Hi,
Since neutral wire is grounded i.e. has the same potential as the ground. Why then we run three wires and not only line neutral and bond the receptacle ground with the neutral?
As in the attached photo, a guy did this an it's working with no problems.
I told him it's wrong but he said why?
You want to keep current on the live and neutral conductors, during normal operation.
You want to prevent the metal parts that don't intentionally carry current, from carrying current during normal operation. Examples would be equipment housings, enclosures, metal conduit, building steel, concrete structures. The EGC's purpose is to bond all metal/conductive parts that do not intentionally carry current, and provide an opportunity for the return path of fault current in the event of a short circuit where a live wire unintentionally contacts non-current-carrying metal.
If you were to share the ground and neutral, you would create a parallel path through the non-current-carrying metal. Current doesn't just take the "path of least resistance". It takes ALL paths, inversely proportional to resistance. It would inevitably energize the equipment housing, if you didn't have a separate ground and neutral.