What is a Miniature Circuit Breaker?

What is a Miniature Circuit Breaker?

A Miniature Circuit Breaker (MCB) is a protective electrical device used in low-voltage power distribution systems. It is primarily used to protect circuits from overload and short circuits. It can automatically disconnect power in the event of an abnormal situation in the circuit, ensuring power safety.

How a Miniature Circuit Breaker works

The small circuit breaker consists mainly of parts such as the tentacle system, the arc-destroying device, the operator mechanism, and the unclasp. When the circuit is working normally, the circuit breaker is closed, and the current flows through the tentacle system to the load. When

MCB Knowledge Base

Understand the definition, classification, working principle, and key parameters of miniature circuit breakers to make informed selection decisions

What is a Miniature Circuit Breaker?

A Miniature Circuit Breaker (MCB) is a mechanical switching device that can make, carry, and break currents under normal circuit conditions, and also make, carry for a specified time, and break currents under specified abnormal conditions such as short circuits.

It is the most widely used terminal protection device in building electrical terminal distribution equipment, mainly used for overload and short circuit protection of lines and electrical equipment.

MCB Classification

Type B Circuit Breakers
Instantaneous tripping current is 3-5 times rated current, suitable for electronic circuit protection, long cable protection

Type C Circuit Breakers
Instantaneous tripping current is 5-10 times rated current, suitable for distribution protection, most commonly used

Type D Circuit Breakers
Instantaneous tripping current is 10-20 times rated current, suitable for equipment with high inrush currents, such as motors, transformers

Working Principle

Overload Protection
When the overload current in the circuit reaches 1.45In as specified in the standard, the bimetallic strip heats up and bends, triggering the mechanism to trip within the specified time, thereby protecting the equipment and circuit.
Short Circuit Protection
When a short circuit occurs in the circuit, the electromagnetic release senses the large current and quickly attracts the ejector rod, causing the circuit breaker to trip instantaneously, usually completed within 2-3ms.
Arc Extinguishing System
When the moving and static contacts open, an arc is generated, which moves towards the arc extinguishing chamber under the action of electric force. The arc extinguishing chamber cools the arc and divides it into many short arcs, eventually extinguishing the arc.

Mcb Working Principle Diagram

Closing Process

The energy storage mechanism releases energy, pushing the moving and static contacts to close and lock, conducting the circuit.

Overload Protection

The bimetallic strip heats up and bends, pushing the tripping mechanism to achieve overload protection.

Short Circuit Protection

The electromagnetic release senses the large current and quickly attracts the ejector rod, causing the circuit breaker to trip instantaneously.

Opening Process

The on command triggers the operator mechanism unlock, the contact points are quickly separated, and the arc extinguishes the medium to extinguish the arc to disconnect the circuit.

Key Parameters Explanation

Parameter NameSymbolDefinitionSelection Suggestion
Rated CurrentInMaximum current that the circuit breaker can carry continuously and stablyIn ≥ Circuit calculated current Ic, with 1.1~1.3 times margin
Rated VoltageUnVoltage used in the circuit breaker designUn ≥ Rated voltage of the line where it is located
Ultimate Breaking CapacityIcuMaximum short-circuit current value that the circuit breaker can breakIcu ≥ Maximum short-circuit current of the circuit where it is located
Service Breaking CapacityIcsMaximum short-circuit current value that the circuit breaker can continue to use after breakingImportant circuits require Ics=Icu to ensure reuse
Conventional Non-tripping CurrentIntMaximum current that the circuit breaker is not allowed to tripUsually 1.3In or 1.45In, depending on the standard
Conventional Tripping CurrentItMinimum current that the circuit breaker must tripUsually n times In, n value varies according to protection type

Frequently Asked Questions

How to select the rated current of a circuit breaker?

The rated current of the circuit breaker should be greater than or equal to the calculated current of the circuit, with a margin of 1.1~1.3 times. For example, if the calculated current of the circuit is 25A, a circuit breaker with a rated current of 32A should be selected. At the same time, the influence of ambient temperature should be considered. When the ambient temperature exceeds 40°C, derating is required.

What is the difference between Type B, C, and D circuit breakers?

The main difference lies in the multiple of instantaneous tripping current: Type B is 3-5 times In, suitable for electronic circuit protection; Type C is 5-10 times In, suitable for conventional loads such as lighting and ordinary sockets; Type D is 10-20 times In, suitable for loads with large starting currents such as motors and compressors.

Is the breaking capacity of the circuit breaker important?

Very important! Breaking capacity is the ability of the circuit breaker to reliably cut off the circuit under short-circuit conditions. If the breaking capacity is insufficient, it may not be able to reliably cut off the circuit in case of a short circuit, leading to circuit breaker explosion, arcing, and fire. Generally, terminal circuits can choose Icu≥6kA, and distribution room incoming lines need to choose Icu≥35kA.

What is the difference between 1P, 2P, 3P, and 4P circuit breakers?

P represents the number of poles, i.e., the number of contacts of the circuit breaker. 1P only disconnects the live wire; 2P disconnects the live wire and neutral wire; 3P is used for three-phase circuits, disconnecting three live wires; 4P disconnects three live wires and one neutral wire. The selection should be determined according to the circuit type and protection requirements.

Product Comparison

Compare technical parameters and performance characteristics of different products to make informed choices

ParameterLM47-63LMB1-63LMB1-63H
BrandLVMALVMALVMA
Number of Poles1P,2P,3P,4P1P,2P,3P,4P1P,2P,3P,4P
Rated Current63A63A63A
Protection TypeType B,C,DType B,C,DType B,C,D
Rated Voltage230/400V230/400V230/400V
Breaking Capacity4kA6kA10kA
Width18mm18mm18mm
Leakage ProtectionNoNoNo
Published On: 2026-02-12

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A Miniature Circuit Breaker (MCB) is a protective electrical device used in low-voltage power distribution systems. It is primarily used to protect circuits from overload and short circuits. It can automatically disconnect power in the event of an abnormal situation in the circuit, ensuring power safety.

