Because we use alternating current, which is a sinusoidal current, that is, its amplitude is plus or minus 220 V. When it comes to household appliances, it needs to be rectified, which means transforming both the plus and minus 220 V. 200 V, so your neutral line and it doesn't matter how the live wire is connected, it's the same thing. Municipal electricity is three-phase and four-wire, that is to say three-phase lines and a neutral line. The voltage of one of the three-phase lines and the neutral line is 220 V, so there is a difference.
Detailed information:
The nature of the mains supply:
I am not talking about the distribution link, just use the 220V generator as a mains supply Let's take the example of the power supply: there are three generator windings, arranged evenly in a circle, so the electricity produced has three phases, with a diff120 degree phase erence. There is a wire for each phase, and there. should be a head and tail of wire. This wire head and tail are 220V. At both ends of the power supply, connect the three wire ends (or wire ends) together. This parallel point is the neutral point and mark these three wire ends. a parallel head like ABCN respectively. Connect N to ground and N to ground. The earth potential difference is 0, and N becomes the working neutral line (the neutral line has a working neutral line and a protective neutral line, commonly known as the neutral line and the grounding line). ABC is called phase A, phase B and phase C (three live wires and three other points can form power supply and fire are called line voltages). , and fire and N are called phase voltages.
Obviously the phase voltage of the light and N is the voltage of 220 V at the end of the wire and at the extremoved from the wire of a winding on the generator. Because it is not a direct current generator, it is divided into positive and negative. There is no difference between positive and negative AC current, so the functions of these two wires are the same, that is, the functions of hot and neutral wires are the same. The difference between hot and neutral wires is artificially adjusted. . For electrical devices they are identical, so they can be inserted from the front or the back.
Reference link: Baidu Encyclopedia-Nature of Mains Power