What is Air Circuit Breaker? How does it Work?

Air Circuit Breaker Working Principle

Air Circuit Breaker (ACB) is an electrical device used to provide Overcurrent and short-circuit protection for electric circuits over 800 Amps to 10K Amps. These are usually used in low voltage applications below 450V.

Circuit breakers are electrical devices used for protection in power systems. Air circuit breakers are a specific type of circuit breaker commonly used in low voltage applications.

All circuit breakers work pretty much the same: they’re passive devices that, under certain physical conditions, open the electrical circuit in order to protect the system and its devices.

When the circuit opens, the electrical current tends to keep flowing so we need a way to extinguish the electrical arc that may form and prevent it from reforming again afterward. We may use different dielectric materials such as oils and gases in order to do that and in the case of air circuit breakers that dielectric is the atmospheric air itself.

In other words, the air works as a resistance to the arc, forcing it to extinguish faster, and then works as an insulator, preventing the arc from reforming after it reaches zero (remember we are talking about alternating current) during its cycle.

It's becoming more and more popular the use of pressurized air so as to allow such circuit breakers to be used in higher voltage applications such as in electrical substations. The reason is that pressurized air has a much higher dielectric constant than atmospheric air.

This type of circuit breaker is the kind of circuit breaker which operates in the air at atmospheric pressure. After the development of the oil circuit breaker, the normal voltage air circuit breaker (ACB) is replaced completely by an oil circuit breaker in different countries. But in countries like France and Italy, ACBs are still preferable choices up to a voltage of 15 kV. It is also a good choice to avoid the risk of an oil fire, in the case of the oil circuit breaker. In America, ACBs were exclusively used for the system up to 15 kV until the development of new vacuum and SF6 circuit breakers.

There are mainly two types of air circuit breakers as below:

1.      Air-Break Circuit Breakers

2.      Air-Blast Circuit Breakers

Air-Break Circuit Breakers

In the Air-Break Circuit breaker, the contact separation and arc extinction occur in the air at atmospheric pressure i.e. the level of Hg under normal conditions. The high resistance principle is employed here. The arc resistance is increased to such an extent that the voltage drop across the arc becomes more than the system voltage and the arc gets extinguished at the current zero of the ac wave. It’s employed in dc circuits and ac circuits up to 12 kV.

Air-Blast Circuit Breakers

In Air- Blast Circuit Breaker require an auxiliary compressed air system that supplies air to the breaker air receiver. When the opening is required, compressed air is admitted to the arc extinction chamber. It pushes away the moving contacts. In doing so the contacts are separated and the air blast takes away the ionized gases along with it and assists arc extinction. Airblast extinguishes the arc within one or two cycles and the arc chamber is filled with high-pressure air, which prevents restrike. The air blast circuit breakers are used for up to 220 kV and above. It uses high-pressure air and is independent of current to be interrupted.

Working Principle of Air Circuit Breaker

The working principle of this breaker is rather different from those in any other type of circuit breaker. The main aim of all kinds of circuit breakers is to prevent the reestablishment of arcing after current zero by creating a situation wherein the contact gap will withstand the system recovery voltage. The air circuit breaker does the same but in a different manner. For interrupting arc it creates an arc voltage in excess of the supply voltage. Arc voltage is defined as the minimum voltage required to maintain the arc. This circuit breaker increases the arc voltage by mainly three different ways,

 
1. It may increase the arc voltage by cooling the arc plasma. As the temperature of arc plasma is decreased, the mobility of the particle in arc plasma is reduced; hence more voltage gradient is required to maintain the arc.

 
2. It may increase the arc voltage by lengthening the arc path. As the length of the 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.

How does Work an Air Blast Circuit Breaker?

The air blast needs an additional compressed air system that supplies air to the air receiver. When opening air is required, compressed air is admitted to the arc extinction chamber. It pushes away the moving contacts. In doing so, the contacts are pulled apart, and the air blast moves away from the ionized gas along with it and assists arc extinction.

 
Airblast extinguishes the arc within one or more cycles, and the arc chamber is filled with high-pressure air, which prevents restrikes. The air blast circuit breakers fall under the category of external extinguishing energy type. The energy supplied for arc quenching is achieved from the high-pressure air, and it is free from the current to be interrupted.

 
Airblast circuit breaker used compressed air or gas as the arc interrupting normal. In the air blast, circuit breaker compressed air is stored in a tank and released through a nozzle to produce a high-velocity jet; this is used to extinguish the arc. Airblast circuit breakers are used for indoor services in the normal high voltage field and normal rupturing capacity. Generally up to voltages of 15 kV and rupturing capacities of 2500 MVA. The air blast circuit breaker is now employed in high voltage circuits in the outdoors switchyard for 220 kV lines.

