The grounding system of low-voltage systems can be classified into three forms: TN, TT, and IT, according to different combinations of distribution systems and electrical equipment grounding. The meaning of their text codes is as follows:
1. The first letter represents the ground relationship of the distribution system:
T: There is a point of direct grounding at the power end;
I: All live parts of the power supply end are insulated from the ground or have one point grounded through impedance.
2. The second letter represents the relationship between the exposed conductive part of the electrical device and the ground:
T: The exposed conductive parts are directly electrically connected to the ground, regardless of any grounding point of the distribution system;
N: The exposed conductive parts are directly electrically connected to the grounding point of the distribution system (in AC distribution systems, the grounding point is usually the neutral point) In the TN system, the exposed conductive parts of all electrical equipment are connected to the protection line and to the grounding point of the distribution system. This grounding point is usually the neutral point of the distribution system