The grounding of power system is directly related to the personal and property safety of users, as well as the normal operation of electrical equipment and electronic equipment. How to choose the appropriate grounding system according to the actual situation and ensure the safe use of the distribution system and electrical equipment is the primary problem faced by electrical designers.
According to the International Electrotechnical Commission (IEC) provisions of the various protective grounding methods of the term concept, low-voltage distribution system according to the different grounding methods called TT system, TN system, IT system. The TN system is divided into TN-C, TN-S and TN-C-S systems.
The grounding form of the low-voltage system
The ground forms of the low-voltage system are IT, TT, TN three categories, and TN is divided into TN-C, TN-C-S, TN-S three forms;
Where the letters represent the meaning:
(1) The first part of the letter indicates the relationship between the neutral points of the distribution system and ground
T: The neutral point of the power supply is directly grounded; I: The neutral point of the power supply is insulated from the ground or grounded through a high impedance point.
(2) The second part of the letter indicates the relationship between the exposed conductive part of the electrical device and the ground
T: The exposed conductive part is directly grounded, and has nothing to do with the ground point of the distribution system;
N: The exposed conductive part is directly connected to the neutral point of the distribution system (also called zero connection system);
(3) The letter after the "-" is an expanded description
C: Protect neutral line and work neutral line with the same wire;
S: The protection neutral line and the working neutral line are completely separated, and two lines are used independently.
C-S: The front part of the protection neutral line and the working neutral line should be the same wire, and the back part of the protection neutral line should be completely separated from the working neutral line, using two wires.
2. TN system
TN system, called protection zero. When the fault makes the metal shell of the electrical equipment live, the phase line and the neutral line are short-circuited, the circuit resistance is small, the current is large, and the fuse can be quickly fused or the protection device can cut off the power supply. In the TN system, it is divided into TN-C, TN-S and TN-C-S three systems.
(1) TN-C system
N wire and PE wire are integrated in the whole system.
(2) TN-S system
The N and PE wires are separate throughout the system.
(3) TN-C-S system
In the whole system, the N wire and PE wire are usually one only in front of the power entry point of the low-voltage electrical device, and the power entry point is divided into two wires.
Third, TT system
The TT system is a system where the neutral point of the power supply is directly grounded and the exposed conductive part of the electrical equipment is also directly grounded. Generally, the grounding of the neutral point of the power supply is called working grounding, and the grounding of the exposed conductive part of the equipment is called protective grounding. In a TT system, the two ground points must be independent of each other. Each device can have its own grounding device, or several devices can share a grounding device.
Fourth, IT system
The IT system is a system in which the neutral point of the power supply is not grounded and the exposed conductive part of the electrical equipment is directly grounded. IT systems can have neutral lines, but IEC strongly recommends against them. Because if the neutral line is set, a ground fault occurs at any point of the N-line in the IT system, the system will no longer be an IT system.
5, TN, TT, IT system applicable environment and place
The grounding form of the low-voltage distribution system should be considered according to the characteristics of the electrical device, operating conditions and service requirements, and the size of the maintenance capacity. As long as the requirements of the installation and operation specifications are met, the three grounding forms are equivalent, there is no priority, the same distribution system can choose one grounding form, or two or three grounding forms can be selected according to needs.
The TN-C system is generally used for general enterprises with basic balance of three-phase load, and the majority of residential users are single-phase users, which is difficult to achieve the balance of three-phase load, and the TN-C system should not be used. TN-S systems are widely used (including communication systems). TT systems can also be used in communication or computer rooms and other places with high power requirements. IT systems are used in coal mines and other places with special requirements for fire prevention.
The principle is as follows:
When a single-phase short-circuit fault occurs in the system, the TN system is equivalent to direct grounding without going through the grounding resistance on the power side, and the loop impedance is small. The TT system circuit is equivalent to the grounding resistance through the power side, and the circuit impedance is large. Because the IT system is not grounded on the power side or is grounded with high impedance, the loop impedance is the maximum.
(1) When the TN-C system is working normally, there is an asymmetric load current passing through the PEN line, and there may be a third harmonic current passing through the PEN line. The voltage drop generated on the PEN line will be presented on the housing of the electrical equipment connected to the PEN line and the metal conduit of the line. When the PEN line is broken, or the PEN line connection terminal is not properly connected, or the phase ground short circuit fault occurs, the fault voltage to the ground will be high, and the fault voltage at one point can be transferred to other parts along the PEN line.
When the electrical equipment housing and metal bushing with this dangerous voltage, there may be one or more places to the ground ignition, arc, ignition nearby flammable materials, resulting in fire. This system is not very safe, generally used for the three-phase load basic balance of the general enterprise, residential users are mostly single-phase users, it is difficult to achieve the balance of three-phase load, should not use TN-C system.
The TN-S system works normally, there is no unbalanced current passing through the PE line, and the equipment housing connected to the PE line is not potentiated, and it is only potentiated when the ground fault occurs, so the risk of the above fault can be greatly reduced. This system is widely used (including communication systems), but ensure the reliability of the ground protection device operation, E connection terminal should be firmly connected.
(2) The metal shell of the TT system equipment is grounded by a separate grounding pole, which has no direct connection with the grounding of the power supply. The equipment shell is a ground potential and will not produce sparks or arcs, so it is safer.
In addition, when the power side or the electrical device is grounded, the fault voltage will not conduct and cross-channel along the PE or PEN line in the electrical device like the TN system, which is better than the TN system.
However, when a ground fault occurs, the fault current needs to pass through the grounding resistance of the equipment and the grounding resistance of the power supply, the circuit impedance is large, and the fault current is smaller than that of the TN system, which reduces the spiritual sensitivity of the line protection device. With the development and application of the leakage protector, the TT system has overcome the weakness of protecting the electrical apparatus.
One additional point: the PE line in TN system is drawn from the neutral point of the power supply. When the transient impulse overvoltage caused by lightning strike or power frequency overvoltage caused by power grid fault occurs, the potential of the phase line and PE line rises at the same time, and the electrical device insulation can withstand the overvoltage to ground. The PE line in the TT system is directly derived from the earth, which is the zero potential of the earth, and the insulation of the electrical device bears a large overvoltage to the ground, which is prone to breakdown and other accidents, and measures should be taken to prevent it.
(3) The neutral point of the power supply in the IT system is insulated to the ground or grounded through the arc suppression coil. When the ground fault occurs, the fault current is the non-fault phase of the ground capacitance current, the fault voltage does not exceed 50V, does not produce electrical spark or arc, the general place does not require immediate removal of the fault circuit, just send an alarm signal, and eliminate the fault within the specified time, you can ensure the reliability of the power supply.
Therefore, the IT system is used in coal mines and other places with special requirements for fire prevention, but the IT system should not be equipped with a neutral line, and the voltage resistance requirements of the power supply and electrical equipment are also high. Can not provide lighting, control and other needs of 220V power supply, need to set 380/220V step-down transformer to provide 220V power supply, making the line structure complicated.