Air Circuit Breaker (or Air Blast Circuit Breaker): What is it?

What is Air Circuit Breaker?

An Air Circuit Breaker uses air at atmospheric pressure around its contacts and arc chute. An Air Blast Circuit Breaker instead uses a stored source of compressed air for interruption. The abbreviation ACBcommonly refers to a low-voltage air circuit breaker, so the two names should not be treated as exact synonyms. When the device opens current, an arc forms between its separating contacts and the designed interruption system extinguishes it.

A low-voltage air circuit breaker uses an arc chute rather than the bulk insulating medium of an oil circuit breaker. Current IEC rules cover low-voltage breakers up to 1,000 V AC or 1,500 V DC. Equipment above that voltage range falls under high-voltage switchgear standards.

Older medium-voltage installations used air-break and compressed-air designs, but the original country-by-country preference claim is not supported as a current rule. Modern application depends on the installed equipment, system duty and replacement programme. Common alternatives include vacuum circuit breakers and SF₆ circuit breakers, with lower-impact technologies increasingly used where suitable.

A fuse normally requires replacement after it interrupts a fault. A circuit breaker can be reclosed after the fault is cleared and the equipment is confirmed safe, subject to its operating instructions. A remote controlled circuit breaker may also support control from a protected location.

Working Principle of Air Circuit Breaker

An AC circuit breaker must extinguish the arc near current zero and restore enough dielectric strength to withstand the recovery voltage. Air-break and air-blast designs achieve this with different mechanisms.

In a current-limiting air circuit breaker, the arc chute raises arc voltage and removes energy from the discharge. This supports interrupting arc current, but a breaker does not universally require arc voltage to exceed the full supply voltage. The frozen list describes three common arc-control effects.

1. One method to increase the arc voltage involves cooling the arc plasma, reducing the mobility of particles within the plasma and requiring a higher voltage gradient to sustain the arc.
2. It may increase the arc voltage by lengthening the arc path. As the length of arc path is increased, the resistance of the path is increased, and hence to maintain the same arc current more voltage is required to be applied across the arc path. That means arc voltage is increased.
3. Splitting up the arc into a number of series arcs also increases the arc voltage.

Types of ACB

This page covers two historically related but technically distinct air-interruption designs:

1. Plain air circuit breaker.
2. Air blast Circuit Breaker.

Operation of ACB

A low-voltage ACB arc chute uses the three effects described in the frozen list below:

1. The first objective is usually achieved by forcing the arc into contact with as large an area as possible of insulating material. Every air circuit breaker is fitted with a chamber surrounding the contact. This chamber is called ‘arc chute’. The arc is driven into it. If inside of the arc chute is suitably shaped, and if the arc can be made conform to the shape, the arc chute wall will help to achieve cooling. This type of arc chute should be made from some kind of refractory material. High temperature plastics reinforced with glass fiber and ceramics are preferable materials for making arc chute.
2. The second objective that is lengthening the arc path, is achieved concurrently with fist objective. If the inner walls of the arc chute is shaped in such a way that the arc is not only forced into close proximity with it but also driven into a serpentine channel projected on the arc chute wall. The lengthening of the arc path increases the arc resistance.
3. The third technique is achieved by using metal arc slitter inside the arc chute. The main arc chute is divided into numbers of small compartments by using metallic separation plates. These metallic separation plates are actually the arc splitters and each of the small compartments behaves as individual mini arc chute. In this system the initial arc is split into a number of series arcs, each of which will have its own mini arc chute. So each of the split arcs has its own cooling and lengthening effect due to its own mini arc chute and hence individual split arc voltage becomes high. These collectively, make the overall arc voltage, much higher than the system voltage.

The following paragraphs apply that working principle of air circuit breaker to a common low-voltage contact and arc-chute arrangement.

An ACB needs a designed interruption system throughout its rated range; the original statement that equipment below 1 kV needs no arc-control device was incorrect. Many power ACBs use main contacts for continuous current and separate arcing contacts to protect the main contact surfaces during opening.

The main contacts carry normal current. In designs with separate arcing contacts, the main contacts open first while the arcing contacts remain closed briefly. The exact contact materials and sequence are set by the circuit breaker manufacturer.

Current transfers to the arcing contacts as the main contacts separate. When the arcing contacts then open, the arc runners guide the discharge into the arc chute through electromagnetic and thermal forces. This sequence limits erosion of the main contacts.

The arc chute cools, lengthens and divides the arc between splitter plates. During the operation of air circuit breaker, these effects increase arc voltage and reduce arc energy until interruption occurs.

Air power circuit breakers remain current products for low-voltage, high-current applications. Older medium-voltage air-break equipment may remain installed, but its condition and replacement needs require model-specific assessment.

Air Blast Circuit Breaker

Compressed-air circuit breaker designs were used at high system voltage, including transmission-class installations. They are legacy equipment rather than the general modern meaning of ACB. The frozen list records historical attributes of the Air blast circuit breaker, but actual performance and maintenance depend on the model.

1. There is no chance of fire hazard caused by oil.
2. The breaking speed of circuit breaker is much higher during operation of air blast circuit breaker.
3. Arc quenching is much faster during operation of air blast circuit breaker.
4. The duration of arc is same for all values of small as well as high currents interruptions.
5. As the duration of arc is smaller, so lesser amount of heat realized from arc to current carrying contacts hence the service life of the contacts becomes longer.
6. The stability of the system can be well maintained as it depends on the speed of operation of circuit breaker.
7. Requires much less maintenance compared to oil circuit breaker.

Historical air-blast systems also have operating constraints, recorded in the frozen list below:

1. In order to have frequent operations, it is necessary to have sufficiently high capacity air compressor.
2. Frequent maintenance of compressor, associated air pipes and automatic control equipments is also required.
3. Due to high speed current interruption there is always a chance of high rate of rise of re-striking voltage and current chopping.
4. There also a chance of air pressure leakage from air pipes junctions.

For the terminology used on this page, plain air circuit breaker means an air-break design at atmospheric pressure. The following frozen list classifies compressed-air designs by blast direction and contact arrangement.

1. Axial Blast ACB.
2. Axial Blast ACB with side moving contact.
3. Cross Blast ACB.

Axial Blast Air Circuit Breaker

In the illustrated axial-blast design, spring force holds the moving and fixed contacts closed. The moving contact blocks a nozzle in the closed position. When a trip command operates the mechanism, stored compressed air enters the interruption chamber according to the breaker’s engineered sequence.

Air pressure and the operating mechanism separate the contacts and open the nozzle path. High-speed airflow then cools and stretches the arc. In AC service, interruption succeeds at current zero when the contact gap recovers enough dielectric strength for the applied recovery voltage.

Axial Blast ACB with Side Moving Contact

In the illustrated side-moving-contact arrangement, the moving contact is connected to a spring-supported piston. Admitted air operates the piston and separates the contacts. The resulting airflow transfers the arc to the arcing electrode and directs it along the contact axis. Timing and pressure values are specific to the breaker model.

Cross Blast Air Circuit Breaker

In a cross-blast air circuit breaker, the blast pipe directs compressed air across the path of contact movement. The air blast circuit breaker places an exhaust chamber opposite the inlet so that airflow passes through the contact gap.

Arc splitters divide the exhaust chamber. When the moving contact separates, cross-flow carries the arc into that chamber, where the splitters lengthen and cool it. The airflow and gap recovery then support interruption at AC current zero.