This depends on how your transformer grounding system, if the transformer grounding system PE wire and N is connected together, then you in the site power distribution box for a PE line with a line; otherwise you from the distribution room to connect a PE line out, of course, it is best to be staked in the field to connect a PE line.
In fact, you can also regardless of the transformer belongs to which grounding system, you directly staked out to do a repeat of the grounding Dan child test stranded lines palpitation surprised Xi Chao Lou ground protection can be, with a length of 2.5 meters 50 * 5 hot-dip galvanized angle iron on the ground (with the digger a pressure on the ground down), connected to a line for the PE line can be
Low-voltage power distribution system TN, TT, IT comparison
According to the definition of the current national standard "Low-voltage Power Distribution Design Code" (GB50054), the low-voltage power distribution system is divided into three, namely, TN, TT, IT three forms. Among them, the first capital letter T indicates that the neutral point of the power transformer is directly grounded; I indicates that the neutral point of the power transformer is not grounded (or grounded through high impedance). The second capital letter T indicates that the shell of the electrical equipment is directly grounded, but there is no connection with the grounding system of the grid; N indicates that the shell of the electrical equipment is connected to the grounded neutral of the system.
TN system: the neutral point of the power transformer is grounded, and the exposed part of the equipment is connected to the neutral wire.
TT system: power transformer neutral grounding, the shell of electrical equipment using protective grounding.
IT system: power transformer neutral point is not grounded (or through the high impedance grounding), and electrical equipment shell electrical equipment shell using protective grounding.
1, ? TN system
Power system of power transformer neutral grounding, according to the exposed conductive part of the electrical equipment and the system connected to a different way can be divided into three categories: that is, TN-C system, TN-S system, TN-C-S system. The following are introduced separately.
1.1, TN-C system
It is characterized by: power transformer neutral grounding, protective zero line (PE) and the work of zero line (N) **** with.
(1) it is the use of neutral point grounding system neutral line (zero line) as a fault current return conductor, when the phase line of electrical equipment touching the shell, the fault current through the zero line back to the mid-point, due to the short-circuit current is large, so can be used to cut off the power supply of the overcurrent protector. tn-c system is generally used in the zero sequence current protection;
(2) tn TN-C system is suitable for three-phase load basic balance occasions, if the three-phase load imbalance, there is an unbalance current in the PEN line, plus some harmonic currents caused by the load equipment will be injected into the PEN, so that the neutral line N charged, and most likely higher than 50V, it not only makes the equipment enclosure charged, causing personal insecurity, but also can't get a stable reference potential.
(3) TN-C system should be repeated PEN line grounding, its role is to zero when the equipment phase contact with the shell, can effectively reduce the zero line to ground voltage.
From the above, it can be seen that the TN-C system has the following defects:
(1) when the three-phase load imbalance, the unbalance current in the zero line, the zero line presents voltage to ground. When the three-phase load is seriously unbalanced, touching the zero line may lead to electric shock.
(2) the zero line through the leakage protection switch can only be used as the working zero line, can not be used as the protective zero line of the electrical equipment, this is due to the working principle of the leakage switch is determined.
(3) butt with a two-pole leakage protection switch for single-phase electrical equipment, such as for the TN-C system in the metal casing of the protective zero line, it is strictly prohibited and the circuit is connected to the work of the zero line, and is not allowed to be connected to the leakage protection switch in front of the PEN line, but in the use of the most likely to occur in the misconnection.
(4) Repeated grounding device connection line, it is strictly prohibited to connect with the working zero line through the leakage switch.
TN-S power supply system, the working zero line and the protective zero line completely separate, thus overcoming the defects of the TN-C power supply system, so now the construction site is no longer used TN-C system.
1.2, TN-S system
The neutral wire (N) and protective wire (PE) of the whole system are separated.
(1) When the phase line of the electrical equipment touches the shell and is directly short-circuited, the overcurrent protector can be used to cut off the power supply;
(2) when the N line is disconnected, such as the three-phase load imbalance, the neutral potential rises, but the shell has no potential, and the PE line has no potential;
(3) the first and last end of the PE line of the TN-S system should be made to repeat the grounding, in order to reduce the danger caused by PE wire breakage.
(4) TN-S system is suitable for industrial enterprises, large civil buildings.
At present, the separate use of a single transformer power supply or power distribution from the construction site closer to the site are basically used in the TN-S system, and level by level leakage protection, does play a role in ensuring the safety of construction power, but the TN-S system must pay attention to several issues:
(1) Protection Zero line is absolutely not allowed to disconnect. Otherwise, in the zero equipment charged part of the shell or leakage, it does not constitute a single-phase circuit, the power supply will not automatically cut off, there will be two consequences: one is to make the zero equipment loss of safety protection; two is to make the back of the other intact zero equipment shell charged, resulting in a wide range of electrical equipment shell charged, resulting in a terrible threat of electric shock. Therefore, in the "JGJ46-88 construction site temporary power safety technical specifications" stipulates that the special protection line must be done at the end of the first repeated grounding.
(2) the same electrical system of electrical equipment is absolutely not allowed to partially grounded partially zero. Otherwise, when the protective grounding of the equipment leakage, will make the neutral grounding line potential rise, resulting in all the equipment using protective zero shell charged.
