First, indirect contact with electric shock protection
(1) Exposed metal parts that are not energized under normal conditions, such as metal casings, metal shields and metal frames, may cause dangerous contact voltages in the event of metal short-circuit faults such as leakage or shelling. The human body touches these exposed metal parts, which is called indirect contact with electric shock.
(2) In the event of failure of electrical equipment, wiring, etc., the protection against accidents caused by personal electric shock is called indirect contact electric shock protection, or protection against indirect contact with live parts.
(3) There are several types of indirect electrical protection measures:
1 Automatically cut off the power supply (ground fault protection).
2 Electrical equipment with double or reinforced insulation (ie Class II electrical products).
3 Insulate a location that is at risk of electric shock to form a non-conductive environment.
4 Use ungrounded local equipotential bonding protection or take equal potential equalization measures.
5 Use safety extra low voltage.
6 Conduct electrical isolation.
Second, the difference between neutral point and zero point, neutral line and zero line
When the power supply side (transformer or generator) or the load side is star-connected, the common contact where the head end (or tail end) of the three-phase coil is connected together is called the neutral point, referred to as the midpoint. The neutral point is divided into the neutral point of the power supply and the neutral point of the load. The wire drawn from the neutral point is called the neutral line, referred to as the neutral line.
If the neutral point is directly connected to the grounding device to obtain the reference zero potential of the earth, the neutral point is called zero point, and the wire drawn from the zero point is called the zero line.
Usually one of the two lines of a 220 volt single-phase loop is called "phase line" or "hot line", and the other line is called "zero line" or "ground line". The term "fire line" and "ground line" is only a common name in practice, especially the "ground line" is not accurate. Strictly speaking, it should be said that if the loop power supply side (three-phase distribution transformer neutral point) is grounded, it is called "zero line"; if it is not grounded, it should be called "neutral line" to avoid "ground" in the grounding device. The line is mixed.
When it is a three-phase line, in addition to the three phase lines, a wire, that is, a neutral line, can be drawn from the neutral point to form a three-phase four-wire line. The voltage between the phase lines in this line is called the line voltage, and the voltage between the phase line and the neutral line is called the phase voltage.
Whether the neutral point is grounded is also called the neutral point system. The neutral point system can be roughly divided into two categories, namely the neutral point grounding system and the neutral point insulation system. According to the provisions of the International Electrotechnical Commission (IEC), the low-voltage power distribution system is divided into three types: IT, TT, and TN. The TN system is divided into three categories: TN-C, TN-S, and TN-CS.
Third, the protection ground
The so-called protective grounding is to make the electrical equipment in the event of a fault. The metal part of the ground voltage (such as the outer casing, etc.) is electrically connected to the earth by wires.
If the electrical equipment is not protected from grounding, when the insulation of one part of the equipment is damaged, the outer casing will be charged. At the same time, due to the capacitance between the line and the earth, the human body will be exposed to the outer casing of the electrical equipment damaged by the insulation, which will be exposed to electric shock. After the protective grounding of the electrical equipment, the grounding short-circuit current will pass through both the grounding body and the human body. The grounding resistance is generally less than 4 ohms, and the human body resistance is about 1000 ohms. Therefore, the current flowing through the human body is almost equal to zero by the shunting action of the grounding body, thus avoiding the risk of human body electric shock under the short-circuit fault current.
Fourth, protection and zero
The protection zero is abbreviated as zero connection, which is to connect the metal parts (housing) of the electrical equipment under normal conditions without wires, and connect the wires directly to the neutral line of the low-voltage distribution network (neutral line) to protect personal safety and prevent occurrence. Electric shock accident.
Protection and zero connection generally cooperate with fuses, trip units, etc., as a low-voltage neutral point direct grounding system and 380/220 volt three-phase four-wire system (referred to as TN-C system in the IEC standard) .
With protection zero, when a short circuit occurs, the short-circuit current flows from the phase line to the neutral line and back to the neutral point of the transformer. Since the resistance and reactance of the fault circuit are very small, the fault current is large enough to make the protection device (fuse or automatic switch) on the line act quickly, thereby disconnecting the leakage device from the power supply, eliminating the danger and protecting it. effect.
Although the protection grounding and the protection zero can ensure personal safety, the protection zero is more advantageous than the protective grounding, because the impedance of the neutral line is small and the short-circuit current is large, thus overcoming the limitation that the protection grounding requires a small resistance value. .
