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 no matter how you connect the live wire, it's the same. 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, uniformly arranged in a circle, so the electricity produced has three phases, with a differencee phase of 120 degrees. 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 other three 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 endfrom 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 distinction 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 AC power
The current direction of AC current is constantly changing. Is it necessary to separate the neutral line? How to determine if the two lines output from the alternator of a power plant are live wires or neutral wires?
Nobel said it very well and exhaustively.
In the image above, whatever wire you cconnect to ground, this is the neutral wire. The remaining chain is the live wire. Remove only the live wire, pass it very far and connect it to your voltage (current) meter. The other terminal of the meter is grounded, which is the neutral wire. This is essentially how power plants provide electricity. But it's actually much more complicated than that. This is actually three-phase electricity. Three live wires are transmitted. The voltage between any wire and the neutral wire is 220, and the voltage between any two wires is 380. This is the actual process, of course, I haven't taken into account the increase and decrease voltage during transmission.
Also, you mentioned the pen testing issue. According to my explanation above, testing electricity with a test pen involves forming a circuit between the wire to be tested, the test pen, the human body and the ground. If this wire is the live wire, there will be current in the middle and the test pen will light up. If this wire is the neutral wire, that is, the ground wire, if you reconnect it, there will be no current. between.
Since there is a large resistance in the middle of the test pen, electricity does not flow during the test. Hell, if the test pen was a light bulb and the resistance was not high , it would electrocute the person. This is why people can be electrocuted if the switch is not connected to the live wire and disconnected.
Both answers above are one-sided. I work in a power plant. I can explain to you: First of all, the electricity produced by general power plants is currently three-phase (divided into three phases: A, B and C). Each phase is 120 degrees (one). circle) different from each other in space. 360 degrees, one phase occupies 120 degrees of space), and the three phases arelive wires.
We all know that to have water flow, there must be water pressure, so if there is current, there must be voltage. Only if there is a live wire and a neutral wire will there be voltage. If there is only one live wire, who makes the difference? As the name suggests, the potential of the neutral wire is zero, that is, it is connected to earth. When the potential of the live wire is positive, current flows out of the live wire through the load and back to the power supply. power supply via neutral wire; When the potential of the live wire is negative (i.e. the potential is currently less than zero), current flows from the neutral wire, through the load, and back to the power source via the wire under pressure. . It can be seen that even though the zero line potential is still zero, that does not mean it is useless.