I also live in the US. Split phase 120V/240V. We had a tree come down on the main line that feeds our side of the street a couple blocks down the road, and it ripped one hot leg off the transformer, but not the other hot and neutral. So we had one leg dead, one leg and neutral live for a two block stretch (my house is the end of that main line). My furnace is fed by all three conductors, as the blower motor is 240V, but the control board runs off 120V. Apparently the leg that went dead was the one feeding the control board and something with how everything is done on that particular furnace, it did not appreciate that and fried several components on the board.
In regards to using optocouplers, that would normally be my go to for this purpose, except I need to sense voltage on the main feed lines from the meter to the main breaker. As I said, I need to sense power in the main feed when the main breaker is disconnected while running on generator during a power outage. This is entirely unprotected, and I have no way myself of making those conductors not live nor any proper location to attach additional wires to them. On top of that, attaching anything before the main breaker is an absolutely no-no and the code inspector would issue a REALLY nasty fine if I got caught with something hooked up that way. It absolutely has to be a non contact sensor, basically reading the tiny current caused by magnetic coupling of the insulated hot leg wire to an insulated antenna wire wrapped around it, similar to how a current CT would be attached.
The last outage we were running on gen power for over two hours after the grid power came back up because none of the neighbors had outside lights on. Typically I have to go out every hour or so to see if the meter has power yet, which is inconvenient, especially in the middle of winter and the middle of the night. And with gas over $3 a gallon and the gen eating a gallon an hour, it adds up quick.