well i've bean reading chapter 3 in vol 1 and bumped into something interesting
http://www.allaboutcircuits.com/vol_1/chpt_3/3.html

what happens if the ungrounded side gets hit by a tree or touched by another person? by then both would be ground fault and the a person touching the ground side would be shock.all making the ground still meaning less.

my alteration in the picture

 

See attachment. The tree or another person are just another load to the source, they don't make a difference for the person who is standing on ground touching ground.

 

In such a situation a large "fault current" would flow. A well-designed system would include components like fuses or automatic circuit breakers, designed to react to this condition and shut off the supply. Circuit breakers reacting to the residual current (the difference between the live and return conductor currents) may be particularly useful here.

The deliberate earth connection is made to have a low resistance, and normally the fault protection arrangements should operate before the local earth voltage can rise high enough for long enough to do much damage.

 

See attachment. The tree or another person are just another load to the source, they don't make a difference for the person who is standing on ground touching ground.

i understood my mistake,thanks.