The differences between air circuit breaker and molded case circuit breaker(ACB&MCCB)

Table of Content

(Last Updated On: 2020年12月15日)

Introduction of MCCB and ACB


1、Frame circuit breaker (universal type)-ACB

The frame breaker is also generally known as an air circuit breaker or universal circuit breaker, mainly used for shut downing the incoming, busbar, and other high-current circuits in the low-voltage distribution system, which consists of contacts, arc extinguishing devices, operating mechanism, and decoupling 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, with larger external dimensions and a lower protection level. Drawer type uses an engineering plastic shell, which has a more compact structure, high protection level, easy access.

two types of air circuit breaker

Intelligent frame circuit breaker built-in single-chip computer, with the display, alarm, self-test, and communication functions to form a monitoring system with the industrial control machine. The biggest feature of the frame circuit breaker is a large capacity, high limit breaking capacity and sufficient short-time withstand current, rated short-time withstand current up to 85KA, which makes the selectivity and stability and excellent features of the air circuit breaker.

In the rated current, the air circuit breaker generally reaches from 630A-6300A. They are installed in an insulated metal frame, can be equipped with a variety of accessories, and more convenient to replace contacts and parts, mostly used in the power end of the main switch.

Air circuit breaker

2、Molded case circuit breaker – MCCB

Molded case circuit breaker is also known as a device circuit breaker, mainly used for the in and out of the line, motor protection in the low-voltage distribution system, because of its grounding terminal outside contacts, interrupter, decoupling and operating mechanism are installed in a plastic shell. Auxiliary contacts, Undervoltage trips, and sub-excitation trips are mostly modular, the structure is very compact, so it generally does not consider maintenance, the protection switch that suitable for use as a branch circuit, mostly operated manually, but the large capacity can choose to divide and close electrically. Due to the application of electronic overcurrent detonators,  

Molded case circuit breaker usually has two kinds of electromagnetic and electronic, general electromagnetic molded case circuit breaker for non-selective circuit breakers, only has two types of protection: long delay and instantaneous, the electronic molded case circuit breaker has four protection functions: a long delay, short delay, instantaneous and ground fault. Some of the new electronic molded case circuit breaker products also come with a regional selective chain function.

Molded Case Circuit Breaker

The difference between air and molded case circuit breakers

The Molded case circuit breaker has a range series of products, which are characterized by compact structure, easy to operate, relatively simple function, generally configured with overcurrent decoupler and instantaneous decoupler, while the frame circuit breakers are functional, especially in recent years, with the application of single-chip computer technology, Frame circuit breaker towards to the high performance, easy maintenance and networking

Briefly, the Obvious difference is the size of the MCCB and ACB, not only the shape and size of the frame circuit breaker is larger than the case circuit breaker, but also the current level, segmentation capacity, etc. are larger than the molded case circuit breaker. This is the essential difference, so frame circuit breakers and molded case circuit breakers are applied to different occasions, the ACB is applied to the upper level into the line, the molded case is applied to the lower level of the frame, the specific as follows:

1.-the appearance and installation size

First of all, from the appearance, it is obvious that the two sizes are different, the volume of frame circuit breaker size is larger, molded case circuit breaker size and volume is relatively small.

2.- the protection function of the trips 

Air circuit breaker intelligent decoupler has the function of protection, measurement, auxiliary, special and communication, 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 phase loss protection/alarm, phase sequence protection, low-frequency protection, high-frequency protection, inverse power protection, voltage harmonic alarm, current harmonic alarm.

While the molded case circuit breaker has long delay protection, short delay protection (i.e. overload protection), and instantaneous protection (i.e. short circuit protection).

In conclusion, the protection function of the frame circuit breaker is more powerful than the molded case circuit breaker.

3.-The scope of application

Frame circuit breakers with a rated current from 400A to 6300A,which are mainly used in power distribution networks to distribute electrical energy, protect lines and power supply equipment from overload, under-voltage, short circuit, single-phase grounding, and other fault hazards.

