What is MCB (Miniature Circuit Breaker) and How Does it Work

double pole mini circuit breaker

Safety is a major consideration in selecting electrical equipment, appliances, or devices. Every piece of equipment and machine at homes, offices or factories needs a well-established electrical system. Without it, electrical devices can become vulnerable to damage from overloads and short circuits in the event of power fluctuations or high inrush currents.

Circuit breaker technology has advanced in recent years, and new techniques have been developed to curb power fluctuations and provide overload protection. One such technique is the use of the MCB (Miniature Circuit Breaker). To get you started, we'll go over some of the basics of a mini circuit breaker in this article.

What is a MCB (Miniature Circuit Breaker)?

MCBs are small electromechanical devices mainly used to protect circuits from overcurrent failures and ensure electrical safety in residences, offices, and industrial sites.

Most circuits now employ MCBs instead of fuse switches because they don't need to be replaced if an overload is detected. Unlike a fuse, an MCB operates as an automatic switch—opening and closing in the event of circuit fluctuations, providing increased operational safety and convenience without increasing running costs.

Standard Circuit Breaker vs. Miniature Circuit Breaker

There are three major differences between a standard circuit breaker and a miniature circuit breaker:

  • Tripping Mechanism: Circuit breakers use a relay mechanism, whereas MCBs use a release mechanism.
  • Breaking Capacity: Circuit breakers have a high rupturing capacity, while MCBs have a low rupturing capacity.
  • Voltage: Circuit breakers are used in systems involving high voltage, while MCBs are used in systems involving low voltage.

How Does a MCB (Miniature Circuit Breaker) Work?

There are two types of tripping mechanisms in MCB (Miniature Circuit Breaker):

  1. Thermal Tripping
  2. Magnetic Tripping

Thermal Trip Unit

magnetic thermal trip unit

The thermal trip unit is made up of a bi-metal thermal device that triggers the activation of a circuit breaker with a delay, depending on the overcurrent value. Its basic purpose is to provide overload protection.

In the event of an overload, the bimetal heats up and bends due to higher current flow. As a result, it moves the trip bar, which unlocks the breaker connections. The time it takes for the bimetal to bend and trip the breaker varies with the current.

Magnetic Trip Unit

magnetic trip unit

The magnetic trip unit provides short circuit protection. It is made up of an electromagnetic device coupled with a fixed or adjustable armature.

The strong current flowing through the coils creates a magnetic field that pulls the moving armature toward the fixed threshold. Then, the hammer trip pushes the moveable contact open.

IEC Standards for Low-Voltage Circuit Breakers

Before we go in-depth in discussing everything about miniature circuit breakers, it is vital that we know their application standards published by the International Electrotechnical Commission (EIC).

  • IEC 60947-2

This standard governs circuit breakers for industrial applications. They safeguard electrical power distribution of up to 1000 Volts AC and 1500 Volts DC, with rated currents ranging from 0,5 to 6300A. Air circuit breakers (ACBs), molded case circuit breakers (MCCBs), and mini circuit breakers are all often used in utilities and manufacturing facilities.

  • IEC 60898-1

According to this standard, the highest rated current is 125A, while the lowest is a few amps, and the maximum value of rated short-circuit capacity (Icn) is 25kA. This standard also refers to AC low-Voltage circuit breakers for households and similar applications, such as the MCBs, which we often find in homes, schools, shops, and office electrical distribution switchboards.

Where are Miniature Circuit Breakers Used?

Miniature Circuit Breakers are frequently used in residential, light-industrial, or commercial settings. Aside from being generally used as a substitute for fuses, the following are the five particular applications of a mini circuit breaker:

1. For Residential Electrical Panels

Unlike the traditional fuse, mini circuit breakers are excellent for residential applications since they are particularly reliable at preventing current overload. In addition to having the capacity to handle great amounts of electricity, they also evenly distribute electricity to all the devices, making power fluctuations in the house less likely.

2. For Lighting System

Because MCBs distribute power evenly throughout all devices, they are particularly helpful in lighting systems that utilize varying quantities of electricity. Furthermore, miniature circuit breakers also serve to ensure the quality and safety of light bulbs.

