WHAT is MCB?

A Miniature Circuit Breaker is an electromechanical device designed to protect an electric circuit from over-current - A term to describe an electrical fault caused by either overload or short circuit.

Types of MCB (Miniature Circuit Breaker)

Several different MCB types are available – types A, B, C, D, K, and Z. However, the three key versions are type B, type C, and type D. Each is designed to be responsive to the likely strength of electrical surges in different settings. These variations are typically known as their ‘trip curve’ but can also be known as their tripping characteristics or overcurrent characteristics.

B Type MCB

Tripping Current: 3-5 times the rated current

Purpose: Protecting the electrical circuits of equipment that does 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 causes surge current

Application: Used in commercial or industrial facilities where there is a risk of excessive 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. 

How to select an MCB?

When selecting an MCB (miniature circuit breaker), the below parameters should be considered:

1) Rated current

Rated current is the maximum current value the circuit breaker can withstand without tripping. MCBs have a rated current from 0,5A to 125A. First, you should determine the total current of the circuit. Then, you should select the proper MCB.

The current rating of MCBs should not be greater than the cable current carrying capacity. It should also be greater than or equal to the maximum current that the system is expected to carry.

Do not use an MCB lower than the total current of the circuit. Otherwise, it trips unnecessarily.

2) Tripping characteristics

Tripping characteristics defines the current vs tripping time relationship. They are differentiated based on the instantaneous tripping range. You should determine the tripping class according to the loads to be protected.

Typical loads by trip curve:

B Curve

Designed for cable protection. Suitable for resistive loads such as electrical heating, water heater and stoves, and long cable runs (fault loop impedance)

C Curve

Designed for medium magnetic startups. Suitable for common loads such as lighting, socket outlets, small motors.

D and K Curves

Designed to allow for high inrush loads. Suitable for high inrush loads and transformers.

Z Curve

Designed to protect circuits that need a very low short circuit trip setting. Suitable for semiconductors.

3) Number of poles

You need to select the appropriate number of poles to provide the necessary protection. Based on the number of poles, the MCBs are classified as:

Single pole (1): Single pole MCBs are used for protecting single-phase circuits, where neutral is taken directly from the incoming supply.

Single pole and neutral (1+N): Single pole and neutral types of MCBs are used when the phase has to be protected, neutral has to be isolated. These are generally used for small loads.

Double pole (2): Double pole MCBs are used when both phase & neutral have to be protected, and complete isolation of supply is required for maintenance. These are generally used for small loads, where only one MCB is used for switching/protecting the entire load.

Triple pole (3): Generally used for the three-phase motor application, where neutral is not required. These are generally used for motor protection.

Triple pole and neutral (3+N): Used for incoming in distribution boards, where neutral isolation is required.

Four pole (4): Used for incoming in distribution boards, where neutral protection is required.

4) Breaking capacity                                                                                                     

Breaking capacity is the maximum current that an MCB can carry and break. High currents above the breaking capacity could damage the MCB. 10kA or 6kA breaking capacities are suitable for residential usage. MCBs with high breaking capacities can be used in industrial applications.

How an MCB Works?

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

1.     Thermal Tripping

2.     Magnetic Tripping

Thermal Tripping:

The thermal trip unit protects against overload currents. 

The thermal unit is based on a bimetal element located behind the circuit breaker trip bar and is part of the breaker’s current-carrying path. 

 

When there is an overload, the increased current flow heats the bimetal causing it to bend. As the bimetal bends it pulls the trip bar which opens the breaker’s contacts.

The time required for the bimetal to bend and trip the breaker varies inversely with the current. 

Magnetic Tripping:

 

The magnetic trip unit protects against a short circuit. The magnetic trip unit is comprised of an electromagnet and an armature.

When there is a short circuit, a high magnitude of current passes through the coils creating a magnetic field that attracts the movable armature towards the fixed armature.

 

The hammer trip is pushed against the movable contact and the contacts are opened.