The grounding of the power system directly relates to the user's personal and property safety, as well as the normal operation of electrical equipment and electronic equipment. How to choose the right grounding system for the actual situation to ensure the safe use of the distribution system and electrical equipment is the primary problem faced by designers. This article briefly analyzes the characteristics and application locations of different grounding systems and is for reference only.
1. The grounding system is classified according to the different grounding combinations of the distribution system and the electrical equipment. According to the IEC60364 regulations, the grounding system generally consists of two letters, followed by subsequent letters if necessary.
The first letter: indicates the relationship between the neutral point of the power supply and the ground. T: Direct grounding I: Not grounded, or connected to ground through impedance. Second letter: Indicates the relationship between the shell of the electrical equipment and the earth. T: Independent of the power supply grounding point. Directly grounded N: Directly connected to the ground point of the power system or the conductor leading to it. Subsequent letters: Indicates the relationship between the neutral line and the protection line. C: Indicates that the neutral line N and the protection line PE are combined into one (PEN line)
S: indicates that the neutral line N is separated from the protection line PE. C-S: indicates that the PEN line is on the power supply side, and the neutral line N and the protection line PE are separated from a certain point. There are three types of low-voltage power distribution systems:
â– TN System â– TT System â– IT System
2. Composition and characteristics of different grounding systems:
â– Composition and characteristics of the TN system In the TN system, the enclosure of all electrical equipment is connected to the protection line (PE) and is connected to the neutral point of the distribution system (if there is no neutral point, ie the transformer is connected to the secondary side of the triangle or Without a neutral point, one phase of the transformer secondary winding can be grounded, but this contact cannot be used as a PEN line). The protection line should be grounded near each substation. When the distribution system is introduced into a building, the protection line is grounded at its entrance. In order to ensure that the potential of the protection line is as close as possible to the ground potential during the fault, connect the protection line to the nearby effective grounding body as far as possible. If necessary, increase the grounding point and evenly distribute it. Its characteristic is that the fault current is larger, only related to the impedance of the cable. In the event of an insulation fault, short-circuit current protection devices are required to momentarily disconnect the circuit.
According to the international standard IEC60364, according to whether the neutral line and the protection line are merged, the TN system is divided into the following three types:
â–¡TN-C
â–¡TN-S
â–¡TN-C-S
Note: For the power grid, when the copper conductor cross-sectional area is ≤ 10mm2 and the aluminum conductor cross-sectional area is ≤ 16mm2, the TN-S system must be used instead of the TN-C system.
The following describes its composition and characteristics:
2.1TN-C system:
In this system, the protection line and the neutral line are combined into one, called the PEN line.
advantage:
â–¡ The TN-C solution is easy to implement, saves one wire, and protects the electric appliance to save one pole, reducing the initial investment cost of the equipment.
â–¡ When a short circuit to ground occurs, the fault current is large, and an overcurrent protection device can be used to shut off the power instantaneously to ensure the safety of human life and property.
Disadvantages:
â–¡ If there is single-phase load in the circuit, or three-phase load is unbalanced, and there is harmonic current in the power grid, due to the current in the PEN, there is a voltage drop between the casing of the electrical equipment and the metal casing of the circuit, which is not favorable for sensitive electronic equipment. â–¡PEN current can cause explosion in potentially explosive atmospheres. â–¡When the PEN wire is disconnected or the phase-to-ground short-circuit occurs, it will present a relatively high ground fault voltage, which may increase the accident range. â–¡Cannot use the residual current protection device RCD (RCD will refuse to operate because no leakage current is detected), so when the insulation fault occurs, the human body and equipment cannot be effectively protected
2.2TN-S system
The system protection line (PE) and the neutral line (N) are separated
advantage:
â–¡ The PE wire does not pass the load current during normal operation. It is suitable for data processing and precision electronic equipment. It can also be used for explosive atmospheres. In civil buildings, most household appliances have separate grounding contact plugs. The TN-S system is convenient. , And safety â–¡ If the circuit impedance is too high or the power short circuit capacity is small, the residual current protection device RCD is required to protect personal safety and equipment to prevent fire hazards.
Disadvantages:
â–¡ Due to the increase of the neutral line, the initial investment is higher. When the TN-S system is relatively short-circuited, the fault voltage to the ground is higher.
