10-10-2015, 06:22 AM
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- 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.
- 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.
- 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 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.