According to the specification for design of low-voltage distribution (GB50054), the definition of low voltage power distribution system is divided into three kinds, namely, TN, TT, IT three forms.Among them, the first capital letter T for power transformer neutral point grounding directly;I say the power transformer neutral point grounding (or through high impedance grounding).The second capital letter N said electrical equipment shell connected to grounding neutral system.T for electrical equipment enclosure grounding directly, but is not connected and power grid grounding system.
TN system: power transformer neutral point grounding, equipment exposed portion connected to the neutral wire.Is the electrical equipment of metal shell and zero line N protection system work, called the system zero protection
TT system: power supply transformer neutral point grounding, electrical equipment shell adopts protective earthing.Electrical equipment of the metal shell directly grounding protection system, called the protective grounding system
IT system: power transformer neutral point grounding (or through high impedance grounding), and electrical equipment enclosure of electrical equipment enclosure protection grounding.
1, TN system,
Power transformer neutral point grounding power system, according to the electrical equipment exposed conductive parts connected to the system of different ways and can be divided into three categories: namely, TN - S system, TN - C system, TN - C - S system.The following is described respectively.
TN 1.1 C system
Its characteristic is: the power transformer neutral point grounding, protective zero line (PE) and work (N) common zero line.
(1) it is the use of neutral point grounding system of neutral line (zero) as a fault current backflow conductor, when phase line to touch the shell of the electrical equipment, electricity flows through the zero line fault back to the point, because of the short circuit current is big, so the overcurrent protector can be used to cut off the power supply.TN - C system generally adopts zero sequence current protection;
(2) TN - C system is suitable for the basic balanced three-phase load situation, if the three-phase unbalanced load, the unbalance current of PEN line, add some load equipment caused by harmonic current will be into the PEN, to neutral N charged, and is likely to be higher than 50 v, it not only makes the equipment casing charged, not for personal safety, but also unable to obtain stable benchmark potential;
(3) TN - C PEN line should be repeated earthing system, its effect is when meet zero equipment shell with contact occurs, can effectively reduce the zero line to ground voltage.
By the knowable, TN - C system exists the following defects:
(1) when the three-phase unbalanced load, an imbalance on the zero line current, the zero line to show the voltage.When serious unbalanced three-phase load, hit zero may result in electric shock accidents.
(2) through the zero line of leakage protection switch, can only work as a zero line, cannot serve as the protective zero line of electric equipment, this is due to the working principle of leakage switch.
(3) docking with diode single-phase electric equipment of leakage protection switch, such as used for the metal case in TN - C system protect the zero line, do not work with the circuit of the zero line is connected, also not allowed to pick up the PEN line in front of the leakage protection switch, but in the use of easily happened misconnection.
(4) repeat grounding device connecting line, and it is strictly prohibited by leakage switch are connected to the work of the zero line.TN - S power supply system, the work is completely separated from the zero line and protection of the zero line, which overcomes the defect of TN - C power supply system, so now is no longer in the construction site use TN - C system.
TN - 1.2 S system
The neutral line of the whole system work/zero line (N) and protect the line (PE) are strictly separated.
(1) when the phase line to touch the shell of the electrical equipment, direct short circuit, over-current protector can be used to cut off the power supply.
(2) when N is broken, such as unbalanced three-phase load, the neutral point potential rise, but shell has no potential, PE line or potential;
(3) TN - S system of PE line, should be repeated earthing end of the first to reduce the risk caused by PE line break.
(4) TN - S system is applicable to industrial enterprises, large-scale civil building.
Currently used alone is alone a transformer power supply or distribution of the construction site near the construction site are mostly adopted TN - S system, to match the leakage protection step by step, does play a role to protect the safety of construction of electricity, but the TN - S system must pay attention to several questions:
(1) the protective zero line PE line is absolutely not allowed to disconnect, also not allowed to enter the leakage switch.Otherwise on the zero equipment charged or touch shell leakage occurs, it is no single-phase circuit, power will not cut off automatically, can produce two consequences: one is the cost by zero equipment safety protection;2 it is to make the back of the other equipment shell intact after zero charged, cause a wide range of electrical equipment enclosure charged, a terrible shock.So in the JGJ46-88 construction site temporary electrical safety technical specifications "order special protection line must be in the end of the first to do repeat grounding.
(2) the same power system of electrical equipment is absolutely not allowed of grounding, parts of zero.Otherwise when leakage protective earthing of equipment, can make the neutral ground potential rise, cause all the equipment enclosure protection by zero charge.
