Low-voltage power distribution grounding systems are divided into three types: IT system, TT system, and TN system, and these three grounding methods are very easy to confuse. Today I will comprehensively talk about the contents of these three systems, hoping to be helpful to everyone.
1. Definition
According to the current national standard "Low Voltage Distribution Design Code" (GB50054), there are three grounding forms of low-voltage distribution systems, namely IT system, TT system, and TN system.
(1). The first letter indicates the relationship between the power terminal and the ground.
T-The neutral point of the power transformer is directly connected to ground.
I-The neutral point of the power transformer is ungrounded or is grounded through a high impedance.
(2) The second letter indicates the relationship between the exposed conductive parts of the electrical device and the ground.
T - Exposed conductive parts of electrical installations are directly connected to earth at a point that is electrically independent of the earth point at the power supply terminal.
N-The exposed conductive parts of the electrical installation have a direct electrical connection to the power terminal ground point.
Then S: the protective line (PE line) and the neutral line (N line) are completely separated; C: the protective line and the neutral line are combined into one; C-S: part is integrated and part is separated;
2. Comprehensive analysis
1.IT system
(1) An IT system is a system in which the neutral point of the power supply is not grounded and the exposed conductive parts of the electrical equipment are directly grounded. IT systems can have neutral wires, but the IEC strongly recommends not setting them up. Because if a neutral line is set and a ground fault occurs at any point on the N line in the IT system, the system will no longer be an IT system.
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IT system wiring diagram
(2) The neutral point of the power transformer is not grounded (or grounded through high impedance), while the electrical equipment enclosure uses protective grounding.
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It is suitable for places with poor environmental conditions and prone to one-phase grounding or fire and explosion, such as 10KV and 35KV high-voltage systems and some low-voltage power supply systems in mines and underground mines.
Note: In IT systems, when a single-phase ground fault occurs in electrical equipment, the current flowing through the human body is mainly capacitive current. Under normal circumstances, this current is not large, but if the insulation strength of the power grid decreases significantly, this current may reach dangerous levels.
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IT system features:
When the first ground fault occurs in the IT system, it is only the non-fault capacitive current to the ground. Its value is very small. The voltage of the exposed conductive part to the ground does not exceed 50V. There is no need to cut off the fault circuit immediately to ensure the continuity of power supply; - Occurrence In the event of a ground fault, the voltage to the ground increases by 1.73 times; - 220V load needs to be equipped with a step-down transformer, or exclusively supplied by a power supply outside the system; - Install an insulation monitor. Place of use: power supply continuity requirements are high, such as emergency power supply, hospital operating room, etc.
When the power supply distance is not very long, the IT power supply system has high power supply reliability and good safety. It is generally used in places where power outages are not allowed, or where continuous power supply is strictly required, such as electric steelmaking, operating rooms in large hospitals, underground mines, etc. The power supply conditions in underground mines are relatively poor, and cables are susceptible to moisture.
Using an IT power supply system, even if the neutral point of the power supply is not grounded, once the equipment leaks, the single-phase ground leakage current will still be small and will not destroy the balance of the power supply voltage, so it is safer than a system with a grounded neutral point of the power supply. However, if it is used over a long power supply distance, the distributed capacitance of the power supply line to the earth cannot be ignored.
When a short-circuit fault occurs in the load or current leakage causes the equipment shell to be electrified, the leakage current forms a circuit through the earth, and the protective equipment may not necessarily operate, which is dangerous. It is safer only if the power supply distance is not too long. This method of power supply is rare on construction sites.
2.TT system
(1) The TT system is a system in which the neutral point of the power supply is directly grounded, and the exposed conductive parts of the electrical equipment are also directly grounded. Usually the grounding of the neutral point of the power supply is called working grounding, and the grounding of the exposed conductive parts of the equipment is called protective grounding.
In a TT system, these two grounds must be independent of each other. Equipment grounding can be that each equipment has its own independent grounding device, or several equipment can share a grounding device.
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TT system wiring diagram
(2) The neutral point of the power transformer is grounded, and the electrical equipment shell adopts protective grounding. Its metal shell is directly grounded at a grounding level that has nothing to do with the power supply terminal grounding point, referred to as protective grounding or grounding system.
