Let's start with the neutral line. If the load is balanced and also three-phase, theoretically only three phase lines are needed, as is the case with three-phase asynchronous motors. A neutral line isn't required. However, in reality, loads aren't always balanced, so a neutral line is designed. The neutral line is a "common line" in a three-phase AC circuit with a "star connection," connecting the three phase lines. It's a common line relative to the three phase lines. The International Electrotechnical Commission (IEC) standard refers to the conductor carrying multi-phase unbalanced current as the neutral line (N line).
Imagine if there were no safety requirements for electricity use, such as not having to worry about electric shocks or damage to devices caused by leakage from electrical loads. In that case, there would be no need to consider grounding measures. As long as there are three phase lines and one neutral line, all electricity requirements can be met, and there would be no need for a grounding wire.
However, people and equipment live, work, and operate on the Earth, which is an equipotential body, and their safety needs to be protected. As long as the potential is equal, people will not be shocked. Therefore, experts have come up with a method to ground these electrical devices or ground the neutral line, so that they are at the same potential as the Earth and will not threaten people's safety.
So, a ground wire is created. By drilling a deep enough hole in the ground and grounding the neutral wire, it's safe for people to touch it because it has the same potential as the ground. Furthermore, from a wiring perspective, this saves on one cabling line, saving a lot of money and resources, so this neutral wire is grounded.
Therefore, in a strictly balanced three-phase AC load, the neutral wire is at zero potential, meaning the voltage is zero. Both the neutral wire and the zero line are conductors drawn from the neutral point of the power supply. The conductor drawn after the neutral point is grounded is called the zero line, thus introducing the concept of a zero line.
In reality, there is a difference between the neutral wire and the ground wire in daily life. The ground wire in our electrical systems is directly grounded near us, primarily for safety protection. For example, if a phase wire leaks current, because the appliance is grounded, you won't get an electric shock if you touch the ground wire. The neutral wire, however, usually passes through a switch before reaching the ground, and in this case, it is different from the protective ground wire. It is represented by PE, but some neutral wires serve as both protective and neutral conductors, so they are marked with PEN.
