This device is called an isolator. It's a mechanical disconnect that opens and closes a 500,000 volt electrical transmission line at the El Dorado electrical substation near Boulder City, Nevada. Another device called a switcher is supposed to open the circuit first, then the isolator opens to physically disconnect the circuit. The switcher has had a history of one of the three phases failing to open. When this happens, the isolator opens with one phase still energized. Since the isolator isn't designed to open hot, there's no arc suppression. and the result is quite spectacular. Because of the history of periodic failures when the switcher has been opened, a video camera had been set up to record the event should it happen again. They were in luck and caught it. The arc ends when an upstream circuit breaker is opened. Full credit goes to Neil Brady who captured this amazing sight. The "wow" at the end of the video says it all.