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SF6 Circuit Breaker and Comparison of Circuit Breakers

  • SF6 Circuit Breaker gas is used as arc quenching medium.
  • SF6 electro-negative gas
    -Has a strong tendency to absorb free electrons
  • Contacts of the CB are opened in a high pressure flow of SF6 gas and arc is struck between them.
  • Conducting free electrons in the arc are captured by the gas.
  • Loss of conducting free electrons in the arc quickly builds up enough dielectric strength.
  • Effective for high power, high voltage service.
Physical properties
  • Colourless, odourless.
  • Non-toxic, no harmful to health.
  • Heavy gas having high density.
  • Liquification starts at low temperature which depends upon pressure.
  • Heat transferability is high as compared to air.
  • Electronegative gas.
Chemical properties
  • Chemically inert and stable upto 500
  • It does not attack metals, plastics, etc.
  • Life of metallic part, contacts is longer in SF6
  • Low reactivity.
  • Components do not get detoriorated.
  • During arc extinction, SF6 is decomposed into SF4 and SF2.
  • These are toxic. Hence proper care should be taken for disposal.
  • Metallic flourides are good dielectric materials. 
Dielectric properties
  • Dielectric strength at atmospheric pressure
    ◦ 2.9 times that of air.◦ 30% less than dielectric oil.
  • Maintains high dielectric strength even diluted by air.
  • At high pressure, dielectric strength of gas increases. Because  of this, smaller clearances and small size of components are required for same kV.
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