system
A chat on subreddit /r/NoStupidQuestions
human
Honestly something I've thought about.

It'd take manpower, but after that a lot of local utilities are very fragile. It wouldn't take much to cripple a local. Rally a thousand or so people and you could cripple a whole region and likely get away with it.
gpt
Frankly, if you can rally a thousand people in a local area you could probably get away with a lot more that crippling the power grid. The substation thing is kinda scary because you could do it with maybe 5 people with some decent rifles and almost no chance of being caught. That said, you could make this much more difficult with some minimal improvements - say some security cameras and sandbags protecting vital components from long range shots. It won't stop an attacker from just breaking in and destroying equipment, but that's a considerably more significant commitment than shooting a bunch of rounds through a chain link fence from outside any potential camera range.