If the rated current, breaking capacity and protection are all met, can the molded case replace the frame? For example, can the T7 molded case circuit breaker of ABB1600 be used on the main incoming line? If so, why is the common practice still to use the frame instead of the much cheaper molded case.
Molded case circuit breakers and frame circuit breakers are both very common products used in circuit breaker products, but many people do not fully understand the difference between the two, and often hesitate when purchasing. Here we first understand the concepts of the two, then understand the difference between the two, and finally understand the selection of the two.
UEM5DH MCCB Breaker
01 What is a frame circuit breaker?
Frame circuit breakers are generally also called air circuit breakers or universal circuit breakers, which are mainly used for the closing of incoming lines, busbars and other high current circuits of low-voltage power distribution systems. It consists of contacts, arc extinguishing devices, operating mechanisms and tripping systems, housings and control units.
According to the installation method, it can be divided into fixed type and drawer type. The fixed type shell is made of metal material, with larger external dimensions and lower protection level. The drawer type uses engineering plastic shell, which is more compact, has a high protection level and is easy to maintain. According to the 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 three protection characteristics of long delay, short delay and instantaneous action protection.
Intelligent frame circuit breakers have built-in single-chip microcomputers, with display, alarm, self-test and communication functions, and can form a monitoring system with industrial computers. The biggest features 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 the frame circuit breaker have good selectivity and stability. It is precisely because of these excellent characteristics that the price of frame circuit breakers is relatively high.
02 What is a 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 and motors in low-voltage power distribution systems. All parts are sealed in the casing, and auxiliary contacts, undervoltage releasers, and shunt releasers are mostly modular. Due to the very compact structure, molded case circuit breakers are basically impossible to repair, and most of them are manually operated. Large capacity can be selected for electric separation and closing. Due to the application of electronic overcurrent releases, molded case circuit breakers can also be divided into Class A and Class B. The use category of molded case circuit breakers is usually Class A. In the case of a short circuit, there is no artificial short delay for selective protection, so there is no short-time withstand current (Icw) value; the characteristics of molded case circuit breakers are 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 dimensions of the two are different
First of all, from the 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 trippers are different
The intelligent tripper of the frame circuit breaker has protection function, measurement function, auxiliary function, special function and communication function, among which the protection function includes 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, current harmonic alarm;
Measurement function: current measurement, voltage measurement, power measurement, frequency measurement, electric energy measurement, harmonic measurement;
Auxiliary function: pre-alarm, self-diagnosis function, fault history recording function, test function;
Special function: 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 (i.e. overload protection), 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 the molded case protection function.
3) The two have different scopes of application
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 ranges from 10 to 800A. It is used to protect the lines and equipment when overload, short circuit and undervoltage occur in the distribution line. It can also provide overload, short circuit and undervoltage protection for infrequent starting of motors.
The first-level distribution equipment is collectively referred to as the power distribution center. They are centrally installed in the substation of the enterprise to distribute electric energy to the lower-level distribution equipment in different locations. This level of equipment is close to the transformer side, so the electrical parameters are required to be higher and the output circuit capacity is also larger. Generally, in the main distribution cabinet, the main component is the frame circuit breaker.
The second-level distribution equipment is a general term for power distribution cabinets and motor control centers. Power distribution cabinets are used in places where the load is relatively dispersed and there are fewer circuits; motor control centers are used in places where the load is concentrated and there are more circuits. They distribute the power of a circuit of the upper-level power distribution equipment to the nearest load. This level of equipment should provide protection, monitoring and control for the load, and the circuit breakers generally selected are: molded case circuit breakers, micro-break switches, etc.
4) Different usage categories
The usage 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, so there is no short-time withstand current (Icw) value;
The usage category of frame circuit breakers is usually Class B. In the case of short circuit, there is an artificial short delay (adjustable) for selective protection, so there is a short-time withstand current (Icw) value;
For specific use, different circuit breaker products need to be selected according to the load conditions.
04 How to choose molded case circuit breakers and frames?
Compared with frame circuit breakers, molded case circuit breakers have lower parameters and relatively simple structures. Usually their 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 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 lower than the frame circuit breaker, which is suitable for large quantity and wide range of installation and use.
UEM6H DC MCCB Moulded Case Circuit Breaker
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 the molded case and micro-breaking products), it is usually installed in places where the load current and fault current are relatively large and require relatively high safety and reliability.
Frame circuit breakers are mostly used for incoming lines, contacts and feeders with high current loads. Molded case circuit breakers can be used at its next level. The low-voltage switch 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 requirements can also be selected according to the requirements of use. If all parameters meet the application conditions, the molded case circuit breaker can also be used as the incoming line switch for connecting the transformer on the low-voltage side.
Post time: 7 月-18-2024