Technical Guides

What is Circuit Breaker?

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Circuit Breaker (CB) is the common name of the automatic switch (automatic breaker).

CB has the function of protecting overload and short circuit in electric system. Some CB lines have additional leakage protection function called aptomat against leakage or shock.

1. Aptomat Classification:

1.1 Classification by structure:

+ Miniature Circuit Breaker (MCB)

+ Moulded Case Circuit Breaker (MCCB)

1.2 Classification by function:

+ CB Overload protection and short circuit: MCB, MCCB.

+ CB leakage protection voltage: Residual Current Circuit Breaker (ECCB), Residual Current Circuit Breaker with Overcurrent Protection ( RCBO ), Earth Leakage Circuit Breaker (ELCB).

1.3 Sorting by phase number / number of poles:

+ 1 Phase / 1 Pole CB

+ 1-Phase + Neutral CB (1P + N): 2 poles

+ 2 Phase / 2 Pole CB: 2 poles

+ 3 Phase / 3 Pole CB: 3 poles

+ 3-Phase + Neutral CB (3P + N): 4 poles

+ 3 Phase / 3 Pole CB: 4 poles

1.4 Classification by short-circuit current:

+ Low cut current: commonly used in civilian.

+ Standard cutting line: often used in industry.

+ High shear line: commonly used in industry and special applications.

1.5 Classification according to the ability to adjust the current:

+ CB has a constant current rating. For example, Mitsubishi’s MCCB NF400-SW 3P 400A has a constant current rating of 400A.

+ CB adjust the rated current. For example, the MCCB NF400-SEW 3P 400A from Mitsubishi has an adjustable rating of 200A – 400A.

In this article, will introduce about MCB and MCCB.

2. MCB / MCCB structure

MCB / MCCB is composed of parts: contact, arc stamping box, MCB shear drive mechanism, protective hook.

+ Contact: MCB / MCCB usually has two types of contacts (main contact and arc) or 3 contacts (main contact, auxiliary contact, arc). The operation of the contact is as follows: when closing the arc contact circuit closed first, followed by the auxiliary contact, finally the main contact. When the main contact circuit breaker opens first, the auxiliary contact opens after and finally the electric arc.

+ Arc stamping box: There are 2 types of arc stamping equipment: semi-closed type arc and open type arc. Features of 2 different types: semi-closed type is located in the enclosure of MCB and has air holes. Open type is used for voltages greater than 50KA or large voltages of 1000V (high voltage). The arc has many steel plates arranged in a mesh into different sections to facilitate the extinguishing of the arc.

+ MCB / MCCB shear transmission mechanism: There are two ways of MCB / MCCB shear drive (manual and electromechanical). For manual shear drives which are carried out with MCB / MCCBs, the rated current is not large. As for the type of mechanical control in the MCB / MCCB has a larger current.

+ Protective hook: Protective hook works to protect electrical equipment from overload and short circuit. There are 2 types of protection hook: electromagnetic hook and thermal relay hook.

3. Principle of operation MCB / MCCB

MCB / MCCB uses temperature-sensitive device (thermal element), current-sensitive electromagnetic device (magnetic element) to provide shut-off mechanism for protection and isolation purposes.

In the case of overloads, a current more than the rated current is driven through the MCB. As the current flows through the bimetallic strip, it gets heated up and deflects by bending and releases the mechanical latch. Deflection time of bimetallic strip depends on the amount of current flowing through the strip. Higher the current faster will be the deflection of the bimetallic strip.

During short circuits, a transient current flowing through the solenoid forces the plunger towards the latch. This action instantaneous releases the mechanical latch and opens the contacts immediately.

Specifications of MCB / MCCB

+ In: Rated current. For example: MCCB 3P 250A 36kA, In = 250A.

+ Ir: the operating current is adjusted within the permissible range of Aptomat. For example, the current regulator 250A can be adjusted from 125A to 250A.

+ Ue: Rated working voltage.

+ Icu: The short-circuit current is able to withstand the maximum current of the contact for 1 second.

+ Icw: Ability to withstand short-circuit currents in 1 unit of time.

+ Ics: the actual ability to cut off when a device malfunction occurs. This ability depends on the manufacturer due to different manufacturing technology. For example, the same manufacturer has two types of MCCB, Ics = 50% Icu and Ics = 100% Icu.

+ AT: Ampe Trip

+ AF: Frame Ampere. For example, NF250A 3P 200A and NF250A 3P 250A both have AF = 250A but one will work when the current exceeds AT = 200A, one will work when the current exceeds AT = 250A. The AT / AF specification indicates the strength of the switch contact. For example, the Aptomat 250AT / 400AF will be more durable than the Aptomat 250AT / 250AF, the size of Aptomat 400AF is also larger, the price is higher.

+ Characteritic cuver: is the protective curve of CB (selected curve of CB). This is a very important parameter, which determines the position of CB in the electrical system.

+ Mechanical / electrical endurace: The number of permitted mechanical switches / permitted number of electrical switches.

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