The simultaneous use of single-phase and three-phase loads can be said to be more common. It can be divided into two categories: one is that the electrical equipment itself has both three-phase load and single-phase load. For some small injection molding machines, the rotary extrusion power is a three-phase motor, and the heating part is a 220V single-phase resistance wire. The other type refers to the need to use both single-phase and three-phase loads in a small area of ​​the workplace. For example, in the factory, there is a separate pump house far away from the plant or workshop. It has a three-phase power pump motor and a working single-phase power supply. If the wiring method of single-phase and three-phase load is mixed, if it is not handled properly, it will affect the normal use of electricity, and it will easily lead to electric shock accidents. This article tries to talk about a rough view for reference.
1 TT distribution system single and three phase load supply
As shown in Figure 1, the neutral point of the power supply is directly grounded, and the exposed part of the electrical equipment is connected to the equipment protection grounding device by the protection line for grounding protection, and there is no electrical connection with the power supply ground. Thus, when a single-phase grounding short-circuit fault such as a phase-carrying shell occurs, the short-circuit current is relatively small, and the conventional protection set by the low-voltage circuit breaker with the fault current as a criterion loses its meaning. Therefore, in this case, a low-voltage circuit breaker with residual current protection and phase-to-phase short-circuit current quick-break protection for one section and overload long-delay protection is generally used as the main switch. The correct wiring is shown in Figure 2.
Figure 1 TT system
Figure 2 TT system with correct wiring with residual current protection circuit breaker
In this case, you should pay attention to the following points:
(1) The N line behind the low voltage circuit breaker cannot be grounded repeatedly, otherwise the low voltage circuit breaker will not be put on.
(2) Fuse or separate circuit breakers cannot be installed on the N line. Otherwise, when the N-line is disconnected, when the three-phase load is unbalanced, the neutral point potential will rise, causing the individual phase line voltage to rise, thereby affecting the power supply quality, resulting in increased line loss and reducing the service life of the device. It also causes batch burnout of single-phase equipment.
(3) The selected low-voltage circuit breaker with residual current protection must be four-pole. It is also necessary to cut off the N-line while cutting the phase line, otherwise the circuit breaker will not work normally.
However, in the low-voltage distribution network of villages and towns, individual processing households in villages and towns, pump houses and small processing workshops in industrial enterprises, low-voltage lines and electrical equipment are all privately invested and constructed and managed by themselves, often with low-voltage distribution network insulation. Poorer, the relative leakage current is larger, and the residual current circuit breaker moves frequently, which will seriously affect the normal production and operation of the enterprise. Therefore, industrial enterprises often use TN systems for low-voltage distribution systems.
Single-phase and three-phase load supply in 2TN distribution system
TN power distribution systems are generally divided into two types: one is the TN-C system, and the TN-C-S system. Most of the new ones use the TN-S system.
(1) TN-C system (see Figure 3):
Figure 3 TN-C system
In the TN-C system, the N-line and the PE line are combined into a PEN line, and the exposed parts of the electrical equipment are connected through the PEN line and connected to the power grounding device. In the event of a ground short-circuit fault such as a phase-to-phase contact, a large short-circuit current is formed, and the low-voltage circuit breaker quickly operates to cut off the power.
Since the fault current is relatively large, it is sufficient to use a normal type of low-voltage circuit breaker with instantaneous (or short delay) overcurrent protection and over-load long-delay action protection.
The precautions in this case are as follows:
The 1PEN line must be reliably grounded repeatedly, and the grounding end of the line and the installation location of the equipment must be firmly and reliably grounded. This can reduce the neutral potential and also reduce the potential on the housing of the consumer.
2 Strictly prevent the neutral line from being disconnected. If the neutral line is disconnected, the neutral point potential will rise when the three-phase load is unbalanced, and the dangerous voltage will be applied to the PEN line, which will cause dangerous voltage to the poorly grounded electrical equipment casing, which will endanger personal safety. .
3 When selecting a low-voltage circuit breaker, consider the single-phase load condition, and the single-phase load should be balanced into the three-phase power supply.
Due to the insecure factors of the TN-C system, most of the factories and residential areas are now transformed into TN-C-S systems.