2.3TN-C-S system
At some point in the system, PEN is divided into protection line and neutral line. After separation, neutral line (N) is insulated from ground (Note: After PEN lines are separated, they cannot be combined)
advantage:
â–¡ Suitable for power supply in industrial and mining enterprises. The TN-C system can meet the needs of fixed equipment. The back-end TN-S system can meet the needs of potential-sensitive electronic equipment. â–¡ In civil buildings, the power line is TN-C and enters buildings. After using TN-S system, ensure the advantages of TN-S system
2.4TT system composition and characteristics:
The neutral point of the transformer or generator of the TT system is directly grounded, and all the enclosures of the electrical equipment are connected together with a protection wire and connected to a grounding point independent of the neutral point of the power supply.
advantage:
â–¡Electrical equipment enclosure has no electrical connection with the grounding of the power supply. It is suitable for potential sensitive data processing equipment and precision electronic equipment. Fault voltage will not spread to ground when fault occurs. â–¡When shorted to ground,
Due to the limitation of the current grounding resistance and the grounding resistance of the electrical equipment, the short circuit current is small and the danger can be reduced.
Disadvantages:
â–¡Short-circuit current is short. When a short circuit occurs, the short-circuit current protection device will not operate, and it will cause electric shock accidents. â–¡Short-circuit protection device's over-current protection cannot provide insulation fault protection, and residual current protector RCD is required to carry out personal and equipment safety protection.
2.5 The composition and characteristics of the IT system:
The IT system's power supply is not grounded or grounded through an impedance. The enclosure of the electrical device can be directly grounded or connected to a separate grounded body via a protective wire.
advantage:
â–¡ Single-phase grounding The first fault, the fault current is small, can not cut off the power supply, alarm equipment alarm, eliminate the fault through the inspection line, the power supply continuity is higher, suitable for large-scale power plant plant power and important production line power â–¡ can be used Residual current protector (RCD) performs personal and equipment safety protection Disadvantages: If the second failure is eliminated before the first failure is eliminated, such as a different phase of the ground short circuit, the fault current is very high, very dangerous, so a fault detection The demand for alarm equipment is high, so that the possibility of double failures can be eliminated and reduced in time to ensure the reliability of IT systems.
2.6 Grounding System Neutral Protection
Use the 4-pole switch to disconnect the neutral wire in the following situations:
■Neutral cross-sectional area of ​​the TT and TN systems is smaller than the phase line. ■Neutral lines are avoided in the terminal power distribution. The neutral line must be protected and can be divided:
■Second failure protection device in IT system to prevent secondary failure after the first fault of the neutral line ■In TT and TN-S systems, the cross-sectional area of ​​the neutral line is less than the cross-sectional area of ​​the phase line ■All In grounded systems, three or more harmonic currents may occur (especially when the neutral cross-sectional area decreases)
In a TN-C system, the neutral line is also a protective line that cannot be disconnected. Due to load current imbalance and insulation fault currents, a dangerous neutral voltage offset can occur. To do this, the user must make equipotential bonding and grounding in each area.
2.7 Selection of grounding system:
The selection of the grounding system should be based on the characteristics of the electrical installations, operating conditions and requirements, and the size of the maintenance capabilities. The opinions of the integrated users and design installers should be selected in accordance with local conditions. As long as the installation and operation specifications are met, the three grounding systems are equivalent and have no priority.
Steps to choose a grounding system:
â– First, to ensure maximum safety and flexibility, three grounding systems can be used in the same power grid.
Series connection and parallel connection of different grounding systems:
â– Must abide by local standards and regulations â– Know the user's requirements and existing maintenance resources:
â–¡ Operational continuity requirements â–¡ Is there a maintenance service â–¡ Is there a fire hazard?
3. System selection and application 3.1 Usually selected as follows:
â–¡ Operating continuity requires high maintenance service: Select IT system â–¡ High operating continuity requires no maintenance service occasions: No completely satisfactory choice, TT system can be selected (its trip selectivity is easy to implement) or TN can be selected System (reduces danger)
â–¡ Operational continuity requirements are not important and maintenance capability: Select TN-S system for easy and quick maintenance and expansion â–¡ Operational continuity is lower. Requirements for maintenance-free service: Select TT system â–¡ Fire-hazardous situation: Select IT system ( Have personnel maintain) or select TT system (use 0.5A residual current protection device)
3.2 Special grid and load selection:
â–¡ For long line, large leakage current grid: Select TN-S system â–¡ Backup power supply grid: Select TT system â–¡ Load sensitive to large fault current (motor): Select TT or IT system â–¡ Poor insulation rating (Electric furnace) or equipment with large-scale high-frequency filtering (large-scale computer): Select TN-S system â–¡ Control and monitoring system: Select TT (equal potential connection between communication devices) or IT system (high operation continuity)
1. The grounding system is classified according to the different grounding combinations of the distribution system and the electrical equipment. According to the IEC60364 regulations, the grounding system generally consists of two letters, followed by subsequent letters if necessary.
