What is the operating principle of an SF6 gas circuit breaker?

• A. Arc is extinguished by air cooling.
• B. Arc is quenched by SF6 gas, which provides high dielectric strength but does not enable rapid interruption.
• C. Arc is quenched by SF6 gas, which provides high dielectric strength and rapid interruption with minimal arcing.
• D. Arc is quenched by oil immersion to increase dielectric strength.

Answer: (C)

SF6 gas circuit breakers rely on the gas as both the insulating medium and the arc-quenching medium. When the contacts separate and an arc forms, the SF6 gas surrounding the arc provides extremely high dielectric strength, so the gap between the contacts and the arc remains well insulated even as the current tries to rise. This high dielectric capability, combined with the gas’s ability to absorb energy from the arc and cool the plasma, causes the arc to be rapidly quenched and the current to interrupt with minimal, short-lived arcing. The gas’s insulating properties also help prevent re-ignition after the current crosses zero, allowing a fast and reliable interruption at high voltages. The other ideas—arc extinguishing by simple air cooling or by oil immersion—do not describe how SF6 breakers operate, which rely on the insulating and cooling action of the SF6 gas itself.