Low voltage power distribution system according to the protective earthing form of different can be divided into: IT system and TT system and TN system.The IT system and TT system equipment exposed conductive parts of the respective protection line directly grounded (previously known as the protective earthing);TN system equipment exposed conductive part via public protection line and neutral power supply direct electrical connection (previously known as the zero protection).
The international electrotechnical commission (IEC) the meaning of the text symbols rules to grounding system are as follows:
The first letter of relations of power system:
T - direct grounding;
Part I - all charged with insulation, or a little by the grounding impedance.
The second letter said device can be exposed conductive part of relationships:
T - exposed conductive parts to ground direct electrical connection, has nothing to do with any ground of the power system;
N - exposed conductive parts location with the contact of the power system, the direct electrical connection (in a communication system, the ground is usually neutral point).
Behind the letters, the letters said the combination of the neutral line and line:
S - the neutral line and line protection is divided;
O to the neutral line and line protection is one.
(1) IT system:
The power of the IT system is neutral point to earth insulation or via high impedance grounding, and the metal casing of electrical equipment directly grounded.Namely: in the past said the protection of the three-phase three-wire system power supply system grounding.
Its working principle is: if the equipment enclosure without grounding, in the event of a single encounter failure, equipment enclosure with the phase voltage, for people to touch the shell, there will be a dangerous current flows through the person with power grid and the distributed capacitance between the earth of circuits.And the metal shell with the protection of the equipment grounding, due to the human body resistance is far larger than the grounding resistance of grounding device, in the case of single collision shell, most of the earth current is grounding device, flows through the body's current is small, thus to personal safety protection.
IT system applicable to adverse environmental conditions, prone to single-phase earth fault, and place of inflammable, explosive.
(2) the TT system:
The power of the TT system neutral point grounding directly;Also directly grounded metal casing of electrical equipment, and has nothing to do with the power of the neutral point grounding.Namely: in the past say three phase four wire system grounding protection in power system.
Its working principle is: when a single encounter shell failure, grounding electricity flows through the protection device and the power supply circuit through the work of grounding device.If someone touch electric shell at this time, because of the protective earthing device resistance is less than the body's resistance, most of the earth current is grounding device, thus for personal protection.
TT systems to ensure the safe there exist shortcomings, mainly displays in:
(1) when a device into single shell failure occurs, the grounding current is not large, often cannot make protective device, which will result in a long-term with fault operation.
(2) when the TT system of electrical equipment is caused by defective insulation leakage, due to the leakage current is often not (only for milliamperes), is impossible for us to make line protection device, this also lead to leakage of equipment shell charged for a long time, increase the risk of the person get an electric shock.
TT system, therefore, must be equipped with residual current protective device, in order to become more perfect protection system.At present, the TT system is widely used in urban, rural residential areas, industrial enterprises and civil construction by public transformer power supply.
(3) the TN system:
In the transformer or generator neutral directly grounded 380/220 v three-phase four-wire low-voltage power grid, will run normally without electrically charged metal casing of electrical equipment through the protection of the public line direct electrical connection with power supply of neutral point.Namely: in the past say three phase four wire system protection by zero in the power supply system.
Single collision occurs when the electrical equipment shell, failure of electricity flowing through the equipment of metal shell forming phase line to protect line of single phase short circuit.This will generate large short-circuit current, circuit protection device on the immediate action, rapid removal of failure part, to ensure the personal safety and normal operation of other equipment or line.
The power of the TN system neutral point grounding directly, and the neutral wire lead.According to its protection line form, TN system is divided into: TN - C system, TN and TN - C - S - S system, etc.
1) TN - C system (three phase four wire system), the system of neutral wire (N) and (PE) is one of the protection line, the line is also known as lines to protect neutral (PEN).It has the advantage of saving a conductor, but on the three-phase unbalanced load or to protect neutral line broken will make all the metal casing of electrical equipment to bring dangerous voltage.In general, such as the protection device and choose appropriate conductor section, TN - C system is able to meet the requirements.
(2) TN -s system (three-phase five wire system), the system of N line and PE line is separated.It has the advantage of PE line in normal circumstances there is no current through, so you don't butt in PE line electromagnetic interference with other devices.In addition, due to the N line and PE line separate, N line disconnect will not affect PE line of protection.But the TN - S system cost of conductive material is more, the investment is bigger.
This system is used for more higher requirements for safety and reliability, the equipment is strict with electromagnetic interference, or poor environmental conditions.For new large-scale civil building, residential area, especially recommended TN - S system.
(3) TN - C - S system (three phase four wire and three-phase five line hybrid system), a portion of the system neutral is unity and protection;And part is separated.It has both TN and TN - S - C system, the characteristics of commonly used in the end of the distribution system environment is poor or have a more strict with electromagnetic interference.
In TN - C, TN and TN - S - S - C system, to ensure that the PE line and PEN line is safe and reliable, except in the power supply working neutral point grounding of PE line and PEN line must also be necessary to repeat grounding.PE line PEN online does not allow installing fuse and switch.
In the same power supply system, not the TT system and TN system protection at the same time.
