[unni111]

Could someone please explain me about the working of SF6 Circuit Breaker?

[rsain2004]

What I know: SF-6 is a high dielectric gas, used in high voltage transformers, rectifiers and switchgear. It is lighter in weight than insulating oils and came about due to the PCB scare. I was working in Iceland when that happened... PCB insulating oils were declared carcenogenic (electricians used them as hand cleaners). The result was PCBs were outlawed, which led to the development of SF-6 insulating gas. It is widely used for air transportable equipment, like radars, due to it's light weight. When exposed to arcing, it smells like rotten eggs... a troubleshooting tip...

[sathe1963]

• Due to the low contact velocity and contact travel, the energy requirement of SF6 breaker using self-extinguishing principle, is very low. This is also because; the energy for extinguishing the arc is drawn from the arc itself. Hence, the operating mechanism employed for this breaker is practically maintenance free.
• No special precautions are necessary for venting out the SF6 gas.
• In SF6 breaker, there is continuous visual as well as electrical monitoring of gas pressure is possible. That is, the internal pressure in the extinction chamber can be read at any time. Also, if the pressure falls below pre-set limit, one can arrange for alarm and / or blocking of tripping / closing. • Even after reaching atmospheric pressure, the external air (due to its lightness) cannot diffuse into the arc chamber with SF6 gas (at atmospheric pressure)(density = 5 times that of air). • SF6 gas is simply the best arc-extinguishing medium.
• In the SF6 gas, arc is maintained during the arcing period by ionization of the SF6 gas, and not by vaporization of metal.
• The resistance of the arc is very small and the arc energy is thus proportionately small. This is very good for contact life and general life expectation of the breaker.
• In the post arc period, the SF6 gas being electro-negative in character absorbs electrons, thus trapping free electrons. Thus, the free flow of electrons is prevented and the rate of build-up of dielectric strength is faster than in any other medium.
• Electro-negative SF6, though ionized cannot move as fast as electrons because of their mass.
• Thus the problems associated with vacuum circuit breaker do not exist in SF6 circuit breaker.