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.

National Electrical Code - NEC

National Electrical Code (NEC)

National Electrical Code is the United States standard for the installation of electric wiring safely; it is the most approved American national standard. Actually it is a series of codes issued by National Fire Protection Association (NFPA). This NEC consists of 19 code making panels and technical correlating committee. NEC work is sponsored by NFPA. It is formally identified as ANSI/NFPA 70.

This code was published in 1897 and is getting updated every three years. The 2008 code is the most recent version. Most states adopt the most recent edition within a couple of years of its publication. As with all national codes some of the portions or sections are omitted usually, but NEC is the least amended model code

NEC can be accessed to the public as a book in early periods but now available in the electronic version. NEC is composed of introduction, nine chapters. The introduction contains purpose, scope, enforcement and rules or information. These chapters deal with voltages, connections, markings, wiring devices, conductors, cables, cords, switches, heaters etc. In general all the chapters enclose numbered articles, parts, sections (or lists or tables) italicized exceptions, and fine print notes (FPN) -- explanations that are not part of the rules. Each code article is numbered based on the chapter which is in.

For eg; Article 210 addresses branch circuits and receptacles. This says about the minimum number of branches, placement of receptacles etc. NEC also has laws about how many circuits and receptacles are to be placed in a residential dwelling. It has separate law enforcement sections on conduit and cable protection, temperature rating etc

NEC consists of codes covering all the sections in electrical field. So in general NEC is the most accepted and least amended electrical code.