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The main advantages of the TT system are:
(a) It can suppress the overvoltage that occurs in the low-voltage power grid when high-voltage lines are connected to low-voltage lines or insulation breakdown occurs between high- and low-voltage windings of distribution transformers.
(b) It has certain leakage capability against lightning overvoltage of low-voltage power grid.
(c) Compared with the case of low-voltage electrical appliances that is not grounded, when an electrical appliance encounters a shell collision accident, the voltage of the shell to ground can be reduced, thus reducing the risk of personal electric shock.
(d) Since the ground current is relatively large when single-phase is grounded, the protection device (leakage protector) can operate reliably and the fault can be eliminated in time.
(e) The single-phase grounded fault point has a low voltage to the ground and a large fault current, which causes the leakage protector to act quickly to cut off the power supply, which is helpful to prevent electric shock accidents.
(f) The PT line is not connected to the neutral line. The line installation is clear and intuitive, and there is no risk of accidents caused by wrong wiring. Large construction sites where several construction units are constructing at the same time can set up PT lines in slices and units. Conducive to safe power management and saving wire usage.
(g) There is no need to bury repeated grounding wires under each electrical equipment, which can save the cost of burying grounding wires. It can also help improve the quality of grounding wires and ensure that the grounding resistance is ≤10Ω, making electrical safety protection more reliable.
The main disadvantages of the TT system are:
(a) When low and high voltage lines are struck by lightning, forward and reverse conversion overvoltage may occur in the distribution transformer.
(b) The protective effect of grounding of low-voltage electrical appliance shells is not as good as that of IT systems.
(c) When the metal shell of the electrical equipment is charged (phase wire hits the shell or the equipment insulation is damaged and leaks), the risk of electric shock can be greatly reduced due to the grounding protection. However, the low-voltage circuit breaker (automatic switch) may not trip, causing the voltage of the shell of the leakage equipment to the ground to be higher than the safe voltage, which is a dangerous voltage.
(d) When the leakage current is relatively small, even if there is a fuse, it may not be able to be blown, so a leakage protector is needed for protection, so it is difficult to promote the TT system.
(e) The grounding device of the TT system consumes a lot of steel and is difficult to recycle, consuming time and materials.
Applications of TT system:
In the TT system, since the grounding device is located near the equipment, the probability of the PE line being disconnected is small and easy to be discovered.
When the TT system equipment is in normal operation, the shell is not charged. When a fault occurs, the high potential of the shell will not be transmitted to the entire system along the PE line. Therefore, the TT system is suitable for powering voltage-sensitive data processing equipment and precision electronic equipment, and has advantages in hazardous locations such as explosions and fire hazards.
The TT system can significantly reduce the fault voltage on leakage equipment, but generally cannot reduce it to a safe range. Therefore, when using a TT system, a leakage protection device or an overcurrent protection device must be installed, and the former is preferred.
The TT system is mainly used for low-voltage users, that is, for small users who are not equipped with distribution transformers and introduce low-voltage power from outside.
3. TN system
The TN system is a system in which the neutral point of the power supply is directly grounded, and the exposed conductive parts of the equipment are directly electrically connected to the neutral point of the power supply.
In a TN system, the exposed conductive parts of all electrical equipment are connected to the protective wire and connected to the ground point of the power supply, which is usually the neutral point of the power distribution system.
The power system of the TN system has a point directly grounded, and the exposed conductive parts of the electrical installation are connected to this point through a protective conductor.
A TN system is usually a three-phase power grid system with a grounded neutral point. Its characteristic is that the exposed conductive part of the electrical equipment is directly connected to the system ground point. When a short circuit occurs due to shell collision, the short-circuit current forms a closed loop through the metal wire. A metallic single-phase short circuit is formed, thereby generating a short circuit current large enough to enable the protective device to operate reliably and remove the fault.
If the working neutral line N is repeatedly grounded and the case is short-circuited, part of the current may be diverted to the repeated grounding point, which will cause the protective device to fail to operate reliably or refuse to operate, amplifying the fault.
In the TN system, that is, the three-phase five-wire system, the N line and the PE line are laid separately and insulated from each other. At the same time, the PE line is connected to the shell of the electrical equipment instead of the N line. Therefore, what we are most concerned about is the potential of the PE line, not the potential of the N line, so repeated grounding in the circuit is not repeated grounding of the N line.