The rated current of the molded case circuit breaker is ranging from 10 to 800A, which is used to protect lines and equipment when overload, short-circuit, and under-voltage occur in distribution lines, and also to provide overload, short-circuit, and under-voltage protection for infrequent starting of motors.

4.- The choice of frame and molded case

Compared with frame circuit breakers, the parameters of molded case circuit breakers are lower and the structure is relatively simple. Usually, its rated current does not exceed 1600 A. As the case circuit breaker is in the next level of the frame circuit breaker, after a period of line, the short-circuit current will be reduced, the parameters and performance of the case circuit breaker can meet the operational requirements, and the small size, flexible installation, also the lower price than the frame circuit breaker, suitable for a large number and a wide range of installation and use.

Accessories and usage of MCCB and ACB

Currently, on the market, air circuit breakers are mainly divided into DW15, DW16, DW17 (ME), DW45, and other series, the products are composed of the body and accessories. There are no accessories by air  circuit breaker itself, but can effectively divide and close the line, when the overload, short circuit, and other accidents happened in the equipment, it could cut off the line to protect the load automatically;

As a derivative and complementary to the function of the circuit breaker, the accessories have added a richer protection function and control mode, which make more abundant in the application, protection, operation, and installation methods. The accessories have become an important part of the circuit breaker. However, users need to apply the accessories according to the actual control and protection line to avoid unnecessary waste, while the selection needs to be divided into voltage level, AC or DC, the number of pairs of auxiliary contacts, etc.. improper selection of the accessories, can not play a protective role, but also will cause great economic losses.

air circuit breaker

Function and application of air circuit breaker. 

1, auxiliary contacts:

Contacts mechanically linked to the main circuit-breaking mechanism of the universal circuit breaker and for the indication of the circuit breaker splitting/closing state. 

Auxiliary contacts are connected to the control circuit of the universal circuit breaker, through the opening and closing of the switch to control or interlock its related appliances. A conventional universal circuit breaker with six pairs of contacts (three normally open, three normally closed), the DW45 series has eight pairs of contacts (four normally open, four normally closed). 

2, alarm contacts:

for the universal circuit breaker accident alarm contacts 

Alarm contacts only act when the universal circuit breaker is tripped, alarm contacts will turn from the original normally open position to the closed position, when the circuit breaker load overload short circuit or under-voltage and other faults tripped freely, connected to the auxiliary lines in the indicator, bell or buzzer, etc., to show or remind the circuit breaker of the fault tripping state. 

3、Shunt-excitation trips:

for the voltage source excitation trips

Shunt excitation decoupler is an accessory for remote breaking operation. When the supply voltage between the rated control voltage between 70% and 110%, can break the circuit breaker reliably. Shunt-excited decoupler is a short-time working system, coil energization time can not exceed 1s, otherwise, the coil will be burned. 

4, Undervoltage trips:

when the end voltage drops to a certain set value, there will be a delay or no delay to disconnect.

Undervoltage trips are used for delay or without delay when the supply voltage drops to within 35% to 70% of the rated operating voltage. When the power supply voltage is equal to or greater than 85% of the rated operating voltage of the Undervoltage release, under thermal conditions, can ensure reliable closure of the circuit breaker; when the power supply voltage is equal to or less than 35% of the rated operating voltage of the release, the Undervoltage release should be able to prevent the circuit breaker from closing. When used, the Undervoltage trips coil is connected to the power side of the circuit breaker, Undervoltage de trigger energized, the circuit breaker can close, otherwise, the circuit breaker can not close.

