Jacob’s Ladder.
My diagnosis is that one of the 3 phases of the circuit breaker failed to open and therefore, they were trying to drop flowing current with a switch that was not designed for that purpose. Notice that the other two phases opened cleanly.
Pretty wild experience. I was on the phone with a guy that was manually cranking a 161 kV line switch open to just de-energize some wire. It had the wrong parts installed, and it made that tell-tale sound all the way until the arc finally broke.
The guy on the phone with me, the foreman, just kept telling the other guy, “Keep cranking, keep cranking, don’t stop...”
What is a Jacob’s ladder?