(3) protective zero PE wire material and connection requirements: protective zero line cross-section should not be less than the cross-section of the work of the zero line, and the use of yellow / green two-color line. Connected with the electrical equipment should be connected to the protective zero line cross-section of not less than 2.5mm2 insulated multi-stranded copper wire. Protective zero line and electrical equipment should be connected with copper nose and other reliable connection, shall not be articulated; electrical equipment terminal should be galvanized or coated with anticorrosive grease, protective zero line in the distribution box should be connected through the terminal board, no joints in other places.
1.3, TN-C-S system
It consists of two grounding systems, the first part of the TN-C system, the second part of the TN-S system, the interface between the N line and PE line at the point of connection.
(1) When the electrical equipment occurs single-phase touching the shell, the same TN-S system;
(2) When the N line is disconnected, the fault is the same as the TN-S system;
(3) PEN in the TN-C-S system should be repeatedly grounding, while the N line should not be repeated grounding.
The equipment shell connected to the PE line is never electrified during normal operation, so the TN-C-S system improves the safety of operators and equipment. Construction sites generally when the transformer station is far from the site or there is no construction of special transformers to take the TN-C-S system.
2, TT power supply system
Power neutral point directly grounded, the exposed conductive part of the electrical equipment with PE wire to the grounding electrode (this grounding electrode and the neutral point of the ground has no electrical connection)
In the use of this system of protection, when a leakage fault occurs in one of the equipment, the equipment metal casing with the fault voltage is larger, while the current is smaller, is not conducive to the protection switch action, harmful to people and equipment. action, which is harmful to people and equipment. In order to eliminate the defects of T-system and improve the reliability of electricity safety and security, according to the principle of parallel resistance, we propose a technical innovation to improve the TT system. The content of the technical innovation is: with not less than the cross-section of the working zero line green/yellow two-color line (referred to as PT line), parallel total distribution box, sub-distribution box, the main mechanical equipment buried under the 4-5 groups of grounding resistance of the protective ground line for the protective ground line, green/yellow two-color line connected to the metal casing of the electrical equipment. It has the following advantages: 1) single-phase grounding fault point to ground voltage is low, fault current is large, so that the leakage protector quickly act to cut off the power supply, help to prevent electrocution accidents. 2) PT line is not connected with the neutral line, the line erection is clear, intuitive, will not be connected to the wrong line of the hidden danger of accidents; a few units of construction at the same time construction of the large construction site can be divided into pieces, set up by the unit of the PT line, is conducive to the safety of electricity management and save wire consumption. (3) do not have to be buried under each electrical equipment to repeat the grounding line, you can save the cost of burying the grounding line expenses, but also conducive to improving the quality of the grounding line and to ensure that the grounding resistance of ≤ 10 Ω, electricity safety protection is more reliable.
TTT system is widely used in foreign countries, in the country is limited to the local grounding requirements of high electronic equipment occasions, the current construction site is generally not used in this system. However, if the public transformer, and there are other users of the TT system, the construction site should also use this system.
3,? IT system
The charged part of the power system and the earth without a direct connection between (or by resistance grounding), while the exposed conductive part of the powered equipment is directly grounded through the protective wire.
This system is mainly used for 10KV and 35KV high-voltage systems and mines, underground certain low-voltage power supply system, is not suitable for application in the construction site, so we will not analyze here.
The Ministry of Construction's newly issued "building construction safety inspection standards" (JGJ59-99) provides that: construction sites dedicated neutral point directly grounded power system must be used in the TN-S zero protection system. Therefore, TN-S zero protection system has been widely used in the construction site, but if the PE wire breaks or is not properly connected to the electrical equipment, the repetitive grounding resistance value is not up to the safety requirements, the same will happen in the electrocution accident, in order to improve the safety of the TN-S zero protection system, the concept of equipotential coupling is proposed here. The so-called equipotential coupling, is the exposed conductive part of the electrical equipment and conductive part of the system outside (such as concrete reinforcement, various metal piping, etc.) through the protection of the zero line (PE line) for the substantive electrical connection, so that the potential of the two tends to be equal. Should pay attention to the difference, that is, the equipotential coupling line normal no current through, only transfer potential, fault current through. The role of equipotential connection. (1) total equipotential coupling can reduce the expected contact voltage; (2) total equipotential coupling can eliminate the danger of electric shock brought about by fault voltage conduction along PE line outside the device. Therefore, the construction site should also gradually promote the technology. Of course, no matter what form of grounding is by no means foolproof absolute safety. Construction site temporary electricity must be strictly in accordance with the JGJ46-88 specification requirements for the system setup and leakage protector use, strict fulfillment of the construction of electricity design, acceptance system, standardized management, in order to put an end to the occurrence of accidents.
You three-phase five-wire since it has been set dead, that PE and N is to be separated, PE line is sure to repeat the grounding, grounding resistance is less than 4 ohms, I think it should be as close as possible to the grounding of the protected equipment, around the outside of the building to play a circle of grounding poles, every 5 meters, 3 a group of Angle steel or flat steel into the ground at least 600, look at the soil resistivity, finish measuring the How much resistance is greater than 4, put a few more grounding pole.