 
Though gasses such as carbon dioxide, nitrogen, Freon, or hydrogen are used as the arc interrupting normal, compressed air is the accepted circuit breaking normal for gas blast circuit breakers. The reasons are given below.

 
The circuit-breaking capacities of nitrogen are similar to compressed air and hence no advantage to using it. Carbon dioxide has the drawback of its being difficult to control owing to freezing at valves and other restricted passages. Feron has high dielectric strength and good arc extinguishing properties, but it is expensive, and it is disintegrated by the arc into acid-forming elements. The desirable features to be found in air blast circuit breakers are:

 
High-Speed Operation – It is very necessary on large interconnected networks so that system stability can be maintained. This is achieved in circuit breakers because the time interval between the discharge of triggering impulse and contact separation is very short.

 
Suitability for frequent operation – Repeated switching by an air blast circuit is possible simply because of the absence of oil, which rapidly carbonizes with the frequent operation and because there is an insignificant amount of wear and tear at the current-carrying contact surfaces. But it must be remembered that if frequent switching is anticipated, then the maintenance of a sufficient air supply is essential.

 
Negligible Maintenance – The ability of the air blast circuit breaker to deal with repeated switching also means that negligible maintenance is required.

 
Elimination of Fire Hazard – Because of the absence of oil the risk of fire is eliminated.

 
Reduced Size – The growth of dielectric strength is so rapid in air blast circuit breakers that the final gap required for arc extinction is very normal. This reduces the sizes of the devices.

 

Types of Air Blast Circuit Breaker

All air blast circuit breakers follow the principle of separating their contacts in a flow of arc established by the opening of a blast valve. The arc which is drawn is usually rapidly positioned centrally through a nozzle where it is kept to a fixed length and is subjected to the maximum range by the airflow. The air blast circuit breakers according to the type of flow of blast of compressed around the contacts are of three types namely axial, radial and cross blast.

 
Axial blast Air Circuit Breaker – In the air blast circuit breaker, the flow of air is longitudinal along the arc. The air blast circuit breaker may be a single blast or double blast. Breaking employing double-blast arrangement is sometimes called radial blast circuit breakers as the air blast flows radially into the nozzle or space between the contacts.

 
The essential feature of the airblast circuit breaker is shown above. The fixed and moving contacts are kept in a closed position by spring pressure under normal operating conditions. The air reservoir tank is connected to the arc chamber through an air valve, which is opened by a triple impulse.

 
When the fault occurs, the tripling impulse causes the opening of the air valve connecting the reservoir to the arcing chamber. The air entering the arc chamber exerts pressure on the moving contacts which move when the air pressure exceeds the spring force.

 
The contacts are separated, and an arc is developed between them. The air flowing at a great speed axially along with the arc cause removal of heat from the edge of the arc and the diameter of the arc is reduced to a very normal value at current zero.

 
Thus, the arc is interrupted, and the space between the contact is flushed with fresh air flowing through the nozzle. The flow of fresh air removes the hot gasses between the contact space and rapidly builds up the dielectric strength between them.

 
Cross Blast Air Circuit Breaker – In such a breaker, an arc blast is directed at right angles to the arc. The schematic representation of the cross principle of the cross blast air circuit breaker is given in the figure below. A moving contact arm is operated in close spaces to draw an arc which is formed by a transverse blast of air into the splitter plates, thereby lightening it to the point when it cannot restrike after zero current.

 
Resistance switching is not normally required as the lightening of the arc automatically introduces some resistance to control the restriking voltage transient but if extra resistance is thought desirable. It is possible to introduce it by connecting it in the section across the arc splitter.

 

The drawback of Air Blast Circuit Breaker

In the air blast circuit breaker, it is necessary that the compressed air at the correct pressure must be available all the time, involving in the largest installation of a plant with two or more compressors. The maintenance of this plant and the problem of air leakages at the pipe fittings are some factors that operate against air blast circuit breakers and it is costly for low voltage as compared to oil or air break circuit breakers.

How Choose an Air Circuit Breaker for Your Project?

The common feature you should consider to choose an air circuit breaker for your project.




01. To IEC 947-2 or BS EN 60947-2, suitable for triple pole service and shall have a breaking capacity of 50 kA symmetrical for 3 seconds at 415 Volts.



ACBs shall be of the horizontal withdrawable, load making, and breaking type with the contacts being of the double break pattern with arcing chutes, shutters, etc. The main arcing contacts shall be of the high-pressure butt type with wipe and roll action on opening and closing. The main contacts shall be of silver alloy.


Removable arc chutes shall be fitted together with an air circuit breaker. ACB shall be complete with a mechanical ON/OFF position indicator, pad-lockable. ACB shall conform toB.S. En 60947-2 (EN 60947-2, IEC 947-2) and B.S. En 60664-1.