3. For Industrial Applications

Among the most common places where miniature circuit breakers can be used are hotels, supermarkets, and bakeries. The majority of these commercial and industrial machinery require up to 30 kA of power supply, necessitating the shift to MCBs to optimize energy flow.

4. For Heater Equipment

As heaters consume a lot of power, they develop high voltage fluctuations easily. Hence, a higher chance of tripping the circuit breaker. This is what miniature circuit breakers are trying to avoid.

5. With Ground Fault Trip Mechanism

In the event of a ground fault, miniature circuit breakers will serve as supplementary protection along with a Ground Fault Circuit Interrupter (GFCI) by automatically initiating its magnetic tripping mechanism and cutting the power off.

What are the Different Types of MCB (Miniature Circuit Breaker)?

miniature circuit breaker infographic

MCBs are classified based on their trip curves to suit different types of applications. The 5 types of miniature circuit breakers are as follows:

B Type MCB

Tripping Current: 3-5 times the rated current

Purpose: Protecting the electrical circuits of equipment that do not cause surge current

Application: Mainly used in residential applications or light commercial applications where connected loads are primarily lighting fixtures and domestic appliances with mainly resistive elements

C Type MCB

Tripping Current: 5-10 times the rated current

Purpose: Protecting the electrical circuits of equipment that cause surge current

Application: Used in commercial or industrial facilities where there are more risks of excessive power surges and short circuit currents. This includes small transformers, lighting, pilot devices, control circuits, and coils

D Type MCB

Tripping Current: 10-20 times the rated current

Purpose: Protecting the electrical circuits that cause high inrush current, typically 12-15 times the thermal rated current

Application: Used in particular industrial/commercial facilities where current inrush can be very high; concerned equipment include transformers or X-ray machines, motors, power supplies, etc.

K Type MCB

Tripping Current: 8-12 times the rated current

Purpose: Protecting windings in motors and transformers and simultaneous overcurrent protection of cables. Also suitable for inductive and motor loads with high inrush currents.

Application: Mainly used in commercial and industrial facilities that have circuits like motors, transformers, and auxiliary circuits

Z Type MCB

Tripping Current: 2-3 times the rated current

Purpose: Protecting and controlling electronic circuits against weak and long-duration overloads and short circuit

Application: Mainly used in commercial and industrial control circuits with high impedance and voltage converter circuits, including highly sensitive semiconductor devices

MCB (Miniature Circuit Breaker) plays an important role in the safe and efficient operation of all electrical systems across households and commercial and industrial facilities. That is why using first-class circuit breakers is so critical.

What are the Advantages of a Miniature Circuit Breaker?

After covering all of the technical aspects of a mini circuit breaker, let's now look at its practical advantages.

1. Highly Responsive to Current

Compared to fuses, MCBs have greater sensitivity for detecting changes in current flow. As a result, even in the event of a sudden short circuit or an overcurrent, electrical devices, equipment, and machines will be safe from damage.

2. Easy to Use

Designed with a simple and trip-free switching mechanism, miniature circuit breakers can instantly turn off energy connections in the case of an overload, even if the operating lever is maintained in the ON position. At the same time, it can promptly recover the power supply immediately after.

3. Broad Model Option

As aforementioned, there are various types of MCBs, each with a unique trip curve to meet the diverse natures of any electrical application. Furthermore, MCBs have varying breaking capabilities in order to cut the maximum current in the circuit.

4. Cost-effective

MCBs are built to last longer and are less maintenance-intensive than fuse switches. Their replacement cost is also significantly lower since these electrical devices may be reused whenever necessary. This makes them environmentally friendly as well.

How to Choose the Right MCB (Mini Circuit Breaker)?

In choosing the right MCB for any industry, you must consider a few steps: First, determine the voltage and frequency at which the device operates, as well as whether it is AC or DC. Following that, take note of the starting current load and trip curves characteristics. It would also be beneficial to learn about any additional features, such as a manual or automated reset, as well as safety approvals.

