In electric system where is neutral line present?is it visible?
In North American practice, neutral is connected to ground in precisely one location.  That is in the main panel, as close as possible to the current source.  The reasons for this are well answered elsewhere on this forum.  

That closest point is considered the main panel.  Anything else fed from the main panel needs to keep ground and neutral totally separate.   

Ground and neutral are not interchangeable.  They should not be tied together anywhere else.  Ground should never be used for current return.  Neutral should never be used as safety protection.  (Those who abuse ground are defeating its protection and wasting the wire.  If they are not serious about grounding, they should just not use grounding at all.  At least then, it will be obvious to electricians that their grounds are no good!)