Why can't three-phase wires be laid separately through pipes?

Many friends are puzzled. Why do you want to make such a rule? The reasons are as follows: "metal pipe" in this paper refers to ferromagnetic pipes such as steel pipes or electric conduits used in building electrical engineering. This kind of pipeline will be caused by the eddy current effect caused by the unbalanced AC current in the pipeline, which will increase the temperature of the pipeline, and the insulation of the insulating wire in the pipeline will age or even fall off, leading to leakage, even short circuit, fire and other failures. Moreover, as long as there is unbalanced current in the metal tube, the wall of the metal tube will heat up due to eddy current effect, which will lead to insulation aging of the wire. Article 52 1.5 of IEC standard also stipulates: "When the wires of AC circuits are installed in ferromagnetic enclosures, all phase wires and neutral wires (if any) shall be encapsulated in the same enclosure. If this condition cannot be met, harmful fever may be caused by the induced effect. " Of course, the three-phase lines will not be laid separately in construction, unless the floor is embedded and the line section is large, the idea of laying small pipes separately will occur.