The first letter: indicates the relationship between the neutral point of the power supply and the ground. T: Direct grounding I: Not grounded, or connected to ground through impedance. Second letter: Indicates the relationship between the shell of the electrical equipment and the earth. T: Independent of the power supply grounding point. Directly grounded N: Directly connected to the ground point of the power system or the conductor leading to it. Subsequent letters: Indicates the relationship between the neutral line and the protection line. C: Indicates that the neutral line N and the protection line PE are combined into one (PEN line)
S: indicates that the neutral line N is separated from the protection line PE. C-S: indicates that the PEN line is on the power supply side, and the neutral line N and the protection line PE are separated from a certain point. There are three types of low-voltage power distribution systems:
â– TN System â– TT System â– IT System
2. Composition and characteristics of different grounding systems:
â– Composition and characteristics of the TN system In the TN system, the enclosure of all electrical equipment is connected to the protection line (PE) and is connected to the neutral point of the distribution system (if there is no neutral point, ie the transformer is connected to the secondary side of the triangle or Without a neutral point, one phase of the transformer secondary winding can be grounded, but this contact cannot be used as a PEN line). The protection line should be grounded near each substation. When the distribution system is introduced into a building, the protection line is grounded at its entrance. In order to ensure that the potential of the protection line is as close as possible to the ground potential during the fault, connect the protection line to the nearby effective grounding body as far as possible. If necessary, increase the grounding point and evenly distribute it. Its characteristic is that the fault current is larger, only related to the impedance of the cable. In the event of an insulation fault, short-circuit current protection devices are required to momentarily disconnect the circuit.
According to the international standard IEC60364, according to whether the neutral line and the protection line are merged, the TN system is divided into the following three types:
â–¡TN-C
â–¡TN-S
â–¡TN-C-S
Note: For the power grid, when the copper conductor cross-sectional area is ≤ 10mm2 and the aluminum conductor cross-sectional area is ≤ 16mm2, the TN-S system must be used instead of the TN-C system.
The following describes its composition and characteristics:
2.1TN-C system:
In this system, the protection line and the neutral line are combined into one, called the PEN line.
advantage:
â–¡ The TN-C solution is easy to implement, saves one wire, and protects the electric appliance to save one pole, reducing the initial investment cost of the equipment.
â–¡ When a short circuit to ground occurs, the fault current is large, and an overcurrent protection device can be used to shut off the power instantaneously to ensure the safety of human life and property.
Disadvantages:
â–¡ If there is single-phase load in the circuit, or three-phase load is unbalanced, and there is harmonic current in the power grid, due to the current in the PEN, there is a voltage drop between the casing of the electrical equipment and the metal casing of the circuit, which is not favorable for sensitive electronic equipment. â–¡PEN current can cause explosion in potentially explosive atmospheres. â–¡When the PEN wire is disconnected or the phase-to-ground short-circuit occurs, it will present a relatively high ground fault voltage, which may increase the accident range. â–¡Cannot use the residual current protection device RCD (RCD will refuse to operate because no leakage current is detected), so when the insulation fault occurs, the human body and equipment cannot be effectively protected
2.2TN-S system
The system protection line (PE) and the neutral line (N) are separated
advantage:
â–¡ The PE wire does not pass the load current during normal operation. It is suitable for data processing and precision electronic equipment. It can also be used for explosive atmospheres. In civil buildings, most household appliances have separate grounding contact plugs. The TN-S system is convenient. , And safety â–¡ If the circuit impedance is too high or the power short circuit capacity is small, the residual current protection device RCD is required to protect personal safety and equipment to prevent fire hazards.
Disadvantages:
â–¡ Due to the increase of the neutral line, the initial investment is higher. When the TN-S system is relatively short-circuited, the fault voltage to the ground is higher.
