Circuit Breakers: A Comprehensive Guide to Understanding Them

Circuit breakers play a crucial role in electrical operations, serving to distribute electrical power, initiate asynchronous motors infrequently, and protect power lines and motors. They can be seen as a combination of fuse-style switches and thermal overload relays, forming a protective mechanism. In the event of severe overload, short circuit, or under voltage faults, circuit breakers have the ability to promptly disconnect and reconnect load circuits, as well as interrupt faulty circuits, preventing accidents from escalating and ensuring safe operation.

MCB, MCCB and ACB photo

Table of Contents

Miniature circuit breakers

Miniature circuit breakers(hereafter MCB) are widely used as terminal protection devices in building electrical distribution systems. They are utilized for the protection of single-phase and three-phase circuits with a current rating of 125A and below, safeguarding against short circuits, overloads, overvoltages, and other electrical faults. They are available in four configurations: single-pole 1P, double-pole 2P, triple-pole 3P, and four-pole 4P.

MCB inner structure description

A MCB consists of an operating mechanism, contacts, protective devices (such as various trip units), and an arc extinguishing system. The main contacts can be closed manually or by electrically closing the circuit. Once the main contacts are closed, the trip-free mechanism locks them in the closed position. The coils of the overcurrent trip unit and the thermal trip unit are connected in series with the main circuit, while the coil of the undervoltage trip unit is connected in parallel with the power supply.

In the market, a MCB rated current(In) can be 10A, 16A, 20A, 25A, 32A, 40A, 50A, 63A and so on. The rated current means the MCB can bear maximum current under normal operating conditions. Generally, when the current is greater than the rated current of the MCB, it will trip and cut off the circuit. However, if the rated current is exceeded for a short period of time, the circuit breaker may not trip, but will return to normal after a certain period of time, which needs to be evaluated on a case-by-case basis.

For all the MCBs, there are 4 types of  trip curves: A/B/C/D.

A-type magnetic trip curve: The magnetic trip current is (2-3) times than In. It is suitable for protecting semiconductor electronic circuits, measurement circuits with small power supply transformers, or systems with long circuits and low current.

B-type magnetic trip curve: The magnetic trip current is (3-5) times than In. It is suitable for protecting residential distribution systems, household appliances, and personal safety.

C-type magnetic trip curve: The magnetic trip current is (5-10) times than In. It is suitable for protecting distribution lines and lighting circuits with higher contact currents.

D-type magnetic trip curve: The magnetic trip current is (10-20) times than In. It is suitable for protecting equipment with high surge currents, such as transformers and solenoid valves.

Molded case circuit breakers

Moulded Case Circuit Breaker

Molded case circuit breakers, also known as MCCB, have their line terminals, arc extinguishing chambers, trip units, and operating mechanisms housed within a single plastic casing. Auxiliary contacts, undervoltage trip units, and shunt trip units are often modular in design, making the structure compact and generally not intended for maintenance, suitable for use as branch circuit protective switches. MCCB typically include thermal-magnetic trip units, while larger models may be equipped with solid-state trip sensors.

The overcurrent trip units of MCCBs come in two types: electromagnetic and electronic. Electromagnetic MCCBs are typically non-selective, offering only long-time and instantaneous protection modes. Electronic MCCBs provide four protection functions: long-time, short-time, instantaneous, and ground fault. Some newly introduced electronic MCCBs also feature zone-selective interlocking capabilities.

MCCBs are commonly used for control and protection of distribution feeders, as main switches for low-voltage side of small distribution transformers, and for terminal control in power distribution systems. They can also serve as power switches for various industrial machinery.

Compared with MCBs, the rated current of MCCBs have the following types: 16A, 25A, 30A, 40A, 50A, 63A, 80A, 100A, 125A, 160A, 200A, 225A, 250A, 315A, 350A, 400A, 500A, 630A. Thus, the MCCB can withstand much higher current than miniature one.

Air circuit breakers

Air circuit breaker

Air circuit breakers, also known frame circuit breakers, have all their components housed within an insulated metal frame. They are typically of the drawout type, allowing for the installation of various accessories and convenient replacement of contacts and parts. They are commonly used as main switches in power supply systems. The overcurrent trip units of frame circuit breakers come in several types, including electromagnetic, electronic, and intelligent trip units. These circuit breakers provide four levels of protection: long-time, short-time, instantaneous, and ground fault. The settings for each protection level can be adjusted within a certain range based on the frame rating.

ACBs are suitable for use in AC 50Hz distribution networks with rated voltages of 380V and 660V, and rated currents ranging from 200A to 6300A. They are primarily used for power distribution and protection of circuits and power equipment against the hazards of overloads, undervoltage, short circuits, and single-phase grounding faults. These circuit breakers feature multiple intelligent protection functions, allowing for selective protection. Under normal conditions, they can be used for infrequent switching of circuits. Circuit breakers with ratings below 1250A can be used for motor overload and short circuit protection in AC 50Hz, 380V voltage networks.

ACBs are also commonly applied as outgoing switches for transformers on the 400V side, busbar interlocking switches, high-capacity feeder switches, and large motor control switches.


In conclusion, circuit breakers are essential components in electrical systems, providing power distribution, motor protection, and safeguarding against faults. Miniature circuit breakers are widely used for terminal protection in building electrical distribution systems, while molded case circuit breakers find applications in control and protection of distribution feeders, transformers, and industrial machinery. Air circuit breakers serve as main switches in power supply systems and offer adjustable levels of protection. Overall, circuit breakers play a critical role in ensuring safe and reliable operation of electrical systems.

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