Measuring High voltage (50V)

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.

Thanks
Tom

Clamping diodes is for when the voltage is below 0V or above 5V. They clamp the voltage between 0V and 5V.

It looks like this:

The Arduino microcontroller has internal clamping diodes on every pin. But they can only have 1mA.

Using seperate grounds is good, but you need something special, like a linear opto-coupler or a digital communication through an opto-coupler.
You might consider a dedicated Arduino connected the power circuit that transmits the voltage wireless.