In an air circuit breaker, we use the air as an arc quenching medium. The air has several advantages over oil as an arc extinguishing medium. These are as follows:
- The risk of fire and maintenance associated with the use of oil is eliminated.
- Mechanical stresses that are set up due to gas pressure and oil movement are eliminated.
- The cost of regular oil replacement due to deterioration of oil with successive breaking operations is absent.
Air Circuit Breaker Working Principle
In the air circuit breakers, we extinguish the arc by increasing its resistance. We lengthen the arc with the help of arc runners, and arc chutes and its resistance is increased by cooling, splitting, and lengthening.
The arc resistance is increased to such a value that the voltage drop across the arc becomes more than the system voltage, and the arc is extinguished at a current zero instant of AC wave. We can classify the ACB breaker into two types, namely:
- Plain Air Circuit Breakers
- Air Blast Circuit Breakers
Plain air circuit breakers further can be subdivided into the following categories:
- Plain Break Type
- Magnetic Blow-out Type
- Arc Chute Type
Plain break type air circuit breaker: Plain ACB is the simplest form of the air circuit breaker. In this type, contacts are made in the shape of two horns. The arc initially strikes across the shortest distance between the horns. It is then moved slowly upwards by the convection currents caused by the heating of air during arcing and the interaction of magnetic and electric fields.
The arc expands from one tip to the other when the horns are fully separated. It results in the lengthening and cooling of the arc. These circuit breakers are used up to 500 V and low power circuits because at higher ratings; the arc may spread to adjacent metal parts.
Magnetic blow-out type air circuit breaker: In this type of circuit breaker, arc extinction is achieved using a magnetic field produced by the current in blow-out coils connected in series with the circuit being interrupted. The magnetic field moves the arc into arc chutes where the arc is lengthened, cooled, and quenched.
The breaking action of such breakers becomes more effective with large currents. Therefore, these circuit breakers are used for high rupturing capacities and where voltage capacity is up to 11KV.
Arc Chute Type: The construction of an air circuit breaker (arc-chute type) used for low and medium voltage is shown in the Figure.
There are two sets of contacts in such breakers named main contacts and arcing contacts. The main contacts are usually made of copper and carry the current in the closed position of the circuit breaker. They are silver coated to make low contact resistance.
The arcing contacts are hard, heat resistant, and usually made of a copper alloy. Arcing contacts are used to reduce the damage of main contacts due to arcing. These contacts can be replaced easily. The arcing contacts close before and open after the main contacts. Here blow-outs consist of steel inserts in the arcing chutes.
These are so arranged that the magnetic field induced in them by the current in the arc moves them upward faster. When the arc comes into contact with the cool surfaces of the steel plates made in the arc chute, it gets rapidly cooled. Thus the arc is quenched by lengthening and excessive power loss. The main problem in this type of circuit breaker is inefficiency at low currents where the electromagnetic fields are weak.
In the occurrence of a fault, the main contacts separate first, and the current is shifted to the arcing contacts. Now the arcing contacts separate, and the arc is struck between them.
This arc is pushed upwards by electromagnetic forces and thermal action. It is split by arc splitter plates. It ends by traveling along with the arc runners. Thus, the arc is quenched by lengthening, cooling, and splitting.
The protection settings for a circuit which should be known are as follows:
Long Time Delay: The long-time delay should be adjusted in such a manner that the inrush current should be allowed to pass through the circuit breaker without tripping when the motor starts.
Ground Fault Pickup: The ground fault pickup should be adjusted to a 20% – 70% rating of the breaker. Ground fault pickup is the amount of ground-fault current which will bring the circuit breaker to the tripping point.
Continuous Amps: The level of the current which will be allowed to pass through the circuit breaker without tripping is called the continuous amp. The continuous amp can be set in the range of 20% to 100% or the circuit breaker’s normal rating.
Instantaneous Pickup: When a circuit breaker is to be tripped without any intentional delay instantaneous pickup value should be adjusted between 2 to 40 times the breaker’s current.
Short Time Pickup: For the selective tripping of the circuit breaker, the short-time pickup is used. The amount of the current which should be allowed to pass through the breaker for a short interval of time is determined by the short-time pickup.
Applications & Ratings of Air Circuit Breaker
Air circuit breakers come in with different ratings as different circuits utilize different current and voltage ratings. However, the standard ratings of the ACBs lie in the range of 400A to 1600A. Similarly, another class of ACB lies in 2000A to 5000A rating. The standard current ratings of the ACB are these; however, the voltage and the physical ratings of the ACB vary from manufacturer to manufacturer.
These circuit breakers are employed in AC circuits as well as DC circuits up to 12 kV. Such circuit breakers are usually of indoor type and installed on vertical control panels. As they have no oil, they are recommended where there are possibilities of fire and explosion.
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