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Miniature Circuit Breaker for Beginners

The miniature circuit breaker, also commonly abbreviated MCB, is a common device in many electrical circuits— low voltage circuits in particular. These devices come in a variety of different sizes and voltage ratings, which are determined by their applications. Get to know more about the miniature circuit breaker, how it works, its applications, and more.

Miniature circuit breaker (MCB)
Miniature circuit breaker (MCB)

What is a Miniature Circuit Breaker?

A miniature circuit breaker (MCB) is a specialized type of breaker that is used to protect low-voltage electrical circuits, such as those in homes or commercial buildings and light industries. MCBs are designed to trip (or open) the circuit when a predetermined amount of current is exceeded.

Most types of miniature circuit breakers today are designed to offer breaking capacities of 6KA up to 10KA for domestic or commercial applications and up to 16KA for industrial power systems. In these installations, the miniature circuit breakers are used in place of fuses.

Miniature Circuit Breaker Function

The miniature circuit breaker function is to provide overcurrent and short-circuit protection of the conductor: when an overload or a short circuit occurs, the MCB trips, or opens, the circuit to prevent damage to the wires. Miniature circuit breakers offer several benefits:

  • You can reset MCBs after an overcurrent event, which eliminates the need to replace them
  • They offer protection against both overload and short-circuit currents
  • MCBs are less prone to failure
  • They are available in a variety of different Amp ratings to suit the specific needs of a circuit or appliance

Applications of Miniature Circuit

Miniature circuit breaker application varies widely and includes residential, commercial, and industrial power systems. In general, MCBs are useful where the load is low and the risk of an overload or short circuit is high. Some of the most common applications of miniature circuit breakers include:

  • Protecting circuits in homes and small businesses
  • Safeguarding industrial equipment
  • Providing overcurrent protection for motors and other loads
  • Protecting against short circuits in electrical systems

In these MCB applications, the device may perform the role of an automatic control switch or isolating device.

Miniature Circuit Breaker Construction

The typical MCB is an enclosed unit that contains several different parts. The miniature circuit breaker enclosure is a molded plastic housing, strong and highly insulation. That said, the most important part of the MCB is the trip mechanism, as this is what allows the breaker to interrupt the flow of current when necessary.

In most MCBs today, the trip mechanism contains both thermal (heat-sensitive), magnetic (current-sensitive) elements. These MCB parts are designed to respond to an increase in current. Other miniature circuit breaker components include the following:

  • An arc quenching chamber: This component contains the arc that is created when the breaker trips. The chamber is designed to extinguish the arc quickly so that it does not cause damage to the breaker or surrounding area.
  • A handle or switch: This component is used to manually open and close the circuit breaker.
  • Contacts: These are the metal parts that complete the circuit when the breaker is in the closed position.
  • Housing: The molded plastic that contains all the other components.

How Does a Miniature Circuit Breaker Work?

The typical miniature circuit breaker (MCB) is a thermal-magnetic device. That means it uses both heat and magnetism to operate. In order to help you understand this miniature circuit breaker working principle, let’s take a look at the two of its main components: the thermal and magnetic tripping units.

Thermal Tripping in MCB

Thermal Tripping in MCB
Thermal Tripping in MCB
Image Resource: https://www.electricaltechnology.org/

The “thermal” part of MCB is a bimetallic strip. This strip is made of two metals with different rates of expansion. When the current flowing through the MCB rises to a certain level, the heat generated causes the two metal strips to expand at different rates. The strip bends, tripping the breaker and opening the circuit.

Different metals may be used in the MCB bimetallic strip to cause thermal tripping in MCB, but the most common combination is steel and brass. In some types of MCBs, the current passes through the bimetallic strip itself, heating it in the process.

In other MCB types, a coil wraps around the bimetallic strip. The current flowing through the coil produces a magnetic field that interacts with and heats the bimetallic strip, thereby causing it to bend and release a mechanical latch.

Magnetic Tripping In MCB

MAGNETIC TRIPPING MCB
Magnetic Tripping In MCB
Image Resource: https://www.electricaltechnology.org/

The “magnetic” part of the MCB is a solenoid. The solenoid is a coil of wire that becomes magnetized when current flows through it. The magnetic field generated by the solenoid helps to trip the breaker faster than the thermal part alone.

Just like a regular circuit breaker, the magnetic tripping in MCB involves separating a set of contacts to control the flow of current. When the current flowing through the contacts exceeds a certain threshold, the magnetic field generated causes the contacts to open.

A miniature circuit breaker has a fixed tripping setting and cannot be adjusted. This setting is determined by the manufacturer and is based on the intended use of the breaker. For example, a domestic MCB may have a tripping setting of 10 amps, while an industrial MCB may have a tripping setting of 100 amps.

Generally, MCBs act as time delay switches, only acting after a timed delay. That means it will not react to short-lived faults such as the inrush current of a motor. These types of faults typically last for only a fraction of a second, but they can cause the MCB to trip if the current is high enough.

Miniature Circuit Breaker Ratings

Miniature circuit breakers are rated by their amperage for both nominal current in Amperes and breaking capacity or KA ratings. The ratings indicate different operational characteristics. These miniature circuit breaker ratings are explained below.

MCB Nominal Rating

The nominal MCB rating is the maximum amperage that the breaker can handle on a regular basis. Typically, this MCB ampere rating ranges from as low as 0.5A to as high as 125A; it depends on the application and the specific needs of the circuit.

MCB KA Rating

The MCB KA rating, also called the breaking capacity rating, is the maximum ampere that the breaker can handle without damage. A common rule of thumb is that the breaking capacity should be several times the nominal rating. MCBs are also rated for voltage. MCB voltage rating varies from 120V/240V for domestic systems to

Miniature Circuit breaker vs. Circuit Breaker

The main difference between a miniature circuit breaker (MCB) and a regular circuit breaker is the size. MCBs are smaller than regular circuit breakers and are designed to protect smaller low-voltage circuits, such as those in homes and offices.

Regular circuit breakers are larger and designed to protect larger circuits, such as those found in industrial and commercial settings.

MCB breakers are also more sensitive than regular circuit breakers and will trip before the current gets too high. Regular circuit breakers can only sense large changes in current.

Conclusion

A miniature circuit breaker, MCB, is a small, sensitive circuit breaker that is designed to protect smaller circuits. MCBs are more precise and sensitive than fuses and regular circuit breakers. And while they may be more expensive than fuses, MCBs are a more cost-effective and convenient alternatives in most applications. So it’s not uncommon to see MCBs used in place of fuses today.

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