Hi
Even though we dont help in h.w. this is kind of logic
question.
anyway.
most of the circuits dont need ground.
ground is connected usually to the case of the circuit to
prevent from EMP to get in or get out.
also if the circuit is not isolated very good, it'll
touch the case, and the swich (I Donnow the english term)
will disconnect your home from electricity.
the main cause we use it at home, is because the 0v line
and the gnd are connect together just before their coming
into your electricity system.
and when it sense that any current (few mA's) is not
coming back from the 0V line, but from the gnd line,
then it shut down your house.
I hope thats answering your Question.
also I'm sure that I'm about to know more ways to use gnd.
in the next replies.
Goodluck
OK