Air Circuit Breakers (ACB): Working & Applications

In modern electrical power systems, ensuring safety and uninterrupted operation is one of the most critical responsibilities of electrical engineers. Whether in substations, industries, commercial buildings, or large mechanical plants, the reliability of power distribution depends heavily on protection devices. One such essential component is the Air Circuit Breaker (ACB). Before advanced vacuum and SF₆ circuit breakers were introduced, ACBs were the backbone of medium-voltage and low-voltage protection. Even today, ACBs remain widely used for their durability, simple construction, and strong arc-quenching capability.

Table of Contents

What is an Air Circuit Breaker (ACB)?

An Air Circuit Breaker is an electrical protection device that uses air as the arc-quenching medium. It is primarily used in low-voltage (LV) and medium-voltage (MV) power distribution systems up to 690V or sometimes 11kV depending on the design. ACBs protect electrical circuits from overload, short-circuit faults, under-voltage, and earth faults by automatically interrupting power flow when abnormalities occur.

Unlike older fuse-based systems, ACBs can be reset and reused, making them highly efficient for industrial systems where downtime is costly. They are mostly used in switchgear, distribution panels, and circuit protection in heavy mechanical loads.

Read also: Types of Circuit Breakers in Power Systems

Why Air is Used in ACBs

Air is an excellent insulating medium when properly compressed and controlled. It allows ACBs to extinguish arcs quickly without the need for costly gases like SF₆. Air’s natural dielectric properties make ACBs environmentally friendly, economical, and easy to maintain.

ACBs use forced air movement, arc chutes, and magnetic blowout coils to stretch and cool the arc until it extinguishes. This makes them suitable for high-power circuits such as motors, generators, and transformer feeders.

Working Principle of an Air Circuit Breaker

The working of an ACB is based on arc interruption and fault detection mechanisms. When the breaker is closed, current flows through the contacts. During a fault, the breaker trips automatically by sensing excessive current through protective relays.

Step-by-step working of ACB

Under normal operating conditions, the ACB remains closed and allows current to pass.
When a fault occurs, the relay sends a signal to the ACB operating mechanism.
The contacts separate, forming an arc between them.
The arc is driven into the arc chute where it is cooled and stretched.
The arc is finally extinguished as its temperature and ionization reduce below the sustaining level.
The circuit becomes open, preventing further damage to equipment.

This entire process happens within milliseconds, ensuring the safety of the electrical system.

Construction of an Air Circuit Breaker

An ACB consists of several mechanical and electrical components designed to interrupt faults safely.

Main parts of an ACB

Arc Chutes

These are metal plates that divide and cool the arc. They force the arc to split into smaller arcs, reducing heat and enabling fast extinguishing.

Contacts (Main, Arcing, and Auxiliary)

ACBs use multiple contact sets. The main contacts carry normal current. Arcing contacts withstand arc heat during opening. Auxiliary contacts are used for control and indication circuits.

Operating Mechanism

Usually spring-charged, this mechanism provides mechanical force to open or close the breaker. It may be manually or electrically operated.

Trip Units

Modern ACBs use electronic trip units that monitor current using microprocessors. They provide overload, short-circuit, earth-fault, and time-delay protection.

Frame and Enclosure

The frame supports all internal components and ensures operator safety.

The combination of these parts ensures smooth operation, high reliability, and efficient arc extinguishing.

Types of Air Circuit Breakers

ACBs can be classified based on construction, operation, and protection features.

1. Plain Break Type ACB

These older types rely solely on the separation of contacts. The arc is blown away using air movement. They are rarely used today.

2. Magnetic Blowout ACB

These breakers use a magnetic field to pull the arc away from the contacts. They are suitable for higher capacities.

3. Air Blast Circuit Breaker

A powerful blast of compressed air extinguishes the arc. These breakers are used at medium and high voltages.

4. Draw-out and Fixed Type ACB

Draw-out ACBs can be removed from service for maintenance without disconnecting cables. Fixed ACBs are permanently mounted.

Modern switchgear mostly uses draw-out type ACBs for safety and ease of maintenance.

Advantages of Air Circuit Breaker

ACBs remain popular because of their useful benefits:

• Simple and sturdy construction
• No risk of harmful gases like SF₆
• High reliability and easy maintenance
• Suitable for repeated operations without replacement
• Adjustable tripping settings for precision protection
• Ideal for industrial power distribution
• Long service life and excellent insulation properties

Their reusability and flexibility make them highly economical in long-term operation.

Disadvantages of Air Circuit Breaker

Although ACBs have many advantages, they also have a few limitations:

• Bulkier compared to MCCBs or VCBs
• Slower arc extinguishing than vacuum breakers
• Not suitable for very high voltage (>12kV)
• Requires periodic cleaning and maintenance
• Sensitive to dust and moisture

Despite these limitations, ACBs remain one of the best solutions for low-voltage switchgear.