(2) The power supply wiring of single-phase and three-phase load equipment in the TN-C-S system is to properly handle the PEN line and repeat the grounding. Generally, the factory is slightly larger and has a transformer and distribution station. It is equipped with an independent distribution transformer . Most of the grounding types use the TN-C-S system. The N-line and PE lines of a low-voltage busbar of the transformer and distribution station are on the workshop. Used together, called the PEN line. Repeated grounding into the workshop, the N line and the PE line are separated after the workshop power distribution box (screen), no longer confused, as shown in Figure 4.
Figure 4 TN-C-S system
What is important here is that the bifurcation point of the N-line and PE line must be at the distribution box (screen) of the workshop, but not to the equipment terminal, and once separated, it is never allowed to merge again, so as to avoid the TN-C-S system. Evolved into a TN-C system. Since the bifurcation point of the N-line and the PE line is closer to the device terminal, the longer the PE line, the more likely it is to break. Once the PEN line breaks, the operating current of the single-phase load enters the earth from the repeated grounding point, and then returns to the neutral point of the transformer through the working ground of the low-voltage side of the transformer. A voltage to ground is generated at the repeated ground in proportion to its corresponding ground resistance. This voltage is also present on the connected PE wire and the conductive housing of the connected device. Of course, if the repeated grounding is good, the voltage is usually small, and the single-phase load current runs along the N line through the ground to form a closed loop operation. However, if the ground fault is repeated or the PEN line is broken after repeating the grounding point, the device connecting the PEN line is now connected. A deadly voltage to ground will appear on the conductive casing, which will seriously threaten personal safety. There are many examples of such electric shock and casualties in reality. Therefore, in the TN-C-S system, the PEN line is absolutely not allowed to be broken, and the repeated grounding must be firm and reliable.
(3) Under the possible conditions, try to cancel the single-phase load in the single-phase and three-phase load coexistence equipment. For the single-phase illumination required for local working illumination attached to the main equipment, due to the low-cost lighting luminaires, the primary and secondary isolated double-coil isolation transformers can be used for power supply, so that only the PE line is introduced into the equipment. There is no problem with the N line.
(4) In the TN-C-S system, the remote isolated single-phase and three-phase load electrical equipment can be handled by a local TT system and a residual current protector. As mentioned above, the pump house is often protected by PEN line according to the principle of singleness of the protection system. However, such isolated electrical equipment far away from the power distribution room, such as TN-C-S system, equipment control A section of PEN in front of the box is likely to break. Especially in the case of poor grounding, the pump motor casing connected to the PEN line may be charged, posing a safety threat to the workers. If the local TT system is used and the residual current protector is installed, only the N-line is used for the single-phase load circuit. The motor casing is grounded and protected. The control box is equipped with a four-pole circuit breaker with residual current protection. In the event of a phase-to-phase short circuit fault such as a motor phase line, the circuit breaker will operate and the power supply will be removed.
1 TT distribution system single and three phase load supply
As shown in Figure 1, the neutral point of the power supply is directly grounded, and the exposed part of the electrical equipment is connected to the equipment protection grounding device by the protection line for grounding protection, and there is no electrical connection with the power supply ground. Thus, when a single-phase grounding short-circuit fault such as a phase-carrying shell occurs, the short-circuit current is relatively small, and the conventional protection set by the low-voltage circuit breaker with the fault current as a criterion loses its meaning. Therefore, in this case, a low-voltage circuit breaker with residual current protection and phase-to-phase short-circuit current quick-break protection for one section and overload long-delay protection is generally used as the main switch. The correct wiring is shown in Figure 2.
In this case, you should pay attention to the following points:
(1) The N line behind the low voltage circuit breaker cannot be grounded repeatedly, otherwise the low voltage circuit breaker will not be put on.
(2) Fuse or separate circuit breakers cannot be installed on the N line. Otherwise, when the N-line is disconnected, when the three-phase load is unbalanced, the neutral point potential will rise, causing the individual phase line voltage to rise, thereby affecting the power supply quality, resulting in increased line loss and reducing the service life of the device. It also causes batch burnout of single-phase equipment.
(3) The selected low-voltage circuit breaker with residual current protection must be four-pole. It is also necessary to cut off the N-line while cutting the phase line, otherwise the circuit breaker will not work normally.
However, in the low-voltage distribution network of villages and towns, individual processing households in villages and towns, pump houses and small processing workshops in industrial enterprises, low-voltage lines and electrical equipment are all privately invested and constructed and managed by themselves, often with low-voltage distribution network insulation. Poorer, the relative leakage current is larger, and the residual current circuit breaker moves frequently, which will seriously affect the normal production and operation of the enterprise. Therefore, industrial enterprises often use TN systems for low-voltage distribution systems.
