In any circuit, when the circuit is not working, the line with voltage (commonly known as live) is called the phase line, and the line without voltage is called the neutral line.
In single-phase circuit, in order to better image, better understanding, for the phase line and neutral line and respectively given two nicknames, called fire line and zero line. Therefore, phase line = fire line, neutral line = neutral line.

Phase line is easy to understand, the power plant, electrical energy output, is by phase line. In a three-phase circuit, the Angle between the three phase lines is 120°, and the voltage between the three phase lines is 380V. They have a common point of intersection, which is called the neutral point. The voltage between the neutral point and any phase line is 220V (calculated by trigonometric function). The line that leads from the neutral point is called the neutral line.

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Therefore, the rated voltage of 380V electrical appliances, use three phase lines; For appliances with rated voltage of 220V, use a phase wire.

There is a problem here, the circuit needs to form a complete loop in order to work properly. Therefore, in a single-phase circuit using only one phase line, there must be a neutral line to work properly. And the use of three phase line in the three-phase circuit, do not need to use the neutral line, also can make the electrical equipment work normally (the three phase circuit of the neutral line, usually protect zero, will be explained below).

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At this point, a question arises — since the voltage between the two phase lines is 380V, why use three phase lines at the same time?

This is because the voltage is 380V only when the Angle between two phase lines is 120 degrees. And if the Angle between the two phase lines changes, the voltage will change. If three phase lines are used, the structure between them is much more stable (triangle stability).

Therefore, although two phase wires can also enable the normal operation of an appliance rated at 380V, they are not usually used in this way.

Both neutral and ground wires need to be grounded
It is actually wrong to say that the neutral line is grounded. The correct term would be neutral ground.
Ground mode
Whether neutral or ground, they are grounded in the same way.
There are many ways to ground, the most common is to use flat steel welded into a “h” font, buried in a 1m deep pit. Salt water is poured into the pit, and a vertical flat steel is welded to the flat steel in the pit, sticking out of the ground. This flat steel is connected to the ground or neutral point to complete the ground.
The specific operation is more complex, interested friends can read my previous article.

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The difference between neutral and ground lines
Since they both need to be grounded, what’s the difference between them?
The structure of ground wire is relatively simple, one end is connected to the ground device, the other end is connected to the user. Its function is relatively simple, is to prevent electrical appliances with the shell electrified. Once electrical leakage occurs, the current on the shell will flow to the earth through the ground wire, thus reducing the harm to the human body.
And the neutral line is the working line, the current can go through the neutral line, directly back to the plant. At the same time, there are the following functions.
The role of neutral grounding
Once the phase line grounding (leakage after contact with the earth and other accidents), due to the connection between the neutral line and the earth, there will be a large short circuit current, promote the protection device (fuse, circuit breaker, etc.) quickly cut off the circuit, to avoid more serious accidents.
At the same time, after the neutral point is grounded, the potential on the neutral line can be guaranteed to be the same as that on the earth, and the neutral point can be locked to zero potential. This will avoid the occurrence of neutral offset, change the Angle between the phase lines, resulting in three-phase voltage imbalance.