How Does a Miniature Circuit Breaker (MCB) Work?

How Does a Miniature Circuit Breaker (MCB) Work?


Miniature Circuit Breaker or shortly MCB offers protection against power surge or faulty currents due to overloads or short circuit.

Correct. But How does a miniature circuit breaker work?

This is one of the rarely answered questions which we have addressed in this article. Everybody knows what it is but not everyone understands how it serves its purpose.

MCBs are time-delay trapping devices where the operating time is set by the magnitude of the current passing through a circuit. If the value of the passing current exceeds, it will automatically trip off the circuit preventing short-circuiting.

MCB doesn’t respond to transients; a motor or a heavy machine starting will not trigger an MCB.

Before we dig deep into the interior of an MCB, let us briefly answer some fundamental questions.

Is MCB the same thing as a FUSE?

The answer to this question is a partial yes. The working principle of an MCB is the same as that of a fuse but MCB is more advanced.

Following are enlisted the major differences between an MCB and fuse.

  1. Fuse is a one-time use device. It needs to be changed each time it gets tripped off. MCB lasts for a long time and does not get damaged after a single trip.
  2. The switch on an MCB is in an entirely different form than that of a fuse.
  3. Unlike a fuse, The metallic conductor in an MCB is covered by plastic wrapping.
  4. The fuse must be changed before it could be restarted, while MCB is easy to restart after tripping.
  5. MCB is more sensitive than a fuse and detects even a minor deviation.
  6. The handling of an MCB is more electrically safe than the fuse.
  7. Unlike fuses, MCBs can be controlled remotely.

Like other circuit breakers, MCB protects wires from overloads and overcurrents. There are two working principles or operational methods that MCBs follow.

  • Thermal operation for overload protection
  • An electromagnetic operation

What Principles does MCB follow?

Thermal Operation

The thermal operation, as the name suggests, works on the principle of heating and bending. A bimetallic strip is used to serve this purpose. When there is a continuous flow of overcurrents, the strip gets heated and deflects by bending, resulting in an open circuit. This deflection of the strip releases a mechanical latch that is connected to the operating mechanism. As a result, the circuit gets opened.

Electromagnetic Operation

When excessive current flows through a circuit, electromechanical displacement of the plunger associated with the MCB solenoid causes the circuit to break. This plunger strikes the trip lever resulting in an immediate release of a latch which leads to the opening of the circuit.

Types of MCB based on number of Poles

MCBs are classified based on the number of poles they possess.

Single Pole: A single-pole MCb has one switch and is used to protect a single-phase power supply.

Double Pole: As the name suggests, the double pole circuit breaker has two switches and is applied to the circuit containing a two-phase system. So it only operates to protect a system with two phases and a neutral.

Triple Pole: There are systems in which three phases are used. The breaker used to protect a three-phase system has three switches. They are called the Triple Pole.

Three Pole with a Neutral: Along with three switches that protect three live phases of the circuit, there is an extra switch called a neutral switch, such a type of breaker is used to protect against short circuit or leakage in any of the three phases and a neutral.

Four Poles: Four poles breaker contain four switches, three phases, and a neutral. It protects all the phases and the neutral.

Characteristics of Miniature Circuit Breakers

Overload Current Ratings (Ampered):

Have you ever experienced a mishap caused by applying too many devices on a system of low capacity?

What happens when multiple devices are connected to a system?

An explosion, right?

So, what if the circuit gets tripped off when the applied load exceeds the rated limit?

It will protect against any sort of problems caused by overloading.

Hence, MCB is classified based on their ampere capacity.

The rated current of an MCB ranges from 0.5 A to 100 A.

Short Circuit Ratings (Kilo Amperes):

Do you know when a short circuit occurs?

When a live wire touches the neutral one. It is rated in Kilo Amperes.

On short-circuiting, the MCB gets tripped off to open the circuit. The fault levels of breakers range from 6,000 A to 10,000 A depending upon the use. A small value MCB will be used commercially but for big industries where the current level is high, a large value MCB would be installed.

