Nothing more than a big light switch

Well, that's what it is, really. A very big light switch. 

This is a three phase air blast circuit breaker working at 400,000 volts. 

You know sometimes, when you flick your light switch, there's a brief flash inside? That's 230 volts complaining about being interrupted by the opening contacts. 

When 400,000 volts is interrupted, it complains harder. 400kV can cross a gap 3m across, and in order to break that electrical arc the interruption is made in sections, and high pressure (400psi) air is explosively vented across the arc to blow it out. It's not a silent operation, in fact it's akin to standing next to a large cannon. Nobody does it by choice and of course ear defenders are worn all the time when one is close to such a monster. Just in case.

This is all a bit low tech nowadays, and modern switch gear uses a highly insulating gas, Sulpher Hexafluoride, to extinguish the arc, making the switch gear very much smaller and cheaper and quieter. Much, much quieter. But SF6 is highly toxic to the environment in terms of greenhouse effect (about 23,900 x CO2e). Of course, the secret is to not let the gas out of that switch.