In a discussion from 6 months ago, a forum questioner was suggested;
"Another suggestion, when reading voltages that may be higher than 5 volts (even with a voltage divider), it is a good idea to clamping diode on the analog input pin. If one of the resistors in your voltage divider slipped loose, it could easily fry your chip." from a user in Texas.
I also need to measure high voltage but do not understand how or where to put this diode to protect the board.
One other question, is there a way of measuring without making Arduino ground common with the ground of the power circuit. Probably should be OK but it seems to provide opportunity for disaster, mixing the two circuits.