If the PE line and the N line are grounded together, since the PE line and the N line are connected at the repeated grounding point, there is no difference between the PE line and the N line in the wiring between the repeated grounding point and the distribution transformer working grounding point. The neutral line current is shared by the N line and PE line, and part of the current is shunted through the repeated grounding point. Since it can be considered that there is no PE line in front of the repeated grounding point, there is only the PEN line composed of the original PE line and the N line in parallel. The advantages of the original TN-S system will be lost, so the PE line and the N line cannot be Commonly grounded.
In the TN system, it is divided into three forms: TN-S system, TN-C system and TN-C-S system according to whether the protective neutral line is separated from the working neutral line.
(1), TN-C system
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TN-C system wiring diagram
(1) In the TN-C system, the functions of the PE line and the N line are combined, and a conductor called the PEN line assumes the functions of both. At the electrical equipment, the PEN wire is connected to both the load neutral point and the exposed conductive parts of the equipment. Due to its inherent technical disadvantages, it is rarely used now, especially in civil power distribution, where the TN-C system is basically not allowed to be used.
(2) The neutral point of the power transformer is grounded, and the protective neutral line (PE) and the working neutral line (N) are shared (referred to as PEN), which is called a three-phase four-wire system. Among them, the role of the neutral line (N line):
One is used to provide phase voltage;
The second is used to conduct unbalanced current;
The third is to reduce the neutral point voltage offset.
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Features of TN-C system:
(a) When the equipment shell is charged, the zero-connection protection system can increase the leakage current into a short-circuit current. In fact, it is a single-phase to ground short-circuit fault. The fuse will blow or the automatic switch will trip, cutting off the power of the faulty equipment, which is safer.
(b) The TN-C system is only applicable when the three-phase load is basically balanced. If the three-phase load is unbalanced, there will be an unbalanced current on the working neutral line and a voltage on the ground, so the metal of the electrical equipment connected to the protection line will The shell has a certain voltage.
(c) If the working neutral line is disconnected, the shell of the energized equipment connected to the zero protection line will be charged.
(d) If the phase wire of the power supply is grounded, the potential of the equipment shell will increase, causing the dangerous potential on the neutral wire to spread.
(e) When using a leakage circuit breaker on the main line of the TN-C system, all heavy-duty grounding behind the working neutral line must be removed, otherwise the leakage switch cannot close, and all repeated grounding behind the working neutral line must be removed, otherwise the leakage switch The gate cannot be closed, and the working neutral line cannot be disconnected under any circumstances. Therefore, in practice, the working neutral line can only be repeatedly grounded on the upper side of the leakage circuit breaker.
(f) When the three-phase load is unbalanced, an unbalanced current will appear on the neutral line, and a voltage will appear between the neutral line and the ground. Touching the neutral line may cause an electric shock accident.
(g) The neutral line passing through the leakage protection switch can only be used as the working neutral line and cannot be used as the protective neutral line of electrical equipment. This is determined by the working principle of the leakage switch.
(h) Single-phase electrical equipment connected to a two-pole leakage protection switch, such as the protective neutral line of its metal shell used in a TN-C system, is strictly prohibited from being connected to the working neutral line of the circuit, nor is it allowed to be connected to The PEN line in front of the leakage protection switch is easy to be misconnected during use.
(i) The connecting wire of the repeated grounding device is strictly prohibited from being connected to the working neutral line passing through the leakage switch.
(2), TN-S system
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TN-S system wiring diagram
(1) The neutral line N of the TN-S system is the same as that of the TT system. Different from the TT system, the exposed conductive part of the electrical equipment is connected to the neutral point of the power supply through the PE line, and shares the ground body with the system neutral point, instead of being connected to its own dedicated ground body, neutral line (N line) It is separate from the protective line (PE line).
The biggest feature of the TN-S system is that after the N line and the PE line are separated at the neutral point of the system, there can no longer be any electrical connection. Once this condition is destroyed, the TN-S system will no longer be established.
(2) Completely separate the working neutral line and the protective neutral line, thereby overcoming the shortcomings of the TN-C power supply system, so the TN-C system is no longer used at the construction site.
TN-S system In this system, the working neutral line N and the protective neutral line PE are completely separated from the neutral point of the power supply end. This system is commonly called a three-phase five-wire system.
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When the phase wire of electrical equipment hits the shell and is directly short-circuited, an over-current protector can be used to cut off the power supply.
