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The short version is that the electrical contacts which open and close to connect or disconnect the circuit are enclosed in a sealed chamber filled with SF6.
But why SF6?
If you open the contacts of a high voltage breaker in air, the voltage is high enough to generate a conductive plasma. This creates a damaging electrical arc between the contacts.
SF6 does not do this. Not at these voltage levels anyhow.
It’s also very inert, nontoxic, easy to contain. etc. good choice for the gas.
All of this is happening down in the bottom parts of the breaker.
The upper parts are just electrical connectors surrounded by ribbed glass insulators. The shape and material of the insulator makes it very difficult for the electricity to ‘jump’ down the outside of the insulator – it’s a pretty typical design, and you’ll see similar insulators if you look at any power lines. There are six, one in and one out for each of the three electrical phases.