....
Yes, it is both dangerous and forbidden to combine or
interconnect
the two.

Is it just me, or do we get this question every day?

--Goedjn

No, it's not you, but I suspect that was rhetorical, right?
Too many people have been taught to look at these things
from a voltage standpoint, (e.g. 0Vac = 0Vac = how you get
it doesn't matter), instead of correctly considering current
flow and direction plus fault control.
I'll never forget my first encounter with ground
currents: HOW could a wire overheat and melt, when there
was 0V measured at BOTH ends of it? I watched 3 wires burst
their insulation into flames before I figured it was a
problem with me preventing knowing what went wrong! I was
an avionics TS at North Island at the time. Of course, if
I'd known to use the right meter scale, I'd have seen the
voltage diff, but ... .

Regards,

Pop

It is comforting and speaks well of this group that such
knowledge and participation exists here.