When the N line is disconnected, such as the three-phase load is unbalanced, the neutral point potential increases, but the shell has no potential, and the PE line also has no potential;
The beginning and end of the PE line in the TN-S system should be repeatedly grounded to reduce the risk caused by PE line breakage.
TN-S system is suitable for industrial enterprises and large civil buildings.
At present, construction sites that use a single transformer to supply power or whose power transformation and distribution stations are close to the construction site basically use the TN-S system. In conjunction with the step-by-step leakage protection, it has indeed played a role in ensuring the safety of construction electricity.
Features of TN-S system:
(a) When the system is running normally, there is no current on the dedicated protection line, but there is an unbalanced current on the working neutral line. There is no voltage between the PE line and the ground, so the zero protection of the metal shell of electrical equipment is connected to the special protection line PE, which is safe and reliable.
(b) The working neutral line is only used as a single-phase lighting load circuit.
(c) The special protection line PE is not allowed to be disconnected, nor is it allowed to enter the leakage switch.
(d) Leakage protectors are used on the trunk lines, so leakage protectors can also be installed on the power supply trunk lines of the TN-S system.
(e) The TN-S power supply system is safe and reliable, and is suitable for low-voltage power supply systems such as industrial and civil buildings.
(f) Protect the neutral line. The PE line is absolutely not allowed to be disconnected, nor is it allowed to enter the leakage switch.
(g) Electrical equipment in the same power system is absolutely not allowed to be partially grounded and partially connected to zero. Otherwise, when the protective grounding equipment leaks, the potential of the neutral point grounding wire will rise, causing the shells of all equipment with protective grounding to be charged.
(h) Materials and connection requirements for the protective neutral PE line: the cross-section of the protective neutral line should not be smaller than the cross-section of the working neutral line, and a yellow/green two-color wire should be used. The protective neutral line connected to the electrical equipment should be an insulated stranded copper wire with a cross-section of not less than 2.5mm2.
The protective neutral line and electrical equipment should be connected with reliable connections such as copper noses, and no hinges should be used; the terminal posts of electrical equipment should be galvanized or coated with anti-corrosion grease. The protective neutral line should be connected through a terminal board in the distribution box and should not be used in other places. The connector appears.
(3) TN-C-S system
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TN-C-S system wiring diagram
(1), TN-C-S
The system is a combination of the TN-C system and the TN-S system. In the TN-C-S system, the section from the power supply uses the TN-C system. Because there is no electrical equipment in this section, it only plays the role of transmitting electrical energy. At a certain point near the electrical load, the EN line is separated to form a separate N line and PE line. From this point on, the system is equivalent to the TN-S system.
(2) In the entire system, the working neutral line and the protection neutral line are partially shared. This system is a local three-phase five-wire system. The first part is the TN-C system, and the second part is the TN-S system. The interface is at the connection point between the N line and the PE line.
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When a single-phase collision occurs in electrical equipment, the same as in the TN-S system
When the N line is disconnected, the fault is the same as that of the TN-S system.
In the TN-C-S system, PEN should be grounded repeatedly, but the N line should not be grounded repeatedly. The equipment shell connected by the PE line will never be charged during normal operation, so the TN-C-S system improves the safety of operators and equipment. Generally, the TN-C-S system is adopted at the construction site when the transformer is far away from the site or there is no construction-specific transformer.
Features of TN-C-S system:
(a) The TN-C-S system can reduce the voltage between the motor shell and the ground, but it cannot completely eliminate this voltage. The size of this voltage depends on the load imbalance and the length of the line. It is required that the load unbalanced current should not be too large, and the PE line should be repeatedly grounded.
(b) PE lines cannot enter the leakage protector under any circumstances, because the action of the leakage protector at the end of the line will cause the front-stage leakage protector to trip and cause a large-scale power outage.
(c) Except for the PE line that must be connected to the N line at the main box, the N line and the PE line are not allowed to be connected at any other sub-box. No switches or fuses are allowed to be installed on the PE line.
In fact, the TN-C-S system is a modification of the TN-C system. When the three-phase power transformer is in good grounding condition and the three-phase load is relatively balanced, the TN-C-S system has good results in construction power consumption practice. However, when the three-phase load is unbalanced and there is a dedicated power transformer at the construction site, the TN-S power supply system must be used.
