What is an Air Circuit Breaker (ACB)? Structure and principle of operation

An Air Circuit Breaker (ACB) is a type of low-voltage electrical switching device widely used in industrial distribution panels and construction projects. This device features a significantly high breaking capacity, far surpassing conventional circuit breakers. In the following overview, BTB Electric will help you understand the structure, working principle, and applications of ACBs.

Definition of Air Circuit Breaker (ACB)

An Air Circuit Breaker (ACB) is a main low-voltage circuit breaker used in 3-phase 380V industrial electrical systems. It has a large physical size, an automatic trip mechanism without manual levers, and a different handle mechanism compared to other types of circuit breakers.

The ACB adheres to the IEC/EN 60947-2 standard, with a rated operational voltage of 690V (AC 50/60Hz), breaking capacity up to 135kA, rated current from 630A up to 10,000A, and impulse withstand voltage of 12kV.

Functions of Air Circuit Breakers

The primary functions of an ACB are switching electrical circuits and protecting the system and equipment from overloads and short circuits. With high technical specifications, ACBs serve as main circuit breakers for load centers or outdoor feeders. This is a critical component ensuring the safety of power systems in industrial plants, large construction sites, and regional substations.

Though typically installed in main switchboards, ACBs are durable enough for outdoor installation without compromising performance.

Construction of Air Circuit Breaker (ACB)

ACBs consist of two main components: external and internal structures.

External Components

• Arc chute
• Control terminal block
• Digital display
• OFF button
• ON button
• Energy storage mechanism indicator
• Main contact status indicator
• Reset (RST) button
• LED indicators
• Controller
• Technical data nameplate
• IN/OUT rocking handle

Other external parts include:

• Mechanical energy storage handle
• Trip lockout button; rocker latch
• Connection lock button
• Position indicator
• Locking mechanisms

Internal Components

• Sheet metal supporting frame
• Current transformer for protection
• Pole group insulation housing
• Horizontal terminal blocks
• Arc chute
• Protective tripping unit
• Terminal box
• Closing spring
• Operating mechanism for opening/closing
• Movable/fixed contact and arc plate assemblies
• Arc chamber
• Fixed terminal connectors (fixed ACBs) – sliding contacts (draw-out ACBs)

Operating Principle of Air Circuit Breakers

The core task of an ACB is to interrupt and prevent arc re-ignition after current zero-crossing. ACBs differ from other switching devices because their contacts operate in free air, and the arc extinction mechanism is unlike that in oil circuit breakers.

During arc interruption, the ACB generates arc voltage to replace the system voltage, maintaining only minimal voltage to sustain the arc. Arc voltage increases through cooling of arc plasma, segmentation into arc strings, or extension of the arc path.

A typical ACB includes two pairs of contacts:

• Main contacts (copper): carry current under normal loads
• Arc contacts (carbon): handle arc interruption

When the breaker trips:

• The main contacts open first, with no arc forming.
• The arc contacts then separate, creating an arc, which is suppressed by arc chutes.
  

Due to thermal and electromagnetic forces, the arc rises and is driven into the arc chute where it is cooled, stretched, and divided. The arc voltage quickly exceeds the system voltage, leading to arc extinction at current zero.

Advantages and Disadvantages of Air Circuit Breakers

Advantages

• Suitable for frequent switching
• Minimizes fire risk
• Space-efficient panel design
• Fast arc quenching ensures system safety
• Long lifespan and low maintenance requirements

Disadvantages

• High initial investment cost
• Less efficient for low-capacity systems
• Potential air leakage in conduits
• Arc movement in chutes may degrade over time

Common Types of Air Circuit Breakers

Air Circuit Breakers are widely applied in high-capacity electrical systems and can also be used in residential applications. Common types include:

• Plain Break Type ACB / Cross-Blast ACB
• Magnetic Blowout Type ACB
• Air Chute Type ACB
• Air Blast Circuit Breaker

Plain Break Type ACB / Cross-Blast ACB

This basic ACB type uses horn-shaped contacts. The arc stretches across the horns, and the arc chute is made from heat-resistant materials, primarily to cool the arc. Inside the chute are metal plates forming compartments, each acting as a mini arc chamber. The arc is split into smaller arcs, raising arc voltage above the system voltage. These are suitable for low-voltage systems.

Magnetic Blowout Type ACB

Used up to 11kV, this ACB utilizes a magnetic field generated by a blowout coil to extend and deflect the arc. The magnetic field controls arc motion into an arc chute for cooling and quenching. Arc displacement is facilitated by discharge current through the blowout coils.

Air Chute Type ACB

This ACB contains main copper contacts with low resistance and silver plating. Arc contacts are made from heat-resistant copper alloys. The breaker has:

• Main contacts: made from copper/silver plates
  
• Arc/supplementary contacts: heat-resistant, easily replaceable

When operated, arc contacts open before and close after the main contacts.

Air Blast Circuit Breaker

Used in systems rated 220kV, 440kV, or higher, particularly during emergencies. There are three types:

• Cross-blast
• Axial-blast
• Axial-blast with moving contacts

Advantages:

• Eliminates fire hazards
• Extremely fast tripping time
• Uniform arc duration across all fault currents
  

Applications of Air Circuit Breakers

ACBs are used to control power systems and auxiliary devices in power plants and industrial electrical networks. They protect vital equipment such as:

• Generators
• Capacitors
• Transformers

ACB arc blowout mechanisms are applicable in both DC and AC circuits up to 12kV. Their high resistance helps increase arc resistance via stretching, cooling, and arc lengthening. ACBs are also used in distribution systems up to 15kV, safeguarding surrounding areas from fire or explosion risks.

BTB Electric's Air Circuit Breaker Solutions

BTB Electric's ACBs comply with IEC/EN 60947-2, offering high flexibility with rated currents from 630A to 6300A and high reliability at voltages up to 12kV. The breaking capacity reaches an impressive 135kA. BTB Electric provides three frame sizes: 2000A, 4000A, and 6300A.

ACB accessories include:

• Shunt trip coils
• Undervoltage trip coils (standard and time-delay)
• Motor charging units
• Auxiliary contacts
• Secondary wiring connectors
• Key locks
• Interlocks
• Lockout testers
• Door label kits

BTB Electric offers comprehensive information to help users identify, differentiate, and select the right ACB for their application, improving overall electrical system safety.