Normally we are not used to seeing accidental explosions or fire blazing in our home or office when the current jumps above the safe level or when there are any other faults in the electrical system. When these accidental incidents are about to happen the circuit breaker cuts the power to the electrical system. Without circuit breakers (or the alternative, fuses), household electricity would be impractical because of the potential for fires and other mayhem resulting from simple wiring problems and equipment failures. It is one of the most important safety mechanisms either in the primary source of electricity or in the electrical distribution systems from where electricity is distributed in our home or in our home.
Air circuit breaker: When the breaker opens the current flow is interrupted that causes an electrical arc to be generated. This arc is cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. When the fault condition is fixed, the contact is again closed to restore power to the interrupted circuit. When a circuit breaker uses compressed air to extinguish the arc the circuit breaker is called air circuit breaker (ACB).
Construction: Each phase of a three-phase Air Circuit Breaker (ACB) consists of 3 types of contacts, namely; main contacts which carry the load current, arcing contacts and the arcing horn.
The main contacts: Under normal operation the main contacts carry the load current. Therefore, main contact resistance must be low enough to prevent overheating, when current is flowing through it. The main contacts are therefore made up of a good conducting material such as copper, silver or copper with silver plating. Since these metals have relatively lower melting points, they can be damaged if arcing occurs. To prevent this damage, the main contacts do not make or break the current.
Arcing Contacts: Since no arcing must occur at the main contacts, arcing contacts are provided, which make or break the circuit current.
Arcing Horns: After the arc is established on the arcing contacts, it is transferred to the arcing horns during the opening of the arcing contacts. Their shape is designed to weaken or extinguish the arc.
Arc chute: The arc chute is a cooling chamber located at the top end of the breaker. It cools the hot gases which are produced when arcing occurs.
Operating mechanism: To understand the sequence of operation of various contacts in the air circuit breaker, one must realize that arcing must never occur at the main contacts.
Opening cycle: When the breaker is closed, the load current is conducted by the low resistance main contacts. But when the breaker opens, the main contacts open first and transfer the current to the arcing contacts. The arcing contacts open and an arc is established across the arcing horns. The arc rises to the top of the arcing horns and enters the arc chute where it is rapidly cooled by heat transferring mechanism in the cooling plates. Cooled gas deionizes that makes it unable to conduct electricity, and consequently arc is extinguished.
Closing cycle: In the closing cycle, the arcing contacts touch first to make the circuit again. Then the main contacts close and enable the circuit to flow electrical current.
Advantages of Air Circuit Breaker (ACB) over the other circuit breakers
* Relatively inexpensive
* Simple installation
* Simple construction
* Simple maintenance requirements
Air Circuit Breakers (ACB) are used in both low and high voltage current. They are commonly used in electrical distribution systems and NGD for voltage up to 15 KV. Air circuit breakers (ACB) which include operating mechanisms are mainly exposed to the environment. They protect for overload, short circuit and earth fault/ground fault. Most of air circuit breakers are used in indoor type substations.
Source: Ezine Articles – Thomas J Scott