopts an engineering plastic shell, with a compact structure, high protection level, and easy maintenance. According to their protection characteristics, frame circuit breakers can be divided into non selective (Class A) and selective (Class B). Class B circuit breakers often use electronic overcurrent release devices and have three protection characteristics: long delay, short delay, and instantaneous action protection.

The intelligent framework circuit breaker is equipped with a built-in microcontroller, which has display, alarm, self-test, and communication functions, and can be combined with an industrial computer to form a monitoring system. The biggest characteristics of frame circuit breakers are 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 frame circuit breakers have good selectivity and stability. It is precisely because of these excellent characteristics that the price of frame circuit breakers is relatively high.

2) Molded case circuit breaker

Molded case circuit breakers, also known as device type circuit breakers, are mainly used for incoming and outgoing lines in low-voltage distribution systems, motor protection. All parts are sealed in the casing, and auxiliary contacts, undervoltage release devices, and shunt release devices are often modularized. Due to its very compact structure, molded case circuit breakers are basically impossible to maintain and are mostly operated manually. For large capacity, electric opening and closing can be selected. Due to the application of electronic overcurrent release devices, 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 event of a short circuit, they selectively protect against artificial short delay and therefore have no short-time withstand current (Icw) value; The characteristics of molded case circuit breakers are small size, good contact protection, easy installation, and low price.

The difference between frame and molded case circuit breaker

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

Firstly, in terms of appearance, the two have different sizes. The frame circuit breaker has a larger size and volume, while the molded case circuit breaker has a relatively smaller size and volume.

2) The two release devices have different protection functions

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

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

Auxiliary functions: pre alarm, self diagnostic function, fault history recording function, testing function;

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

Communication function: communication protocol function, etc.

Plastic case circuit breakers mainly have long delay protection, short delay protection (i.e. overload protection), and instantaneous protection (i.e. short circuit protection).

From this, it can be seen that the protection function of the frame circuit breaker is more powerful than that of the plastic shell protection.

3) The two have different scopes of application

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

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

As shown in the figure:

The primary distribution equipment, collectively known as power distribution centers, is centrally installed in the enterprise's substations to distribute electrical energy to lower level distribution equipment in different locations. This level of equipment is located close to the transformer side, so the electrical parameters are high and the output circuit capacity is also large. Generally, in the main distribution cabinet, the main component frame circuit breaker is used.

Secondary distribution equipment refers to the general term for power distribution cabinets and motor control centers. The power distribution cabinet is used in situations where the load is relatively dispersed 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 from a certain circuit of the higher-level distribution equipment to nearby loads. This level of equipment should provide protection, monitoring, and control for loads, and generally uses circuit breakers such as molded case circuit breakers and micro circuit breakers.

4) Using different categories

Molded case circuit breakers are usually classified as Class A. In the event of a short circuit, they selectively protect against human induced short delay and therefore have no short-time withstand current (Icw) value;

The use category of frame circuit breakers is usually Class B. In the event of a short circuit, selective protection has an adjustable short delay, resulting in a short-time withstand current (Icw) value;

Specific usage requires selecting different circuit breaker products based on the load situation.

(3) Selection of frame and plastic shell

Compared to frame circuit breakers, molded case circuit breakers have lower parameters and a relatively simple structure. Usually, its rated current does not exceed 1600A. Due to the fact that the molded case circuit breaker is located at the next level of the frame circuit breaker, the short-circuit current will decrease after passing through a section of the circuit. The parameters and performance of the molded case circuit breaker can meet the operational requirements, and the size should be small, the installation should be flexible, and the price should be lower than that of the frame circuit breaker. It is suitable for installation and use in large quantities and a wide range.

Usually in use, due to the high parameters of the frame circuit breaker (rated voltage and rated current, as well as corresponding breaking capacity, etc., which are higher than those of plastic shell and micro break products), it is usually installed in places where the load current and fault current are relatively large, requiring high safety and reliability. Frame circuit breakers are commonly used for incoming lines, connections, and feeders for high current loads. Its next level can use a molded case circuit breaker.

As shown in the above figure, the low-voltage switches 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, products that meet the usage requirements can also be selected according to the usage requirements. If all parameters meet the application requirements, a molded case circuit breaker can also be used as the incoming switch for connecting the low-voltage side of the transformer.