Air circuit breaker working principle; Air circuit breaker

Air Circuit Breaker (ACB): Working Principle, Operation, Advantages & Disadvantages

Air Circuit Breakers (ACB) are among the most widely used types of circuit breakers, particularly in low-voltage (LV) and medium-voltage (MV) systems. As the name suggests, these breakers operate in atmospheric air pressure and use air as the arc-extinguishing medium.

Traditionally, ACBs were also deployed in medium-voltage networks up to 15 kV, but with advancements in vacuum and SF₆ circuit breakers, their use in MV systems has declined. However, for low-voltage high-current applications (above 800A), ACBs still remain a preferred and reliable choice.

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Working Principle of Air Circuit Breaker

The primary objective of any circuit breaker is to interrupt current safely and prevent the arc from restriking once the current reaches zero. This is achieved by creating conditions such that the contact gap can withstand the system recovery voltage.

In simple terms, arc quenching is done by ensuring that the arc voltage exceeds the system voltage. For ACBs, this is achieved through three main mechanisms:

1. Cooling the Arc Plasma – As temperature decreases, particle mobility reduces, requiring a higher voltage gradient to maintain the arc.
2. Lengthening the Arc Path – A longer arc increases resistance, thus requiring higher voltage for continuation.
3. Splitting the Arc into Series Arcs – Dividing the arc into smaller arcs using arc splitters collectively increases the overall arc voltage.

When the arc voltage exceeds the supply voltage, the arc is extinguished.

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Operational Procedure of ACB

ACBs are designed with arc chutes to manage and extinguish arcs. The operation process is as follows:

• Arc Chute Design:
• Constructed from refractory materials like glass fiber–reinforced plastics or ceramics.
• Equipped with metallic arc splitters that divide the arc into multiple smaller arcs.
• Each mini arc chute cools and lengthens the arc, raising arc voltage beyond system voltage.

• Contact Arrangement:
• Main Contacts – Made of copper, carry current under normal load.
• Arcing Contacts – Made of carbon, carry current during arc formation.
• During opening:
• Main contacts separate first (no arc at this stage due to parallel arcing path).
• Arc forms only when arcing contacts separate.
• The arc is guided upward into the arc chute by thermal and electromagnetic effects.
• Arc Quenching:
• Once inside the arc chute, the arc cools, splits, and lengthens.
• Arc voltage rises beyond system voltage at current zero, leading to successful quenching.

For low-voltage ACBs (<1 kV), arc control devices are often not required. But for higher short-circuit currents, ACBs with arc chutes are essential.

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Applications of Air Circuit Breakers

• Low-voltage power distribution systems in commercial and industrial plants.
• High-current LV switchgear (above 800A).
• Backup protection for MCCBs (Molded Case Circuit Breakers).
• Replacement of oil circuit breakers in environments where fire hazards must be avoided.

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Advantages of Air Circuit Breaker

• ✅ No fire hazard (unlike oil circuit breakers).
• ✅ High breaking speed and faster arc quenching.
• ✅ Reduced heat stress on contacts, improving service life.
• ✅ Stable system operation due to fast fault clearing.
• ✅ Lower maintenance requirements compared to oil circuit breakers.

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Disadvantages of Air Circuit Breaker

• ❌ Requires high-capacity air compressor for frequent operations.
• ❌ Maintenance of compressor, air pipes, and control systems is necessary.
• ❌ Risk of high rate of rise of restriking voltage (RRRV) and current chopping.
• ❌ Possibility of air leakage from joints and pipes.

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Air Circuit Breaker vs Other Circuit Breakers

Feature	Air Circuit Breaker (ACB)	Oil Circuit Breaker (OCB)	Vacuum Circuit Breaker (VCB)	SF₆ Circuit Breaker
Arc Medium	Air at atmospheric pressure	Mineral Oil	Vacuum	Sulphur Hexafluoride Gas
Voltage Range	LV & up to 15 kV	Up to 132 kV	Up to 36 kV	Up to 765 kV
Fire Hazard	None	High	None	None
Maintenance	Moderate	High	Low	Moderate
Applications	LV switchgear, industrial plants	Old substations	Medium voltage systems	EHV & UHV substations

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Conclusion

Air Circuit Breakers, though obsolete in medium-voltage applications, are still a reliable choice for low-voltage high-current systems due to their fast operation, safety, and low maintenance. With the advancement of VCBs and SF₆ breakers, their usage in higher voltage levels has decreased, but in industrial LV power systems, ACBs continue to hold significance.

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⚠️ Disclaimer

This article is for educational and informational purposes only. The design, operation, and maintenance of circuit breakers should be carried out strictly as per the manufacturer’s guidelines and relevant electrical standards (IEC/IS/IEEE). Always consult a certified electrical engineer before making decisions regarding circuit breaker selection and operation.

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