0v voltage is normal, the standard is can be regarded as 220V as a battery, the positive pole as the fire wire, the negative pole as the zero wire, the negative pole with the earth good contact, the ground wire also with the earth good contact.
With voltage, charge flows in the wire to form an electric current. It's the same thing as water flowing from a high place to a low place. Water does work as it flows and electricity does work as it flows .
Expanded:
The zero wire being energized is a reflection of not being well grounded, if it is well grounded, the current will flow into the ground and will not be detected with an electric pen. If you detect a charged zero wire with an electric pen, either the zero wire is broken or the contact is bad. But that's really the result, not the cause of the energized zero wire.
Normally, the trunk portion of the zero wire will pass a smaller unbalanced current, but the current passing through the zero wire on each phase is equal to the current passing through the fire wire on that phase.
And the ground line is not passed by any current, and the metal casing of the electrical equipment connected to the ground line is not charged with potential during normal operation. Zero line and ground line can not be adjusted, if adjusted leakage protection switch will operate, cut off the power supply.
Baidu Encyclopedia - Zero Wire
Baidu Encyclopedia - Ground Wire