The electric
field strength E is defined as the ratio of change in
voltage dV to the change in length dr.

_{
}

If the electric
field strength is zero i.e. E = 0 then relation 1 becomes:

Integrating
both sides, we get:

If the electric
field strength is zero i.e. E = 0 at a point, the
electrostatic potential is not necessarily be zero at that
point but the electrostatic potential will be constant
according to the relation 2.