Keep in mind that while selecting the right MCB for industrial applications, operational variables such as temperature, corrosion, and radiation must also be taken into account.

Miniature Circuit Breaker Lockout Devices

Miniature circuit breakers are one of the critical power sources that must be monitored in an industrial setting, particularly during repair or maintenance. Because MCBs are so simple to operate, they may be accidentally switched ON, which can lead to accidental shocks and injury. This happens when a circuit prematurely reactivates while maintenance or repair is being carried out in the area.

Using mini circuit breaker lockout tagout devices can help avoid these incidents. These devices work by isolating hazardous energy from the system; physically locking the system in a safe mode.

Here are the most common types of miniature circuit breaker lockout tagout devices:

Miniature Circuit Breaker Lockout Device – Pin Out Standard (POS)

tradesafe miniature circuit breaker pos

As standard, the Pin Out Standard (POS) design features outward-facing pins and a push-button mechanism for faster deployment. Generally, it accommodates up to 0.5” hole spacing and accepts 1 circuit breaker padlock. Its nylon circuit breaker lock body is also electrically resistant and impact-resistant. No need to use tools for installation.

Looking for a POS mini circuit breaker lockout? Order here.

Miniature Circuit Breaker Lockout Device – Pin Out Wide (POW)

tradesafe miniature circuit breaker pow

The Pin Out Wide (POW) with an outward-facing pins design is excellent for locking out wider-spaced toggle holes of miniature circuit breakers. It commonly features a push-button mechanism for faster deployment and accepts 1 circuit breaker padlock.

Its nylon body is also electrically resistant and impact-resistant. No need to use tools to install it.

Looking for a POW mini circuit breaker lockout? Order here.

Miniature Circuit Breaker Lockout Device – Pin In Standard (PIS)

tradesafe miniature circuit breaker pis

The Pin In Standard (PIS) design features inward-facing pins with a push-button mechanism for faster deployment. Generally, it is designed to accept 1 circuit breaker padlock, while its nylon body is electrically resistant and impact-resistant. No need to use tools to install it.

Looking for a POW mini circuit breaker lockout? Order here.

Miniature Circuit Breaker Lockout Device – Tie Bar Lockout (TBLO)

tradesafe miniature circuit breaker tblo

A Tie Bar Lockout (TBLO) is used for cases where pin-in or pin-out devices are not suitable to lockout non-standard mini circuit breakers. These LOTO devices are typically made of industrial-grade plastic and have a thumbscrew locking mechanism.

Its design also allows for quick and secure lockout of most tie bars and multi and single pole miniature circuit breakers (MCBs).

Looking for a POW mini circuit breaker lockout? Order here.

Miniature Circuit Breakers are revolutionary electrical devices that continue to make living better and safer. From their advanced working and operation to their practical benefits, MCBs are far better than fuses and switches at every level.

For your lockout circuit breaker needs, partner with TRADESAFE; a trusted USA-based and industry-leading supplier. Every lockout tagout device we make is built to last and is compliant with government-mandated LOTO standards, which we adhere to in both the design and production process. At TRADESAFE, our top priority is to help every workplace comply with OSHA safety requirements.

Find out more about TRADESAFE or shop your lockout tagout devices here.

The material provided in this article is for general information purposes only. It is not intended to replace professional/legal advice or substitute government regulations, industry standards, or other requirements specific to any business/activity. While we made sure to provide accurate and reliable information, we make no representation that the details or sources are up-to-date, complete or remain available. Readers should consult with an industrial safety expert, qualified professional, or attorney for any specific concerns and questions.


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Author: Herbert Post

Born in the Philadelphia area and raised in Houston by a family who was predominately employed in heavy manufacturing. Herb took a liking to factory processes and later safety compliance where he has spent the last 13 years facilitating best practices and teaching updated regulations. He is married with two children and a St Bernard named Jose. Herb is a self-described compliance geek. When he isn’t studying safety reports and regulatory interpretations he enjoys racquetball and watching his favorite football team, the Dallas Cowboys.