Fifth, the difference between protective grounding and protection
The main differences between protective grounding and protective zeroing are:
(1) Protection principle Different protection grounding is to limit the ground voltage after leakage of equipment so that it does not exceed the safe range. In the high-voltage system, in addition to limiting the ground voltage, in some cases, there is also the role of causing the grid protection device to operate; the protection of the zero is to make the equipment leakage by means of the zero-connected line to form a single-phase short circuit, which promotes the line The protection device operates and the power to the faulty device is turned off. In addition, in the protected zero-connected grid, the protection neutral and repeated grounding can also limit the ground voltage when making a leakage.
(2) Scope of application Different protective earthing is applicable to both high and low voltage power grids that are generally ungrounded, and to low voltage power grids that have adopted other safety measures (such as installing leakage protectors); protection zero is only applicable to neutral points directly. Grounded low voltage grid.
(3) Different line structure If protective grounding measures are taken, there may be no working zero line in the power grid, only the protective grounding line; if the protective zeroing measures are taken, the working zero line must be set, and the working zero line is used for zero protection. The protective zero line should not be connected to switches or fuses. When installing a disconnector such as a fuse on the working zero line, a protective ground wire or a zero wire must be additionally installed.
6. Which metal parts of electrical equipment should be grounded or zeroed
Where there is no electricity under normal conditions, and when the insulation is damaged, the shell is short-circuited or other faults occur, the metal parts of the electrical equipment and its accessories that may be charged shall be grounded or connected to zero. These metal parts or accessories include:
(1) Metal enclosures or pedestals for motors, transformers, circuit breakers and other electrical equipment.
(2) The frame of the power distribution panel (disc) and the control panel (stage), the metal frame of the transformer, the power distribution station, and the metal barriers and metal doors near the live parts, and the steel bars in the reinforced concrete frame.
(3) Metal protective tubes and metal sheaths for wires and cables, junction boxes for AC and DC power cables, metal casings for terminal boxes, protective covers for bus bars, and protective nets.
(4) Metal bases and casings for lighting fixtures, fans and electric heating equipment, and rails for cranes.
(5) Metal pole towers for overhead ground lines and overhead lines, and casings and brackets for switches, capacitors, etc. mounted on the tower.
(6) Secondary windings of current transformers and voltage transformers.
(7) Hand-held power tools that do not use isolation transformers or enclosures of mobile electrical equipment that exceed the safety voltage.
(8) Transmissions for electrical equipment.
(9) Bases for lightning arresters, protective gaps, lightning rods and coupling capacitors.
7. Which metal parts of electrical equipment may not be protected or grounded
The following metal parts of electrical equipment, except as otherwise specified, may generally not be protected or grounded:
(1) The safety equipment used in the dry place or the electrical equipment casing below the safe voltage (AC 50 volts or less, DC 110 volts or less).
(2) Enclosures for electrical measuring instruments, relays and other low-voltage electrical appliances mounted on switchboards (screens), control panels (stages) and metal frames for electrical distribution devices.
(3) Insulating porcelain bottle fittings on overhead wooden poles and outdoor wooden frames of substations, and insulator metal bases that do not endanger personal safety when insulation is damaged.
(4) Fittings for insulators and bushings on the grounded metal frame, grounded on the motor and electrical enclosure of the machine.
(5) The outer casing of low-voltage electrical equipment (including mechanical equipment with which metal is connected) in a non-conducting place.
(6) Electrical installations with high-sensitivity leakage protectors to ensure personal safety.
Eight, the color of the protective ground wire
The electrical wires produced in China in the past have their grounding lines marked with black, and this mark has been eliminated. At present, China has implemented international standards, using yellow and green two-color insulated wire as the protective grounding wire. The yellow and green colors are the special color code for the protective grounding wire specified by the International Electrotechnical Commission. They have been internationally used, and China has clearly specified the use of this color standard in the corresponding standards. However, some countries in Japan and Western Europe use a single green line as a protective grounding wire. Therefore, some electrical products exported to these countries in China also use a single green line as a protective grounding wire. If you use this export-to-domestic electrical product, you must be careful not to connect the wrong line due to the different color of the grounding wire, resulting in electric shock. If it is difficult to judge the color code of the protective grounding wire, you should check the manual, or disassemble it, or use a multimeter to judge.