Each circuit breaker shall be enclosed in sheet steel and provided with three-phase manual and automatic isolating devices suitably interlocked so as to prevent isolation except when the circuit breaker is in an open position. It shall be arranged in such a manner that it will not be possible to withdraw the breaker or remove the front cover unless the ACB is in an isolated position.


Provision shall be made for locking the ACB in this position (isolated position).


Shutters shall be provided to protect the live terminals against accidental touch when the ACBsare in a fully withdrawable position. The ACB shall have solid-state overload and short circuit protection devices along with an earth fault trip mechanism. The solid-state circuit shall be provided with proper LED indication to indicate the trip status of the ACB. The ACBs shall be equipped with a shunt trip of 30V DC trip mechanism for tripping the breaker with a resistive earth fault relay to be provided separately with suitable sizes of CTs.


02. The Air Circuit Breakers (the conventional type and not molded case circuit breaker) shall be 500 V, 50 Hz, triple pole with a neutral link for incoming and outgoing ACB or four poles for bustier ACB only with ratings as shown on the Drawings. They shall be air break, trip-free, draw-out type with mechanical and electrical ON/OFF indicators.


3. Where air circuit breakers are to be electrically operated by an automatic motor wound spring mechanism, a standby manual operating handle shall be provided for operating the circuit breaker in case of power or motor failure.


4. The air circuit breaker shall be provided with built-in overcurrent, short circuit, and earth fault protections having the following characteristics:-


a. adjustable long time delay current setting between 50–200% with variable tripping time characteristics.



b. adjustable short time delay current setting 200–800% with variable tripping time characteristics.


c. instantaneous tripping for heavier over current applications adjustable from 400–1600% of the base current.


d. adjustable earth fault trip current setting 20–80% with variable tripping time characteristics.


5. The circuit breaker shall have three positions on the draw-out mechanism, namely the service position where all main and auxiliary contacts are made, the test position where main contacts are open but auxiliary contacts are closed, and the isolated position where all contacts are open. An indicator shall clearly show these positions and provisions shall be made for locking the breakers in any position. ON/OFF indicator shall be provided.


6. Mechanical Interlocks shall be provided to prevent withdrawing or inserting of the breaker when it is „ON’. Any attempt to do so shall trip the breaker automatically.


7. The withdrawable part of the circuit breaker shall be effectively connected to the earth through scraping contacts that shall make before and break after the main and auxiliary contacts.


8. The moving contacts comprising the main and arcing contacts shall be of the spring-loaded, self-aligning type. The arc contacts shall be arranged to make before and break after the main contacts.


9. The air circuit breaker shall include but is not be limited to the following as a minimum:-


(a) 8NO/8NC auxiliary contacts;
(b) arc chutes;
(c) folding extension rail;
(d) charging handle;
(e) open and close push buttons;
(f) trip indicator;
(g) spring charge motor;
(h) spring charge indicator;
(i) breaker position indicator mechanically and electrically;
(j) microprocessor-based protection and management unit that provides the following control and monitoring features:-

(i) overcurrent protection; 

 (ii) short circuit protection; 

 (iii) earth fault protection; 

 (iv) neutral protection; 

 (v) thermal memory; 

 (vi) alarm logging; 

 (vii) field selectable Manual or Auto reset; 

 (viii) microprocessor malfunction watch dog; 

 (ix) programmable input/outputs; 

 (x) load monitoring; 

 (xi) operation counter; 

 (xii) serial communication; 

(k) carriage/Lifting Truck for ACB exceeding 25 Kg in weight [One carriage for each site/project regardless of the number of breakers provided; 

(l) shunt trip and under-voltage release.




10. The Main Incoming Circuit Breakers shall be provided with cable terminal boxes to suit the upcoming cables from the transformer/source supply. The gland plate for the incoming cables shall be non-ferrous material brass compression type glands, earthing tags, and shrouds.



11. Circuit breakers shall be tropicalized to operate continuously at an ambient temperature of 55°C and high relative humidity.


12. Type test certificate for each size of circuit breakers and MCCB from an internationally recognized testing authority acceptable to the Engineer shall be provided.13 The ACB shall have adjustable settings and the following facility:


Long time current setting and tripping delay.
Overload signal.
Short-time pick-up and tripping delay.
Instantaneous pick-up.
Earth leakage test button.
Long time rating plug screw.
Test connector
Lamp test, reset, and battery test.
Indication of tripping cause.
Digital display.
Three-phase bar graph and power display.
Setting / programmable buttons.


14. The ultimate breaking capacity (ICU) shall be a minimum of 44KA. The rated service breaking capacity (ICS) and rated short-time withstand current (ICW) shall be equal to or greater than ICU.


15. The ACB section of the switchboard shall be in a separate cubicle separated from other parts of the switchboard. This section shall not have any outgoing feeders.