Applications of Air Circuit Breakers

Air Circuit Breakers are widely used across industries due to their reliability and simplicity. Their major applications include:

• Electrical distribution panels
• Power plants and substations
• Industrial motor control centers
• Marine and railway electrical systems
• Large commercial buildings
• Generator and transformer protection
• Mechanical and HVAC plants

ACBs are ideal for high-power systems where safe and frequent switching is needed.

Importance of ACB in Electrical Power Systems

ACBs play a critical role in power protection by preventing damage to equipment and ensuring system stability. Engineers rely on ACBs because they offer flexible protection settings, remote control features, and high breaking capacity.

In mechanical industries, ACBs ensure that motors, compressors, and pumps are protected from overloads and faults. In electrical infrastructures, ACBs minimize downtime and contribute to smooth operation.

Conclusion: The Air Circuit Breaker is a vital component in modern electrical engineering, ensuring safe and reliable operation of power distribution systems. Its ability to break high currents, withstand mechanical stress, and operate repeatedly makes it an essential device for industrial and commercial installations. Even with the evolution of vacuum and gas circuit breakers, ACBs continue to hold significant importance due to their simplicity, reliability, and environmental friendliness.

For students, professionals, and enthusiasts, understanding ACBs is crucial for designing, maintaining, and troubleshooting safe electrical systems.

ACB vs MCCB – Comparison Table

Parameter	ACB (Air Circuit Breaker)	MCCB (Molded Case Circuit Breaker)
Full Form	Air Circuit Breaker	Molded Case Circuit Breaker
Operating Medium	Air used for arc quenching	Molded insulating case with arc chutes
Voltage Range	Mainly Low Voltage (LV) up to 690V	LV protection, typically up to 1000V
Current Rating	Very high (up to 6300 A and above)	Medium range (up to 1600 A)
Breaking Capacity	High	Moderate
Application Area	Industrial switchgear, substations, power plants	Commercial buildings, small industries, distribution panels
Protection	Advanced: overload, short circuit, earth fault, under-voltage	Standard: overload and short circuit; earth-fault optional
Trip Unit	Mostly electronic (advanced settings)	Thermal-magnetic or electronic
Use Case	Ideal for main power distribution	Used for outgoing feeders / branch circuits
Reset/Reuse Capability	Easily resettable, suitable for frequent operations	Resettable but not ideal for frequent switching
Maintenance	Higher, requires periodic servicing	Low, minimal maintenance required
Size	Larger and heavier	Compact and lightweight
Breaking Time	Slightly slower	Faster response time
Cost	Higher	Economical
Installation Type	Mostly draw-out type	Fixed or plug-in type
Communication / Monitoring	Supports advanced communication, metering and automation	Limited communication options

Frequently Asked Questions (FAQ)

1. What is an Air Circuit Breaker (ACB)?
An ACB is a protective electrical device that uses air to extinguish the arc and protect circuits from overload, short circuit, and other faults.

2. Where are ACBs commonly used?
They are used in power plants, substations, industrial distribution systems, commercial buildings, and generator panels.

3. What is the working voltage range of an ACB?
Most ACBs operate in low-voltage systems up to 690V, though some industrial types may reach 1kV.

4. How does an ACB interrupt a fault?
When a fault occurs, the relay sends a signal to trip the breaker. The contacts open, forming an arc. The arc chute cools and stretches the arc until it extinguishes.

5. Why is air used in ACBs?
Air is a natural, safe, and cost-effective insulating medium that helps cool and quench the arc without environmental impact.

6. What are the main parts of an ACB?
Key components include contacts, arc chute, operating mechanism, tripping unit, frame, and auxiliary contacts.

7. What is the difference between ACB and MCCB?
ACBs are used for higher currents (up to 6300 A) with advanced protection, while MCCBs are compact and used for currents up to around 1600 A.

8. Can ACBs be remotely operated?
Yes, modern ACBs support electrical ON/OFF commands and remote monitoring via communication modules.

9. What protections does an ACB provide?
It offers overload, short circuit, earth fault, under-voltage, and sometimes over-voltage protection depending on the trip unit.

10. What is an arc chute in an ACB?
It is a structure of metal plates that divide and cool the arc so it can be extinguished safely.

11. How often should an ACB be serviced?
Periodic maintenance is usually recommended every 6–12 months depending on usage, load, and environment.

12. What causes an ACB to trip frequently?
Common reasons include overload, short circuit, mechanical wear, poor contact pressure, or a faulty trip unit.

13. What is the breaking capacity of an ACB?
Breaking capacity refers to the maximum fault current the ACB can safely interrupt without damage.

14. Are ACBs environmentally safe?
Yes. Unlike SF₆ circuit breakers, ACBs do not use greenhouse gases and are environmentally friendly.

15. Can an ACB replace an MCCB?
Not usually. ACBs are designed for main incoming feeders, while MCCBs are meant for branch or outgoing feeders. Selection depends on system rating and design.