9. In the low-voltage power distribution system, matters needing attention in the protection of zero
In the power distribution system below 1 kV, the following matters should be noted when using protective zero:
(1) The neutral point of the three-phase four-wire low-voltage power supply must be well grounded, and the working grounding resistance should meet the specified requirements to ensure the zero line work and protection.
(2) The neutral wire must be grounded repeatedly at the specified location to prevent the danger of the neutral wire breakage.
(3) No switches or fuses shall be installed on the neutral circuit to prevent dangerous ground voltages from being connected to the zero-equipment housing when the neutral wire is disconnected; however, for the protection of the neutral wire or the additional protective neutral wire At the same time, switches and fuses can be installed on the phase and neutral lines of the circuit at the same time.
(4) The laying requirements for the neutral wire are the same as those for the neutral wire to prevent the wire breakage failure.
(5) The protective connection of all electrical equipment shall be connected to the zero mains in parallel.
(6) No protective earthing is allowed in any equipment in the zeroing system.
(7) The current carrying capacity of the neutral line should be at least 1/2 of the current carrying capacity of the phase line.
(8) The minimum cross-section of the neutral wire shall not be less than the minimum cross-section specified in the regulations to ensure that the neutral wire can withstand the short-circuit fault current and automatically remove the power supply of the faulty device.
X. Basic principles for automatically cutting off the power supply
The protection measure for automatically cutting off the power supply means that the appropriate switchgear is used. When the insulation of the equipment is damaged, the power supply of the damaged equipment is automatically cut off within a specified time to prevent the electric shock and death caused by the human body being exposed to dangerous voltage for a long time. . Therefore, the basic principle is that the fault current generated by any failure of the electrical equipment is cut off in time to ensure the safety of the person.
The adoption of this protection measure is based on the following two interrelated conditions:
(1) The path through which current flows, or "faulty loop", that is, the faulty power source circulates in the loop. The composition of such a loop is related to the type of distribution system (or neutral point system). Generally, the fault loops of various systems of TN, TT, and IT used are different.
(2) The time to cut off the fault current, that is, after the fault occurs, cut off the fault current for a certain period of time to ensure personal safety. The time to cut off the fault current is related to many factors. For example, the probability of a fault occurring, the probability that the human body will touch the outer casing of the device in the event of a fault. But the key is the amount of contact voltage that the human body may suffer when it touches the charged casing.
XI. Neutral point work system in accordance with IEC standards
According to the IEC standard, there are three types of neutral point working systems for low-voltage power distribution systems:
(1) The neutral point of the TN system power supply is directly grounded, and the exposed conductive part (metal case) of the load device is connected to the ground point through the protective conductor. Depending on how the neutral conductor (working neutral) is connected to the protective conductor (protective ground), the TN system is divided into three types: the TN-S system. In the whole system, the neutral conductor and the protective conductor are strictly separated. The so-called single-phase three-wire system and three-phase five-wire system; in the TN-CS system, the system has a part of the neutral conductor and the protective conductor function on one conductor, and the other part of the neutral conductor and the protection body is Separate; TN-C system, in the whole system, the function of neutral conductor and protective conductor is combined on one conductor, which is the common zero-protection system in China.
The meaning of the above letters is: the first letter "T" indicates that a point (or neutral point) in the power system is directly grounded; the second letter "N" indicates that the exposed conductive part of the device is directly connected to the power system ground point. Electrical connection; the letter "S" indicates that the neutral conductor and the protective conductor are separate; the letter "C" indicates that the function of the neutral conductor and the protective conductor is combined on one conductor.
(2) The power system of the TT system is directly grounded. The grounding of the exposed conductive part of the equipment is electrically unrelated to the grounding of the power supply system. It is called the protective grounding system in China.
The first letter "T" indicates that a point in the power system is directly grounded; the second letter "T" indicates that the ground of the exposed conductive portion of the device is not electrically related to the ground of the power system.
(3) The power supply end of the IT system is not grounded or grounded through impedance, and the exposed conductive part (metal case) of the electrical equipment is grounded.
The first letter "I" indicates that all live parts of the power supply are not grounded or grounded through impedance; the second letter "T" indicates that the grounding of the exposed conductive part of the device is electrically uncorrelated with the grounding of the power system (whether or not grounded). .
12. When the protection measures for automatically cutting off the power supply are taken, the time limit for cutting off the power supply is specified.