What are the Tripping Curves?

Ever heard of the circuit tripping curves?

Breakers are classified based on their tripping curves.

Let’s take a look at their types by keeping in view the tripping curves.

Hold on.

What are the meanings of a tripping curve?

The tripping curve is the highest amount of current a miniature circuit breaker can withstand before tripping off.

Classification of MCBs based on Tripping Curves

Type B Miniature Circuit Breaker: This type of circuit breaker trips instantly when the current is 3 to 5 times the rated value of current. They are suitable for residential appliances and some commercial uses.

Type C Miniature Circuit Breaker: These breakers are used for commercial areas as they trip when the current value is 5 to 10 times the rated current value of the breaker. They are used in big commercial areas where there are more chances of short-circuiting and overloads.

Printers, PCs, and transformers are a few devices that these breakers protect.

Type D Miniature Circuit Breaker: Big industries use this type of circuit breaker. Their tripping value is 10 to 20 times the rated value. X-ray, big transformers, and winding motors have a large in-surge current so the circuit must contain a type D breaker.

Type K Miniature Circuit Breaker: this type trips when the current value is 10 to 12 times the current rating. They are suitable for inductive motors and high inrush currents.

Type Z Miniature Circuit Breaker: very sensitive to short circuits and strongly recommended for small devices/equipment such as semiconductors and chips, etc. as they trip only if the current is 2 to 3 times above the current rating.

Don’t you think that now is the time to open an MCB circuit and know what is inside?

Parts of MCB

Simply manufactured and maintenance-free breaker can’t be repaired or maintained, it has to be replaced once tripped off. The constructions of MCb mainly contain three parts. These three main construction parts are briefly explained below.

The frame of Miniature Circuit Breaker: the frame of MCB is strong, rigid, and insulated in which the other electrical components are placed. It is a molded case.

Operating Mechanism of Miniature Circuit Breaker: ‘ON’, ‘OFF’, and ‘TRIPPED’ are the three positions that an MCB contains. When the breaker is tripped due to over currents, then the external switching latch can be in the ‘TRIPPED’ position. However, when manually switching off the MCB, it will be in the ‘OFF’ position. In the close condition, it will be in ‘ON’ positions.

Trip Unit:  it is called the backbone of a breaker as it is responsible for the proper working of miniature circuit breakers.

Can MCB be operated manually?

Is it possible to manually switch ON/OFF the miniature circuit breaker?

The answer to the above-mentioned question is a YES. It’s quite easy to operate it by hand. When the operating lever of the miniature circuit breaker is operated manually, it means we are setting it at ON/OFF positions by hand and the latch point is displaced which results in opening the circuit.

However, no matter what operating principle MCB is following, the general rule of thumb is the latch point is displaced and the deformed spring is released.

Advantages of MCB

  • Easy to operate and user-friendly
  • It comes in a compact size so it saves time and space.
  • MCBs have a long lifetime.
  • It is recyclable so it is environmentally friendly.
  • It has less maintenance and replacement cost.
  • It is universal, offering protection and disconnector properties in one unit.
  • It is resettable.
  • It causes a complete shutdown by disconnecting all poles.
  • They are highly sensitive and offer full protection.

Disadvantages of MCB

  • One of its demerits is that a limited number of accessories could be mounted on MCBs.
  • It reacts slowly to short circuits as compared to semiconductor fuses.
  • Some types of MCB cost more than the others.

Wrapping up

Protection of a circuit is imperative, without it the smooth supply of electricity is next to impossible. From several circuit breakers, miniature circuit breakers stand apart in terms of quality, reliability, and accuracy. A short circuit, overload, and a sudden touch of a human with the live war can be disastrous but thanks to the very quick and efficient breaker that breaks the circuit immediately and opens the contact. Hence, if you want your circuit to be safe and secure; be it a commercial, domestic, or industrial use, miniature circuit breakers serve at all locations.


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