2.3TN-C-S system
At some point in the system, PEN is divided into protection line and neutral line. After separation, neutral line (N) is insulated from ground (Note: After PEN lines are separated, they cannot be combined)
advantage:
â–¡ Suitable for power supply in industrial and mining enterprises. The TN-C system can meet the needs of fixed equipment. The back-end TN-S system can meet the needs of potential-sensitive electronic equipment. â–¡ In civil buildings, the power line is TN-C and enters buildings. After using TN-S system, ensure the advantages of TN-S system
2.4TT system composition and characteristics:
The neutral point of the transformer or generator of the TT system is directly grounded, and all the enclosures of the electrical equipment are connected together with a protection wire and connected to a grounding point independent of the neutral point of the power supply.
advantage:
â–¡Electrical equipment enclosure has no electrical connection with the grounding of the power supply. It is suitable for potential sensitive data processing equipment and precision electronic equipment. Fault voltage will not spread to ground when fault occurs. â–¡When shorted to ground,
Due to the limitation of the current grounding resistance and the grounding resistance of the electrical equipment, the short circuit current is small and the danger can be reduced.
Disadvantages:
â–¡Short-circuit current is short. When a short circuit occurs, the short-circuit current protection device will not operate, and it will cause electric shock accidents. â–¡Short-circuit protection device's over-current protection cannot provide insulation fault protection, and residual current protector RCD is required to carry out personal and equipment safety protection.
2.5 The composition and characteristics of the IT system:
The IT system's power supply is not grounded or grounded through an impedance. The enclosure of the electrical device can be directly grounded or connected to a separate grounded body via a protective wire.
advantage:
â–¡ Single-phase grounding The first fault, the fault current is small, can not cut off the power supply, alarm equipment alarm, eliminate the fault through the inspection line, the power supply continuity is higher, suitable for large-scale power plant plant power and important production line power â–¡ can be used Residual current protector (RCD) performs personal and equipment safety protection Disadvantages: If the second failure is eliminated before the first failure is eliminated, such as a different phase of the ground short circuit, the fault current is very high, very dangerous, so a fault detection The demand for alarm equipment is high, so that the possibility of double failures can be eliminated and reduced in time to ensure the reliability of IT systems.
2.6 Grounding System Neutral Protection
Use the 4-pole switch to disconnect the neutral wire in the following situations:
■Neutral cross-sectional area of ​​the TT and TN systems is smaller than the phase line. ■Neutral lines are avoided in the terminal power distribution. The neutral line must be protected and can be divided:
■Second failure protection device in IT system to prevent secondary failure after the first fault of the neutral line ■In TT and TN-S systems, the cross-sectional area of ​​the neutral line is less than the cross-sectional area of ​​the phase line ■All In grounded systems, three or more harmonic currents may occur (especially when the neutral cross-sectional area decreases)
In a TN-C system, the neutral line is also a protective line that cannot be disconnected. Due to load current imbalance and insulation fault currents, a dangerous neutral voltage offset can occur. To do this, the user must make equipotential bonding and grounding in each area.
2.7 Selection of grounding system:
The selection of the grounding system should be based on the characteristics of the electrical installations, operating conditions and requirements, and the size of the maintenance capabilities. The opinions of the integrated users and design installers should be selected in accordance with local conditions. As long as the installation and operation specifications are met, the three grounding systems are equivalent and have no priority.
Steps to choose a grounding system:
â– First, to ensure maximum safety and flexibility, three grounding systems can be used in the same power grid.
Series connection and parallel connection of different grounding systems:
â– Must abide by local standards and regulations â– Know the user's requirements and existing maintenance resources:
â–¡ Operational continuity requirements â–¡ Is there a maintenance service â–¡ Is there a fire hazard?
3. System selection and application 3.1 Usually selected as follows:
â–¡ Operating continuity requires high maintenance service: Select IT system â–¡ High operating continuity requires no maintenance service occasions: No completely satisfactory choice, TT system can be selected (its trip selectivity is easy to implement) or TN can be selected System (reduces danger)
â–¡ Operational continuity requirements are not important and maintenance capability: Select TN-S system for easy and quick maintenance and expansion â–¡ Operational continuity is lower. Requirements for maintenance-free service: Select TT system â–¡ Fire-hazardous situation: Select IT system ( Have personnel maintain) or select TT system (use 0.5A residual current protection device)
3.2 Special grid and load selection:
â–¡ For long line, large leakage current grid: Select TN-S system â–¡ Backup power supply grid: Select TT system â–¡ Load sensitive to large fault current (motor): Select TT or IT system â–¡ Poor insulation rating (Electric furnace) or equipment with large-scale high-frequency filtering (large-scale computer): Select TN-S system â–¡ Control and monitoring system: Select TT (equal potential connection between communication devices) or IT system (high operation continuity)
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