Q: What is the ground voltage with neutral? How many Volts should it be?
A: The ground voltage with neutral is the potential difference between PE and N wires. Ideally, it is zero. Usually, it is 1-3 V depending on the conditions of the system. 5 V is the maximum allowable limit.
Q: Why does it occur? What can be done to prevent and eliminate it?
A: It may occur for different reasons in three-phase networks: short-circuit that was not stopped when phase wire touches metal grounded parts of equipment, third harmonics generated by electronic equipment, high resistance of N or PE wires. Compensators and/or periodical maintenance will help to prevent and eliminate this voltage. The solution, of course, depends on the reason.