When a ground fault occurs, it is best to quickly cut off the power supply at the point of failure. Typically, the cut-off time is related to the magnitude of the contact voltage and the environmental characteristics of the location. For the general environment, the contact voltage should be no more than 50 volts for the power frequency AC; for the three-phase AC 380/220 volt system, after the ground fault occurs, the time for cutting off the power supply has the following provisions:
(1) Lines and distribution lines that supply power only to fixed electrical equipment should be disconnected from the power supply within 5 seconds of the fault.
(2) In the TN system, the power supply to the power tool and the mobile electrical equipment should be cut off within 0.4 seconds. At this time, the cross section of the protective conductor should not be less than 1/2 of the phase line cross section, and the grounding must be repeated at the receiving end of the power receiving end.
(3) In the TN system, if the same switchboard is used to supply power to hand-held power tools, mobile electrical equipment and stationary electrical equipment, the power cut-off time of all lines powered by the switchboard shall comply with item (2) above. Provisions.
(4) In the TT system, the power-off time for powering handheld power tools and mobile electrical equipment should generally not exceed 0.1 seconds.
When the protective device used can not meet the requirements of the power-off time, the leakage protection device can be used. At this time, the power-off time should not exceed 0.1 second.
13. Measures and requirements for automatic power-off protection when using TN system
In the TN system, in order to make the automatic cut off power supply protection more reliable, the measures taken and the requirements to be met are as follows:
(1) The metal casing of all electrical equipment should be reliably connected to the grounding point of the power supply terminal through the protective ground wire or through the working neutral wire. If other metal components (such as pipes) are used as natural grounding bodies in the vicinity, connect the protective earthing wire as much as possible, and repeat the grounding of the protective earthing wire or neutral wire at the incoming line of the building.
(2) In the NT-C system (ie, the zero-connect system), the general electrical equipment should use an overcurrent protection device as a protection device for automatically cutting off the power supply. For hand-held power tools and mobile electrical equipment, leakage protectors should be installed (to meet the cut-off time requirements and other reasons), so the protective ground wires of these tools and equipment must be grounded separately (constituting a local TT system); If there is no separate grounding electrode, it should be connected to the protective grounding wire on the power supply side of the leakage protector.
(3) In order to ensure that the power supply can be automatically cut off, in addition to the requirements of item (2) above, the following requirements shall be met: the section of the protection trunk shall be not less than 10 mm <sup>2</sup> (copper line) Or 16 mm<sup>2</sup> (aluminum wire), protect the cross section of the branch line (if multi-core cable or protective ground wire and phase wire are worn together), not less than 1.5 mm<sup>2</sup > (copper wire) or 2.5 mm <sup>2</sup> (aluminum wire).
14. Measures and requirements for automatic power cut protection when using TT system
Generally, the power grounding point (neutral grounding) of the TT system is separate from the protective grounding point of the powered device, that is, the device uses protective grounding. Therefore, when all electrical equipment is protected by the same protective device, the metal casings of these electrical equipment should be connected together with protective earthing wires, and the following requirements should be met:
Re·Id≤Us
In the formula, Re is the sum of the grounding resistance and the protective grounding resistance, and the Id is the operating current for ensuring that the protective device automatically cuts off the power supply, and the Us is the specified limit contact voltage, volt.
In the TT system, the leakage protector should be preferred. If the above requirements are met, an overcurrent protection device can also be used.
Fifteen, the requirements for electric shock protection when using IT systems
When the TT system is used, the power supply terminal (neutral point) is generally not grounded; the neutral point of the transformer is insulated and is not taken out. However, the exposed part of the electrical equipment (metal enclosure) must be grounded. In this case, when a single-phase ground fault occurs, the ground current is small, that is, when the electric shocker touches the outer casing, the current that enters the ground through the human body is returned to the power supply through the insulation leakage of the line conductor and through the coupling capacitor. If the wire is well insulated (has a large insulation resistance), it is much less dangerous than a phase line that touches the neutral grounding line (TN or TT system). This means that the line is fault free and the capacitance to ground is small.
However, in the event of a fault, that is, when single-phase grounding occurs, the voltage to the ground of the other two phases will rise to the line voltage. Therefore, monitoring and alarming measures to prevent single-phase grounding should generally be taken (no power supply is required) to avoid double grounding (two-phase grounding). Considering the situation of double grounding, the same measures to automatically cut off the power supply in the TT system (when the electrical equipment is grounded separately) or the measures to automatically cut off the power in the TN system (when the common grounding of the electrical equipment is used) .
