Placed a power substation to power up an existing part of my city.
Removed power poles because they weren’t needed but failed to remove all of them.
When I hit the go button again I got an alert that a power network didn’t have any power, the power poles I hadn’t removed got the lightning symbol on them, and the iron mines I had there stopped working despite being within range of the power substation.
Removal of the power poles got the iron mines working again.
What seems to have happened is that the iron mines checked for power, found the power poles, found that the power poles weren’t connected and triggered the no power state.
Have units that need power check to see if they’re in range of a power substation first.
Mark all unnecessary power poles within range of a power substation marked for removal.