AIR BLAST CIRCUIT BREAKERS

Circuit breakers are mechanical devices designed to close or open contact members, thus closing or opening of an electrical circuit under normal or abnormal conditions. Air blast circuit breakers are using compressed air or gas as the circuit breaking or interrupting medium. Gases such as nitrogen, carbon dioxide, hydrogen or Freon can be used as arc interrupting medium. But compressed air is the most accepted arc interrupting medium. The reasons are, nitrogen has circuit breaking properties similar to compressed air and there is no advantage of using it. Carbon dioxide has a draw back as it is difficult to control owing to freezing at valves and other restricted passages. Hydrogen has increased breaking capacity but it’s costlier.

Freon has high dielectric strength and good arc extinguishing properties, but it is expensive and it is decomposed by the arc into acid forming elements.

Desirable features of air blast circuit breakers are High speed operation, which is very necessary on large inter connected networks in order that system stability can be maintained and in the air blast circuit breaker this is achieved because the time interval between the receipt of a tripping impulse and contact separation is very short. Suitability for frequent operation, repeated switching by an air blast circuit breaker is possible simply because of absence of oil, which rapidly carbonizes with frequent operation, and because there is an insignificant amount of wear and tear at the current carrying contact surfaces. High speed reclosure by automatic means is an advantage on hv inter connected networks to assist and maintain system stability during the clearance of transient faults, a type of fault which is perhaps in majority on over head line. Negligible maintenance, the ability of the air blast circuit breaker to cope 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 final gap required for arc extinction is very small. This reduces the size of the device.

The air blast circuit breaker requires an auxiliary compressed air system which supplies air to the breaker air receiver. When 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 assist arc extinction.

VACUUM CIRCUIT BREAKER

VACUUM CIRCUIT BREAKERS

Circuit breakers are devices which can automatically break a circuit under fault conditions and it can make the circuit manually after the fault. Circuit breakers are classified into different types. One of the main circuit breaker among them is Vacuum circuit breaker. Here vacuum is used to extinct the arc. Arc extinction is the main process of a circuit breaker

Before that, it is needy to explain how an arc is produced. Circuit breaker has two contacts- Fixed and movable, otherwise called as electrodes. Normally the contacts remain closed but if any fault occurs these contacts open automatically and of course can be opened manually.

We know that circuit breakers work under the instructions from relay. When the trip coils from relay energize, circuit breaker identifies fault occurred and hence the contacts are pulled apart. A high intensity current flows through the contacts of the circuit breaker before they are opened by the system. At the same instant contacts begin to separate and causes heavy heat and temperature in between the contacts. This is termed as an arc. This heat is highly dangerous and can cause heavy damage to the whole system. So it has to be extinguished in no time.

Vacuum is having superior qualities over other arc quenching medium. It has high insulating strength. When the contacts are opened in vacuum, the arc is produced in between them. It got extinguishes quickly since the metallic vapours and ions produced during the arc rapidly condense on the surface of the circuit breaker contacts, resulting in quick recovery of dielectric strength.

The contacts of Vacuum circuit breaker and arc shied is mounted inside a vacuum chamber. The chamber is connected to the control mechanism by means of stainless steel bellows. A glass vessel is used as the outer insulating body. This is all about its construction.

Vacuum circuit breaker is really the most reliable one with low cost, no fire hazards, no gas generation, low arc energy, little maintenance and low inertia.