Components of air circuit breaker

Function and application of molded circuit breaker

With the development of technology, the performance of the molded case circuit breakers is becoming more and more advanced. It is applied to the occasion of the power distribution system, mainly plays the role of distributing electrical energy and can reliably act to isolate the fault when the fault occurs in the distribution system and reaches the action current value of molded case circuit breaker to ensure the normal operation of the distribution system, the accessory part of molded case circuit breaker also plays a vital role

1.-Auxiliary contact

It is mainly used for circuit breaker when a fault occurs to provide signals, which alarm contacts to issue fault indication signals for the following reasons: overload or short-circuit detonation

Under-voltage, under-voltage, residual current action detonation, manual free detonation

2.-Leakage alarm non-disconnecting module

The main application occasions are firepower, such as elevators for fire fighting. Fire pumps, fire alarms and other equipment, fire fighting, and lighting. Anti-theft alarm power supply, hospital operating room power supply and special equipment that does not allow power failure and equipment where power failure will cause huge economic loss

Which alarm contacts can be connected with indicators, buzzers, alarms, etc. When the alarm signal response should be promptly checked whether the line has a fault.

3.-Under-voltage detonator

The main function is to realize the under-voltage protection function of the circuit breaker and to disconnect the circuit breaker when the power supply voltage is too low, to protect the power-using equipment.

● When the power supply voltage drops (or even slowly) to the range of 70% to 35% of the rated control power supply voltage, the Undervoltage striker should make the circuit breaker reliably disconnect.

When the power supply voltage is equal to or greater than 85% of the rated control power supply voltage of the Undervoltage derailer, the circuit breaker should be able to ensure closure.

● When the supply voltage is lower than 35% of the rated control supply voltage of the Undervoltage derailer, the Undervoltage derailer shall prevent the circuit breaker from closing.

4.-Shunt-excitation decoupler

When the power supply voltage is equal to any voltage between 70% and 110% of the rated control power supply voltage, the shunt-excited release should be able to make the circuit breaker operate reliably.

Whether it is a molded case circuit breaker or frame circuit breaker, its accessories play a vital auxiliary role in the circuit, but it does not mean that the more accessories the better to protect the product, we have to choose to add the necessary accessories according to the actual situation, to ensure the safety of the circuit and do not waste.


Applications and problems of ACB and MCCB

Air circuit breakers are especially used in the development of widely used power systems and power system applications that integrate system visualization, automation, networking, real-time, requiring increasingly high precision, complex network structures, and widely used automated digital process control, the high failure rate of low-voltage circuit breakers also exists a high demand for the existing circuit breakers can no longer meet the modern power integrated automation system needs.

Some common problems will happen when MCCB in the actual applications, specifically including the following types. First, the manual operation is invalid. When the closure operation is done of the molded case circuit breaker and found that the equipment failure, you should replace the damaged coil as well as the voltage applied in the detonator to perform the closure operation of the molded case circuit breaker. Second, the molded-case circuit breaker breaks automatically. When using the molded-case circuit breaker, the device executes the breaking on its own in a sudden, the most frequent cause of the problem is due to the damaging of the operating mechanism, which leads to the self-braking of the molded-case circuit breaker. Third, the noise is large. When you use the molded-case circuit breaker and found that the noise emitted by this product is obviously much louder than the usual use, which may due to dirt or short circuit cracks of the internal core of the device, you just need to clean or replace a core.


Through the above content, we have a clear view of the difference between air circuit breakers and molded case circuit breakers, so how to choose and use the right circuit breaker will be clear. Because of the different current, choose the right one will prevent accidents.

Besides, low-voltage circuit breakers are an essential part of the distribution network, but thermal-magnetic circuit breakers mainly rely on thermal double gold and magnetic system, it is increasingly challenging to meet the requirements of reliability, accuracy, and safety of the distribution system, and can not realize the communication network. The intelligent circuit breaker fully applies computer control technology, which not only improves the protection accuracy but also makes the distribution system more reliable, safe, and accurate. At the same time, it can browse and record the grid operating parameters in real-time, providing a detailed database for power load analysis. Therefore, more and more attention is paid to the application of intelligent circuit breakers in the modern power grid. But all choice is according to the real application.



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