Single-phase and three-phase load supply in 2TN distribution system
TN power distribution systems are generally divided into two types: one is the TN-C system, and the TN-C-S system. Most of the new ones use the TN-S system.
(1) TN-C system (see Figure 3):
In the TN-C system, the N-line and the PE line are combined into a PEN line, and the exposed parts of the electrical equipment are connected through the PEN line and connected to the power grounding device. In the event of a ground short-circuit fault such as a phase-to-phase contact, a large short-circuit current is formed, and the low-voltage circuit breaker quickly operates to cut off the power.
Since the fault current is relatively large, it is sufficient to use a normal type of low-voltage circuit breaker with instantaneous (or short delay) overcurrent protection and over-load long-delay action protection.
The precautions in this case are as follows:
The 1PEN line must be reliably grounded repeatedly, and the grounding end of the line and the installation location of the equipment must be firmly and reliably grounded. This can reduce the neutral potential and also reduce the potential on the housing of the consumer.
2 Strictly prevent the neutral line from being disconnected. If the neutral line is disconnected, the neutral point potential will rise when the three-phase load is unbalanced, and the dangerous voltage will be applied to the PEN line, which will cause dangerous voltage to the poorly grounded electrical equipment casing, which will endanger personal safety. .
3 When selecting a low-voltage circuit breaker, consider the single-phase load condition, and the single-phase load should be balanced into the three-phase power supply.
Due to the insecure factors of the TN-C system, most of the factories and residential areas are now transformed into TN-C-S systems.
(2) The power supply wiring of single-phase and three-phase load equipment in the TN-C-S system is to properly handle the PEN line and repeat the grounding. Generally, the factory is slightly larger and has a transformer and distribution station. It is equipped with an independent distribution transformer . Most of the grounding types use the TN-C-S system. The N-line and PE lines of a low-voltage busbar of the transformer and distribution station are on the workshop. Used together, called the PEN line. Repeated grounding into the workshop, the N line and the PE line are separated after the workshop power distribution box (screen), no longer confused, as shown in Figure 4.
What is important here is that the bifurcation point of the N-line and PE line must be at the distribution box (screen) of the workshop, but not to the equipment terminal, and once separated, it is never allowed to merge again, so as to avoid the TN-C-S system. Evolved into a TN-C system. Since the bifurcation point of the N-line and the PE line is closer to the device terminal, the longer the PE line, the more likely it is to break. Once the PEN line breaks, the operating current of the single-phase load enters the earth from the repeated grounding point, and then returns to the neutral point of the transformer through the working ground of the low-voltage side of the transformer. A voltage to ground is generated at the repeated ground in proportion to its corresponding ground resistance. This voltage is also present on the connected PE wire and the conductive housing of the connected device. Of course, if the repeated grounding is good, the voltage is usually small, and the single-phase load current runs along the N line through the ground to form a closed loop operation. However, if the ground fault is repeated or the PEN line is broken after repeating the grounding point, the device connecting the PEN line is now connected. A deadly voltage to ground will appear on the conductive casing, which will seriously threaten personal safety. There are many examples of such electric shock and casualties in reality. Therefore, in the TN-C-S system, the PEN line is absolutely not allowed to be broken, and the repeated grounding must be firm and reliable.
(3) Under the possible conditions, try to cancel the single-phase load in the single-phase and three-phase load coexistence equipment. For the single-phase illumination required for local working illumination attached to the main equipment, due to the low-cost lighting luminaires, the primary and secondary isolated double-coil isolation transformers can be used for power supply, so that only the PE line is introduced into the equipment. There is no problem with the N line.
(4) In the TN-C-S system, the remote isolated single-phase and three-phase load electrical equipment can be handled by a local TT system and a residual current protector. As mentioned above, the pump house is often protected by PEN line according to the principle of singleness of the protection system. However, such isolated electrical equipment far away from the power distribution room, such as TN-C-S system, equipment control A section of PEN in front of the box is likely to break. Especially in the case of poor grounding, the pump motor casing connected to the PEN line may be charged, posing a safety threat to the workers. If the local TT system is used and the residual current protector is installed, only the N-line is used for the single-phase load circuit. The motor casing is grounded and protected. The control box is equipped with a four-pole circuit breaker with residual current protection. In the event of a phase-to-phase short circuit fault such as a motor phase line, the circuit breaker will operate and the power supply will be removed.
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