At high voltages the electricity may be able to arc between the steps. Electricity will do some pretty incredible things at high voltages. Check out this video of some idiots turning a powerline into a Jacob's ladder.
At the voltage needed to arc between aluminum steps you've got a lot bigger problems than worrying about your ladder. A quick search shows overhead trolley cables deliver 600V DC. Not exactly "turning air to plasma" level there. The rest of the overhead cables firefighters should encounter are the same normal residential 34kV lines you see everywhere (data tables show an air arc distance under 2 inches) unless I'm missing something.
That video was awesome.
I'm guessing that the types of ladders SFFD are using aren’t going to be anywhere near power lines of that high a voltage.
I can't tell what, if anything, they were doing besides filming.
i think a weight tied to thin uninsulated wire, thrown like a baseball over the lines