Neutral and Ground Potential Difference:
The ground is always consider at zero potential.Whereas the neural potential is the sum of ground potential and voltage drop in the Resistance or Reactance between ground and neutral.
The neutral may or may not be at the same potential as ground.
In case of unbalanced condition of the circuit, zero sequence currents flow through the grounded impedance and cause voltage drop.
So Neutral Potential is not equal to the potential at Ground
However in case of balanced condition there is no current flows through the Resistance or Reactance. In this case Ground Potential is equal to Neutral Potential which is zero.