20th September 2020

Miniature Circuit Breaker(MCB) construction and Types

Miniature Circuit Breaker(MCB)
Photo Credit: ABB products
What is a Miniature Circuit Breaker (MCB)?

Miniature Circuit Breaker(MCB) is used for making and breaking an electric circuit, breaking is done under an abnormal condition like overcurrent(when circuit carried current more than the anticipated/rated ampere capacity) and short circuit condition.

MCBs are commonly used in Low Voltage system and it is more sensitive to faults when compared to fuse.

Construction of Miniature Circuit Breaker(MCB)

To understand the construction of Miniature Circuit Breaker(MCB), ABB MCB is selected for better visualization and understanding

Thermal / Magnetic trip units

During Overcurrent event, the electromagnetic trip unit is activated to open the circuit breaker. Thermal trip unit is temperature sensitive and the magnetic trip unit is current sensitive. Both trip unit will function independently.

MCB Thermal / Magnetic trip units
Photo Credit: ABB products

During short circuit, a high magnitude of current flows through the coils creating a magnetic field that attracts the movable armature towards the fixed armature as shown in the below image. The hammer trip is pushed against the movable contact and the contacts are opened. Commonly operating time of breakers contacts during a short circuit is completed in .5 milliseconds.

MCB fixed contact moving contact
Arc runners / Arc chutes

While overload or short circuit occurs and during the opening of the contact arc is formed between the contacts. to encounter this arc, the arc will be moved into the arch chute by running the arc down the interior of the breaker along the arc runner. When the arc reaches the arc chute it is broken into small segmented arcs and arcs split the overall energy level into segments

 Arc runners / Arc chutes MCB
Thermal Trip Unit

The thermal unit is consists of 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. Because of this, the tripping time becomes quicker as current increases in magnitude.

MCB Thermal Trip Unit

Types of MCB based on Number of Poles

MCB
 Single Pole (SP) MCB:

Suitable for protection against overload & short circuit for high inrush current loads for single-phase circuit(only phase is connected to Circuit breaker).

Double Pole (DP) MCB

Suitable for protection against overload & short circuit for high inrush current loads for single-phase circuit(phase and neutral are connected to Circuit breaker).

Triple Pole (TP) MCB

Suitable for protection against overload & short circuit for high inrush current loads for the three-phase circuit(all 3 phase are connected to Circuit breaker and neutral not connected to Circuit breaker ).

Three Pole with Neutral(TPN MCB)

Suitable for protection against overload & short circuit for high inrush current loads for the three-phase circuit(all 3 phase are connected to Circuit breaker and neutral also connected to Circuit breaker but without any protection).

Four Pole (4P) MCB

Suitable for protection against overload & short circuit for high inrush current loads for the three-phase circuit(all 3 phase and neutral are connected to Circuit breaker).

Types of MCB based on Trip Curves

MCB triping curves

Circuit breaker(MCBs) are classified into various tripping characteristics depending on the type of component or equipment to be protected and over the range of fault current. The important MCB tripping characteristics are as follows as per IEC/EN 60947-2 standards:

Type B MCB

Type B MCBs are used for overcurrent protection of cables. It trips as soon as the current rises between 3 to 5 times its rated current as shown in the above graph.

Type C MCB

Type B MCBs are used for overcurrent protection of cables. It trips as soon as the current rises between 5 to 10 times its rated current as shown in the above graph.

Type D MCB

Type C MCBs are used for overcurrent protection of cables. It trips as soon as the current rises between 10 to 20 times its rated current as shown in the above graph.

Type K MCB

Type K MCBs are used for protecting motors, transformers and simultaneous overcurrent protection of cables with overload tripping. It trips as soon as the current rises between 10 to 14 times its rated current as shown in the above graph.

Type Z MCB

Type Z MCBs are used for control circuits with high impedances, voltage converter circuits, semicable protection and simultaneous overcurrent protection of cables with overload tripping. It trips as soon as the current rises between 2 to 3 times its rated current as shown in the above graph.

LV Switchgear or Low Voltage Switchgear

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