1, in which TT grounding mode is applicable: TT system is suitable for large villages with neutral line output and single-phase and three-phase power sharing. Better safety effect can be obtained by installing leakage protection device.
At present, some construction units adopt TT system. When the construction unit borrows its power supply for temporary power consumption, it applies special protection wires to reduce the steel consumption of grounding devices. Separate the newly added special protection line PE from the working zero line n.
2. Where TN grounding mode is applicable: buildings with internal substations. Because in buildings with substations, it is troublesome to set system grounding and protective grounding separately for TT system.
Even if the system grounding of neutral line in substation is led out by insulated conductor and a separate grounding electrode is set, the distance between it and outdoor underground metal pipeline connected with protective grounding PE line is often difficult to meet the requirements.
However, if TN-C-S system is used in this building, the voltage drop caused by neutral line current on the front-end pen line will lead to potential difference in the building and cause adverse consequences, such as interference with information technology equipment.
Therefore, TN-S system is the best choice for grounding system in buildings with substations, especially in explosive dangerous places, and TN-S system is more suitable for avoiding electric sparks.
3. Where IT grounding mode is applicable: Due to the unfavorable factors of IT system, the application of IT grounding mode is limited. In China, due to the ignorance of IT system, except in mines, metallurgical enterprises and some local areas, IT system is hardly used for the distribution of electrical equipment in buildings.
(1) Surgical equipment and life-sustaining equipment in the hospital. Some electrical medical equipment that maintains patients' lives, such as cardiopulmonary bypass equipment, will soon lead to the danger of patients' lives if the power supply is interrupted, and the high reliability IT system with uninterrupted power supply can avoid this danger.
(2) Safety lighting in some assembly places. This can be divided into two situations: one is a gathering place with a large number of people, and it system should be used for evacuation lighting to ensure the safe evacuation of people during power failure; The other is an important meeting place with great political influence, and the interruption of power supply will have serious political influence. The distribution system in this place, including lighting electricity, should adopt IT system.
3 mine. Whether it is a coal mine or a metal mine, it is a special place with great electrical danger. There is groundwater under the mine, so it is necessary to ensure the power supply of the pump to prevent the underground personnel from being flooded. The exhaust fan of coal mine must also ensure power supply to discharge gas. In addition, people need to rely on reliable power supply for electric vehicles and mining and excavation production, and it systems can ensure uninterrupted power supply.
(4) glass factory. If the glass factory loses power in an accident for a long time, the glass melt will solidify in the glass melting furnace, which will make the glass melting furnace scrapped.
Second, the low-voltage distribution box with leakage protection, pe line and N line cannot be repeatedly grounded.
If the PE line and the N line are grounded together, because the PE line and the N line are connected to the repeated grounding point, there is no difference in the wiring between the repeated grounding point and the working grounding point of the distribution transformer, and the neutral line current originally borne by the N line becomes shared by the N line and the PE line.
Some current is shunted through repeated grounding points. Because of this, it can be considered that there is no PE line in front of the repeated grounding point, and only the PEN line composed of the original PE line and the N line in parallel will lose the advantages of the original TN-S system, so the PE line and the N line cannot be grounded together.
Extended data:
Main advantages of 1 and TT grounding mode:
(1) can suppress overvoltage in low-voltage power network when high-voltage lines are connected to low-voltage lines or insulation between high-voltage and low-voltage windings of distribution transformer breaks down;
(2) It has certain leakage ability to lightning overvoltage of low-voltage power grid;
(3) Compared with the case that the low-voltage electrical appliance is not grounded, when the electrical appliance collides with the shell, the voltage between the shell and the ground can be reduced, thus reducing the danger of personal electric shock;
(4) Because the grounding current is large when single-phase grounding occurs, the protection device (leakage protector) can act reliably and remove the fault in time.
2. The main disadvantages of 2.TT system are:
(1) When the low-voltage and high-voltage lines are struck by lightning, there may be forward and reverse operating overvoltage in the distribution transformer;
(2) The protective effect of low-voltage electrical appliance shell grounding is not as good as that of IT system;
(3) When the metal shell of electrical equipment is charged (the phase wire collides with the shell or the insulation of the equipment is damaged and leaks), the risk of electric shock can be greatly reduced due to grounding protection. However, the low-voltage circuit breaker (automatic switch) may not trip, resulting in the leakage equipment shell voltage higher than safe voltage, which is a dangerous voltage.
(4) When the leakage current is relatively small, even if there is a fuse, it may not blow, so the leakage protector is needed for protection, so TT system is difficult to popularize.
(5) The grounding device of 5)TT system consumes a lot of steel, and it is difficult to recover it, which is time-consuming and laborious.
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