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air circuit breaker

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An air circuit breaker (ACB) is an electrical device used in low voltage applications (typically below 450V) to protect circuits from overcurrents and short circuits. Here's a breakdown of how it works:


  • Fixed and Moving Contacts: These contacts are responsible for opening and closing the circuit. The fixed contacts are mounted within the breaker housing, while the moving contacts are connected to a tripping mechanism.
  • Arc Chute Chamber: This chamber is filled with insulating material and designed to extinguish any electrical arc that forms when the contacts separate.
  • Tripping Mechanism: This mechanism triggers the opening of the contacts based on various factors like overload current or short circuit. It can be thermal or magnetic depending on the type of fault it's designed to detect.

Working Principle:

  1. Normal Operation: Under normal conditions, the contacts of the ACB remain closed, allowing current to flow freely through the circuit.
  2. Overload Protection: If the current exceeds a predetermined safe limit for a certain duration, a thermal trip unit within the tripping mechanism heats up and triggers the release of the moving contacts. This sudden separation creates an arc, but the arc chute chamber extinguishes it quickly.
  3. Short Circuit Protection: In case of a sudden surge in current due to a short circuit, the magnetic trip unit in the mechanism reacts almost instantaneously. The magnetic force generated by the high current overcomes the holding force on the moving contacts, causing them to separate rapidly. The arc chute chamber again extinguishes the resulting arc.
  4. Resetting: Once the fault is cleared, the ACB can be manually reset by moving the operating handle to the "on" position. Some ACBs may also have automatic reset features.

Benefits of Air Circuit Breakers:

  • High Speed Operation: ACBs can interrupt faults very quickly, minimizing damage to electrical equipment.
  • Wide Current Range: They can handle a wide range of current ratings, making them suitable for various applications.
  • Arc Quenching: The arc chute chamber effectively extinguishes arcs, reducing the risk of fire and explosions.
  • Relatively Less Maintenance: Compared to some other breaker types, ACBs require less frequent maintenance.


  • Distribution Panels: ACBs are commonly used as main breakers in distribution panels to protect branch circuits from overcurrents.
  • Motor Control Centers: They play a crucial role in protecting motors from starting current surges and overload conditions.
  • Industrial Plants: ACBs are essential for safeguarding electrical equipment and ensuring safety in industrial settings.


  • Voltage Range: ACBs are limited to low voltage applications. For higher voltage circuits, different types of breakers are needed.
  • Current Interrupting Capacity: While they can handle high currents, there's a limit to their fault current interrupting capacity.

Overall, air circuit breakers are a vital component in ensuring safe and reliable operation of electrical systems by protecting circuits from overcurrents and short circuits.