Three-phase four-wire system and three-phase five-wire system

Three-phase four-wire system
In low-voltage distribution networks, transmission lines generally use three-phase four-wire system , in which three lines represent A, B, and C phases respectively, and are not split, and the other is the neutral line N (different from the zero line. In the single-phase transmission line entering the user, there are two lines, one of which is called the live line and the other is called the zero line. Under normal circumstances, the zero line must pass current to form a loop of current in the single-phase line. In a three-phase system, the three phases form a loop by themselves, and under normal circumstances, the neutral line is without current), so it is called a three-phase four-wire system; in a 380V low-voltage distribution network, in order to obtain a 220V line voltage from a 380V phase-to-phase voltage, an N line is set. In some occasions, it can also be used for zero-sequence current detection to monitor the balance of three-phase power supply.

Three-phase five-wire system
The three-phase five-wire system refers to A, B, C, N and PE wires, among which the PE wire is the protective ground wire, also called the safety wire, which is specially used to connect to the equipment casing to ensure the safety of electricity use. The PE wire is connected with the N wire on the power transformer side, but it cannot be used as the neutral wire after entering the user side, otherwise, it will be no different from the three-phase four-wire system after confusion. However, since this confusion can easily make people lose their vigilance, it may be more likely to cause electric shock accidents in practice. Now it is stipulated that the power supply of civil residences must use the three-phase five-wire system. If yours is not, you can request rectification. For safety, you must make a firm request!
Whether it is the N wire or the PE wire, repeated grounding must be used on the user side to improve reliability. However, repeated grounding is just repeated grounding. It can only be connected together at the grounding point or near the grounding position, but it does not mean that it can be connected together at any location, especially indoors. This must be remembered, and you should also pay attention to whether your friends have violated it! !
It is best to use standard/standard wire colors in the application: yellow for A wire, blue for B wire, red for C wire, brown for N wire, and yellow-green for PE wire

In a three-phase four-wire low-voltage power distribution system, why is it emphasized that the neutral line cannot be disconnected?