(3) the zero line of PE material and connection of protection requires: protect the section work should not be less than zero, zero line cross section, and use the yellow/green double color line.Connected to the electrical equipment protective zero line should be the insulation of the cross section of not less than 2.5 was more strands of wire.Protect the zero line connected to the electrical equipment should use the reliable connection, such as copper nose shall not adopt any articulated;Electrical equipment terminal should be galvanized or painted anticorrosive grease, protect the zero line in distribution box should be through the wiring terminal board, joint shall not occur elsewhere.
The characteristics of TN - S power supply system as follows.
1) the system normal operation, special protection online don't have a current, imbalance current work is zero.PE line to ground no voltage, so the electrical equipment metal case of by zero protection is on the protection of the special line of PE, safe and reliable.
2) work zero N only be used for single-phase circuit lighting load.
3) to use the leakage protector, the main work of zero line shall not repeat grounding, and PE line has repeated earthing, but without leakage protector, so TN - S system leakage protector can also be installed on the main power supply.
TN - C - 1.3 S system
It is composed of two grounding system, the first part is TN - C system, the second part is the TN - S system, the interface in the junction of N line and PE line.
(1) single collision occurs when the electrical equipment shell, with TN - S system;
(2) when N is broken, fault with TN - S system;
(3) the PEN in TN - C - S system should repeat grounding, and N line shoulds not be repeat grounding.PE line connection device shell always won't be charged during normal operation, so the TN - C - S system improves the operation safety of personnel and equipment.Construction site when the change from "the general far away or there is no special transformer construction take TN - C - S system.
2, TT power supply system
Power neutral point grounding directly, exposed conductive part of the electrical equipment with PE line from grounding (no electrical contact with the grounding neutral grounding)
When using the system protection, leakage fault occurring when a equipment, equipment for metal shell with the breakdown voltage is larger, the electric current is small, is not conducive to protection switch action, harm to people and equipment.To eliminate the defect with TT system, improve power security reliability, according to the principle of shunt resistance, the Suggestions to consummate the TT system of technology innovation.Technical innovation content is: work with not less than zero line cross section of the green/yellow double-color wire line (PT), total distribution box, distribution box in parallel, the main mechanical equipment of buried under 4-5 set of protective earthing ground resistance line to protect the earth, with green/yellow double-color wire connection electric equipment metal shell.It has the following advantages:
1) the single-phase grounding fault points of low voltage, fault current is larger, the leakage protector action quickly cut off power supply, to prevent to get an electric shock accidents.
2) PT line not connected with the neutral line, line construction and clear and intuitive, and there won't be wrong line accident hidden danger;Several construction units and construction site, the unit can be divided the PT lines, is conducive to the safe management and save wire consumption.
Don't 3) each buried under repeated earthing wire electric equipment, can save the earth wire embedding expenses, but also to improve the quality of ground wire and ensure the grounding resistance of 10 or less Ω, electric safety protection more reliable.TT system is widely used in foreign countries, in China is limited to local docking to the electronic equipment situation of high demand, at present in the construction site generally do not adopt the system.But if it is public transformer, and there are other users are using TT system, the construction site should also adopt the system.
3, IT system
Electric part of power system with no direct connection between the earth (or the grounding resistance), while the exposed conductive part of the electrical equipment by protecting line directly grounded.
This system is mainly used for high voltage 10 kv, 35 kv system and underground mines, some of the low voltage power supply system, is not suitable for application in the construction site, therefore no longer in this analysis.
The ministry of construction issued by the new "building construction safety inspection standards" (JGJ59-99) regulation: the neutral directly grounded power system dedicated to the construction site must be used in TN - S zero protection system.Therefore, TN - S zero protection system has been widely used in the construction site, but if the PE line break or with electrical equipment for electrical connection, repeat the grounding resistance could not reach the requirements of safety, can also be electric shock accidents, in order to improve the TN -s zero safety protection system, put forward the concept of equipotential connection here.So-called equipotential connection, is the electric equipment can be exposed conductive part and the system can be conductive parts (such as the primary reinforcement in the concrete and various metal pipe, etc.) by protecting zero line (PE) for the electrical connection, essentially equal to the potential of both.Differences should be paid attention to, that is, such as normal when no current through the equipotential connection line, only transfer potential, failure is an electric current passes through.
The role of the equipotential connection:
(1) the total equipotential connection can reduce the expected contact voltage;
(2) total equipotential connection can eliminate offsite conduction along a PE line voltage electric shock of the risk of failure.Therefore the construction site should be phased in the technology.Of course, no matter what earthing form is by no means foolproof absolutely safe.Construction site temporary power must be strictly in accordance with codes for JGJ46-88 system Settings and the use of the leakage protector, strictly fulfill the construction electricity design, acceptance system, standardize the management, to prevent accidents.
