A circuit breaker in which the contacts open and close in the air at atmospheric pressure is known as an air circuit breaker. The principle of arc interruption followed in an air circuit breaker are different from those in any other type of circuit breaker. While both types of circuit breakers have the same objective ie. to prevent the resumption of arcing after current zero by creating a situation wherein the contact gap will withstand the system recovery voltage. In the air circuit breaker, the situation is achieved by creating an arc voltage in excess of the supply voltage.
This can be done in the following way:
1. Intensive cooling of the arc plasma, so that the voltage gradient is very high.
2. Lengthening the arc path to increase the arc voltage.
3. By splitting up the area into a number of series of the arc.
The principle of operation of these breakers is based on the high resistance method. They are an interruption in oil takes place due to hydrogen gas generated by the decomposition of oil. The are interruption properties of hydrogen gas is much better than -the air, but the air has several advantages as an arc extinguishing medium as compared to oil.
The following distinct merits of air over oil while interrupting the are:
1. The risk from fire and maintenance associated with the use of oil are eliminated.
2. Arcing products in air arc generally completely removed. On the other hand, oil deteriorates with successive breaking operation.
3. There is no requirement of regular oil replacement.
4. Heavv mechanical stress set up by gas pressure and oil movement is absent in air circuit breakers.
5. Relatively inferior are extinguishing properties of air may be offset by using various principles of our control and operating air at high pressures.
Therefore except for a certain medium range of voltages, air circuit breakers are commonly used for low voltage circuits as well as the highest transmission voltages.
Simple air circuit breakers which do not incorporate any are-control devices are used for low voltages. Below 1 kV. The oil circuit breakers are not used for heavy fault currents on low voltages due to carbonization of oil and unduly rapid current collapse.
The popular type of air circuit breaker (ACB) is are chute air circuit breaker. These breakers usually have two pair of contacts per phase. The main pair of contacts which are made up of copper carries the current under normal operating conditions. The additional pair becomes the arcing electrode as the circuit breaker is opened and are made of carbon. The repolarization and distortion of the contacts due to the heat of the are confined to these contacts. Therefore the materials required for these contracts should be nonvolatile. The main contacts separate while the arcing pair is still in contact. Hence the arc is initiated only when the arcing pair get separated.
In arc-chute ACB, the arc is extinguished by applying the principle of lengthening and increasing
the power loss of the are.
Figure 4.25 explains how the arc discharge moves upward with the help of the effects of both electromagnetic and thermal. These are discharges are then directed towards the chute. The chute consists of splitters and baffles. Baffles improve cooling while splitters lengthen the arc. The splitters increase the length of the are even further. In this breaker relatively high in arc resistance is obtained near current zeros. This effect plays an important role in obtaining high breaking capacity by modifying the circuit power factor near current zero such that the voltage available to restrike the arc is less than the p peak value.
Air circuit breaker Disadvantage
The disadvantage of arc-chute ACB is that if the electromagnetic fields are weak such as at low current, the breaker becomes less efficient. At low current, the movement of the arc towards chute becomes slower and at high-speed interruption is less assured.
Air circuit breaker Advantage
Air circuit breakers are exclusively used for the systems up to 15 KV until the arrival of the new vacuum and SF6 breaker technologies. The air circuit breaker use is restricted to low voltage applications or high-security installations where the risk of an oil fire of oil contamination of the environment is too high to be tolerated.