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.