Can plastic case replace the frame when rated current, breaking capacity and protection are all met? For example, can a 1600A molded case circuit breaker be used on the main incoming line? If you can, why is the common practice still using frames instead of using much cheaper molded cases?
Moulded case circuit breakers and frame circuit breakers are both very commonly used products in circuit breaker products. However, many people do not fully understand the differences between the two, and often have uncertainty when purchasing. Here, first understand the concept of the two, secondly understand the differences between the two, and finally understand the choice of the two.
01 What is a frame circuit breaker?
Frame circuit breakers are also generally known as air circuit breakers or universal circuit breakers, and are mainly used for closing incoming lines, bus connections and other high-current circuits in low-voltage 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 is made of metal material, has a large external size and a low level of protection. The drawer type adopts an engineering plastic shell, which is relatively compact in structure, high in protection level and convenient for maintenance. According to protection characteristics, frame circuit breakers can be divided into non-selective (Class A) and selective (Class B). Class B circuit breakers mostly use electronic overcurrent releases, which have long delay, short delay and instantaneous action protection. Three-stage protection characteristics.
The intelligent frame circuit breaker has a built-in microcontroller, which has display, alarm, self-test and communication functions, and can form a monitoring system with an industrial computer. The biggest characteristics of frame circuit breakers are their large capacity, high ultimate breaking capacity and sufficient short-term withstand current. For example, the rated current of the JONCHN HW1 circuit breaker is as high as 6300A, and the rated short-term 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 higher.
02 What is a molded case circuit breaker?
Moulded case circuit breakers, also known as device circuit breakers, are mainly used for incoming and outgoing lines and motor protection of low-voltage distribution systems. All parts are sealed in the housing. Auxiliary contacts, undervoltage trippers and shunt trippers are mostly modular. Due to its very compact structure, molded case circuit breakers are basically impossible to maintain. Most of them are operated manually, and electric switching can be selected for large capacity. Due to the application of electronic overcurrent trippers, 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, selective protection is provided for no short delay. Therefore, there is no short-term withstand current (Icw) value; molded case circuit breakers are characterized by small size, good contact protection, easy installation and use, and low price.
03 What is the difference between molded case circuit breakers and frame circuit breakers?
1）The appearance and installation size of the two are different.
First of all, in terms of appearance, the size of the two are different. 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 protection functions are different.
The intelligent trip of frame circuit breakers 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, ground protection, low voltage protection/alarm, overvoltage protection/alarm, voltage imbalance and 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, power measurement, harmonic measurement;
Auxiliary functions: pre-alarm, self-diagnosis function, fault history function, test function;
Special functions: load monitoring, area joint control, high and low temperature range;
Communication functions: Communication protocol functions, etc.
Moulded case circuit breakers mainly have long delay protection, short delay protection (i.e. overload protection), and instantaneous protection (i.e. short circuit protection).
It can be seen that the protection function of frame circuit breakers is more powerful than that of molded case protection.
3）The scope of application of the two is different
The limit current of frame circuit breakers ranges from 400A to 6300A and are mainly used in distribution networks to distribute electrical energy and protect lines and power supply equipment from overload, undervoltage, short circuit, single-phase grounding and other faults;
The limit current of molded case circuit breakers ranges from 10 to 800A. They are used to protect lines and equipment when overload, short circuit, or undervoltage occur to lines and equipment in distribution lines. They can also provide overload, short circuit, and undervoltage protection for infrequent starts of motors.
As shown in the figure:
Primary power distribution equipment, collectively referred to as power distribution centers, is installed centrally in enterprise substations to distribute power 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 large. Generally, in the main distribution cabinet, the main components frame circuit breakers.
Secondary distribution equipment is the collective name 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; motor control centers are used in situations where the load is concentrated and there are many circuits. They distribute power from a circuit of the higher-level distribution equipment to nearby loads. This level of equipment should provide load protection, monitoring and control. Generally, circuit breakers are selected: molded case circuit breakers, micro-interrupters, etc.
4）Different usage categories
The use category of molded case circuit breakers is usually Class A. In the case of short circuit, selective protection has no short delay, so there is no short-term withstand current (Icw) value;
The use category of frame circuit breakers is usually Class B. In case of short circuit, it selectively protects artificial short delay (adjustable), so it has a short-term withstand current (Icw) value;
For specific use, different circuit breaker products need to be selected according to the load situation.
04 How to choose molded case circuit breakers and frame circuit breakers?
Compared with frame circuit breakers, molded case circuit breakers have lower parameters and relatively simple structure. Generally, 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 be reduced after passing through a section of line. The parameters and performance of the molded case circuit breaker can meet the operation and use requirements. Moreover, the size is small, the installation is flexible, and the price is lower than that of the frame circuit breaker. It is suitable for installation and use in large quantities and a wide range.
Generally in use, due to the high parameters of frame circuit breakers (rated voltage and rated current, as well as corresponding breaking capacity and other performance are higher than molded case and micro-breaking products), they are usually installed in places where the ratio of load current to fault current is large and requires higher safety and reliability.
Frame circuit breakers are mostly used for incoming lines, connections and feeders with high current loads. The next level can use molded case circuit breakers. 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-micro circuit breaker. In some special environments, products that meet the use requirements can also be selected according to the use requirements. For example, if various parameters meet the application conditions, molded case circuit breakers can also be used as the incoming switch of the low-voltage side connection transformer.
Post time: Jul-16-2024