How a Miniature Circuit Breaker works

The small circuit breaker consists mainly of parts such as the tentacle system, the arc-destroying device, the operator mechanism, and the unclasp. When the circuit is working normally, the circuit breaker is closed, and the current flows through the tentacle system to the load. When

MCB Knowledge Base

Understand the definition, classification, working principle, and key parameters of miniature circuit breakers to make informed selection decisions

What is a Miniature Circuit Breaker?

A Miniature Circuit Breaker (MCB) is a mechanical switching device that can make, carry, and break currents under normal circuit conditions, and also make, carry for a specified time, and break currents under specified abnormal conditions such as short circuits.

It is the most widely used terminal protection device in building electrical terminal distribution equipment, mainly used for overload and short circuit protection of lines and electrical equipment.

MCB Classification

Type B Circuit Breakers
Instantaneous tripping current is 3-5 times rated current, suitable for electronic circuit protection, long cable protection

Type C Circuit Breakers
Instantaneous tripping current is 5-10 times rated current, suitable for distribution protection, most commonly used

Type D Circuit Breakers
Instantaneous tripping current is 10-20 times rated current, suitable for equipment with high inrush currents, such as motors, transformers

Working Principle

Overload Protection
When the overload current in the circuit reaches 1.45In as specified in the standard, the bimetallic strip heats up and bends, triggering the mechanism to trip within the specified time, thereby protecting the equipment and circuit.
Short Circuit Protection
When a short circuit occurs in the circuit, the electromagnetic release senses the large current and quickly attracts the ejector rod, causing the circuit breaker to trip instantaneously, usually completed within 2-3ms.
Arc Extinguishing System
When the moving and static contacts open, an arc is generated, which moves towards the arc extinguishing chamber under the action of electric force. The arc extinguishing chamber cools the arc and divides it into many short arcs, eventually extinguishing the arc.

Mcb Working Principle Diagram

Closing Process

The energy storage mechanism releases energy, pushing the moving and static contacts to close and lock, conducting the circuit.

Overload Protection

The bimetallic strip heats up and bends, pushing the tripping mechanism to achieve overload protection.

Short Circuit Protection

The electromagnetic release senses the large current and quickly attracts the ejector rod, causing the circuit breaker to trip instantaneously.

Opening Process

The on command triggers the operator mechanism unlock, the contact points are quickly separated, and the arc extinguishes the medium to extinguish the arc to disconnect the circuit.

Key Parameters Explanation

Parameter NameSymbolDefinitionSelection Suggestion
Rated CurrentInMaximum current that the circuit breaker can carry continuously and stablyIn ≥ Circuit calculated current Ic, with 1.1~1.3 times margin
Rated VoltageUnVoltage used in the circuit breaker designUn ≥ Rated voltage of the line where it is located
Ultimate Breaking CapacityIcuMaximum short-circuit current value that the circuit breaker can breakIcu ≥ Maximum short-circuit current of the circuit where it is located
Service Breaking CapacityIcsMaximum short-circuit current value that the circuit breaker can continue to use after breakingImportant circuits require Ics=Icu to ensure reuse
Conventional Non-tripping CurrentIntMaximum current that the circuit breaker is not allowed to tripUsually 1.3In or 1.45In, depending on the standard
Conventional Tripping CurrentItMinimum current that the circuit breaker must tripUsually n times In, n value varies according to protection type

Frequently Asked Questions

How to select the rated current of a circuit breaker?

The rated current of the circuit breaker should be greater than or equal to the calculated current of the circuit, with a margin of 1.1~1.3 times. For example, if the calculated current of the circuit is 25A, a circuit breaker with a rated current of 32A should be selected. At the same time, the influence of ambient temperature should be considered. When the ambient temperature exceeds 40°C, derating is required.

What is the difference between Type B, C, and D circuit breakers?

The main difference lies in the multiple of instantaneous tripping current: Type B is 3-5 times In, suitable for electronic circuit protection; Type C is 5-10 times In, suitable for conventional loads such as lighting and ordinary sockets; Type D is 10-20 times In, suitable for loads with large starting currents such as motors and compressors.

Is the breaking capacity of the circuit breaker important?

Very important! Breaking capacity is the ability of the circuit breaker to reliably cut off the circuit under short-circuit conditions. If the breaking capacity is insufficient, it may not be able to reliably cut off the circuit in case of a short circuit, leading to circuit breaker explosion, arcing, and fire. Generally, terminal circuits can choose Icu≥6kA, and distribution room incoming lines need to choose Icu≥35kA.

What is the difference between 1P, 2P, 3P, and 4P circuit breakers?

P represents the number of poles, i.e., the number of contacts of the circuit breaker. 1P only disconnects the live wire; 2P disconnects the live wire and neutral wire; 3P is used for three-phase circuits, disconnecting three live wires; 4P disconnects three live wires and one neutral wire. The selection should be determined according to the circuit type and protection requirements.

Product Comparison

Compare technical parameters and performance characteristics of different products to make informed choices

ParameterLM47-63LMB1-63LMB1-63H
BrandLVMALVMALVMA
Number of Poles1P,2P,3P,4P1P,2P,3P,4P1P,2P,3P,4P
Rated Current63A63A63A
Protection TypeType B,C,DType B,C,DType B,C,D
Rated Voltage230/400V230/400V230/400V
Breaking Capacity4kA6kA10kA
Width18mm18mm18mm
Leakage ProtectionNoNoNo

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