There are three types of grounding forms for low-voltage distribution systems:
TN system
The power system has a direct grounding point, and the exposed conductive parts of the receiving equipment are connected to the grounding point through protective wires. According to the combination of neutral wire and protective wire, it can be divided into three forms:
TT system
The power system has a direct grounding point, and the exposed conductive parts of the power receiving equipment are connected to the grounding electrode that is not directly related to the power system grounding point through protective wires, as shown in Appendix E1 Figure E.1-4.
The live parts of the power system are not directly connected to the ground (or have a point grounded through a sufficiently large impedance), and the exposed conductive parts of the power receiving equipment are connected to the grounding electrode through protective wires, as shown in Appendix E.1 Figure E.1-5.
1. In the grounding form of the TN system, all exposed conductive parts of the power receiving equipment must be connected to the grounding point of the power system with protective wires (or shared neutral wires, i.e. PEN wires), and the exposed conductive parts that can be simultaneously touched must be connected to the same grounding device.
2. When using the TN-C-S system, when the protection line and neutral line are separated from a certain point (usually at the entrance), they cannot be merged again, and the insulation level of the neutral line should be the same as that of the phase line.
3. Protective and isolation devices should not be installed on the protection line, but it is allowed to set up contacts that can only be disconnected with tools for testing purposes. The isolation of PEN line is detailed in Chapter 8 of this specification.
4. In the TN system, in addition to meeting the requirements of formula 8.6.4.6 in Chapter 8 of this specification, when a direct short circuit fault occurs between the phase line and the ground, in order to ensure that the ground voltage of the protection line and the exposed conductive parts connected to it does not exceed the agreed contact voltage limit of 50V, the following requirements should also be met: In the formula, RB represents the parallel effective grounding resistance of all grounding electrodes (Ω);
U0- rated phase voltage (V);
RE - The minimum ground contact resistance of the conductive part outside the device that is not connected to the protective wire (which may cause short-circuit faults between the phase wire and ground). When the Re value is unknown, it can be assumed to be 10 Ω.
If the requirements of formula 14.2.5 are not met, leakage current operated protection or other protective devices should be used.
5. In the TT system, all exposed conductive parts that share the same grounding protection device must be connected to the grounding electrode shared by these parts with a protective wire (or connected to the protective grounding bus or the main grounding terminal).
The grounding resistance of the grounding device should meet the requirement of cutting off the power supply within the specified time in case of single-phase grounding fault, or limiting the contact voltage to below 50V, that is, meeting the requirement of formula 8.6.4.10.
The grounding fault protection method for TT system distribution lines is specified in Chapter 8 of this specification.
6. Any live parts (including neutral wires) in the IT system are strictly prohibited from being directly grounded. The power system in the IT system should maintain good insulation to the ground. Under normal circumstances, the short-circuit current to ground measured from each phase shall not exceed 70mA (AC RMS). If continuous power supply is the main purpose, this value can be relaxed with the limit of not damaging the equipment. All exposed conductive parts of the equipment should be connected to the grounding electrode (or protective grounding busbar, main grounding terminal) through protective wires. The connection method is specified in Chapter 8 of this specification.
7. The IT system must be equipped with insulation monitoring and grounding fault alarm or display devices.
8. Unless otherwise specified, the IT system shall not lead out neutral wires.
9. When selecting the grounding form of the system, one should be determined based on the conditions of system safety protection and combined with the actual situation of the project.
Low voltage power grids powered by the same generator, distribution transformer, or busbar should not use two grounding systems simultaneously (for example, TN and TT systems should not be used simultaneously in the same low-voltage distribution system).
When it is difficult to fully adopt the TN system in the same low-voltage distribution system, the TT system grounding type can also be partially adopted. However, the power supply part of the TT system should be equipped with devices that can automatically switch to ground faults (including leakage current protection devices) or powered through isolation transformers. The time for automatic fault removal must comply with the relevant provisions of "Grounding Fault Protection" in Chapter 8 of this specification.