First, what is grounding?
There are two kinds of grounding, one is working grounding and the other is protective grounding. The purpose of working grounding is to obtain the zero potential of the earth, and the purpose of grounding protection is for personal safety.
Second, the international and national standards on grounding.
The international standard is IEC 60364 and the national standard is GB16895. These two standards are mandatory standards, that is, any distribution system must be unconditionally 100% implemented.
Third, what is the zero line?
The zero line refers to the PEN line, and its correct name is also called the protective neutral line.
Let’s look at the well-known diagram of TN-C grounding system in IEC60364, as follows:
I believe this picture is familiar to everyone, at least I have quoted it n times in Zhihu. Now, let’s take a closer look at this picture.
The grounding system in this picture is TN-C. T here means that the neutral point of the transformer is directly grounded. Pay attention to the left side of the picture, we see that the neutral point of the transformer is directly grounded, and its purpose is to obtain the ground potential, that is, zero potential. Therefore, the direct grounding of transformer neutral point is also called working grounding.
Starting from the neutral point grounding of the transformer, this line is called PEN line, that is, the zero line.
The term zero line is quite controversial. However, the popularity of the zero line is quite high, even people at the grandmother level know it, which shows that this term will exist for a long time.
Fourth, what is protection by zero?
Note that there are two loads in the middle and lower part of the above figure, one on the left needs to introduce three phase lines and neutral lines, and the other on the right only needs to introduce three phase lines. Let’s see how the PEN line is connected to the load.
We can see that the PEN wire is first connected to the shell of the load, and then connected to the neutral input terminal. This is the protection to zero.
What is the purpose of doing this?
As we all know, the correct name of the zero line is to protect the neutral line, and the protection is the first and the neutral line is the second. It can be seen that in the international standard IEC60364, the significance of the PEN line of TN-C grounding system being led to the equipment shell first is to protect personal safety.
Note: the shell of the load, that is, the exposed conductive part, is not directly grounded, but connected to the protective neutral line PEN or the protective line PE, which is represented by the symbol n in IEC60364. Therefore, the grounding system in the above picture belongs to TN grounding system, and the PEN line, that is, the zero line, is directly connected to the user. This grounding system is called TN-C system.
In the actual wiring, the PEN wire is first introduced into the neutral terminal, and then into the special terminal of the exposed conductive part, which is just the opposite of the original TN-C drawing of IEC60364.
Fifth, the problems of TN-C system.
If the PEN wire breaks in the middle of two loads, there will be no problem with the shell of the first load, but there will be a problem with the shell of the second load.