If the branch line of the line is long and long, and the capacitance current between the phase line and the earth is also large, the risk of touching a phase line is very large. Therefore, when using an IT system, an insulation monitoring device and a protective device (overcurrent protection device or leakage protector) that automatically cuts off the power supply when double grounding should be installed.
XVI. Basis for selecting the grounding method (or neutral point system) of the distribution system
The choice of the grounding method of the power distribution system should take into account both economic and security requirements.
From the economic point of view, low-voltage distribution systems are commonly used in three-phase four-wire or three-phase five-wire systems, and the two operating voltages of 380 and 220 volts are used for power load and lighting load, respectively. In this way, not only the number of transformers is small, but also conductors of smaller cross-section can be used, thereby saving investment.
From a safety perspective, if the line can maintain a high level of insulation and the capacitance to ground is small, an IT system can be used. For example, in some distribution networks where the branch line is small and short, if the line is not easily corroded by corrosive media, and conditions are subject to frequent insulation monitoring and maintenance (for example, mobile machinery and safety requirements are required) This system can be used in higher locations.
The following locations should first use a neutral grounded system:
(1) The production site is very humid and has corrosive media. It cannot guarantee that the electrical equipment has good insulation.
(2) It is difficult to detect and quickly eliminate damaged insulation in time.
(3) There are many branch lines, and the capacitance current of the line is very large.
Therefore, large-scale workshops in the enterprise and production plants that are difficult to conduct insulation monitoring should adopt a neutral grounding system.
According to statistics, from the perspective of security, the operational reliability of the above two systems is basically the same.
For systems of 1000 volts or more and 35 kilovolts or less, there are many cases of neutral point insulation for technical reasons. At present, there is a tendency to ground the neutral point through a resistor. For systems above 35 kV, the neutral point is best grounded.
17. TT system to prevent electric shock and what measures should be taken to eliminate the defects
In the TT system, the metal casing of the equipment is generally grounded and is not associated with the neutral grounding of the system. When the insulation of electrical equipment is damaged, if the outer casing is not grounded, the phase voltage is applied to the outer casing, and it is dangerous for the human body to touch it. If protective grounding is applied, most of the current will flow through the ground. Because the body resistance and the grounding resistance are in parallel, and the body resistance is much larger than the grounding resistance, the current through the human body is small.
However, such systems still have some shortcomings that need to be taken to eliminate them. For example, in a TT system where the neutral point below 1000 volts is directly grounded, the neutral point grounding resistance is 4 ohms. If the protective earthing resistance is also calculated as 4 ohms, the short-circuit current is only used when the single-phase shell is short-circuited. 27.5 amps, ie I=220/(4+4)=27.5, to ensure that the power supply can be automatically cut off in the event of a fault, the rated current of the fuse and the automatic switch is limited, that is, the setting current of the automatic switch should not exceed 18 amps, the fuse rated current should not exceed 7 amps. If the electrical equipment is slightly larger, the setting value or rated value of the selected protective appliance exceeds the above value, the power supply will not be cut off automatically, and the ground voltage is 27.5×4=110 volts on the outer casing for a long time, which is very dangerous for the human body. of.
When using a single TT system, the grounding point on the power supply side is separate from the protective grounding of the powered device. If the grounding resistance of the grounding point on the power supply side is 4 ohms, when a single-phase short circuit (clamshell) occurs, the grounding resistance must be reduced to 1 when the ground voltage on the electrical equipment casing is reduced below 50 volts. Under the circumstance, even if the voltage of the equipment casing is reduced to less than 50 volts in the event of a fault, the risk of electric shock still exists, and the protection device cannot be guaranteed to operate. Therefore, in such systems, the use of leakage protectors should be prioritized. The overcurrent protection device is used only under the condition that the protection device can be operated. Therefore, when using the TT system, the following considerations should be made:
(1) The electrical equipment protected by the same protective device (electrical appliance) is connected to the grounding casing by a protective earthing wire.
(2) The limit contact voltage of the equipment should be less than 50 volts. At this time, the limit contact voltage ≥ the automatic cut-off current of the protection device × [grounding resistance + protective ground wire (connecting the outer casing of each electrical equipment) resistance].
(3) When using an overcurrent protection device, if an appliance with inverse time characteristics is used, there should be a current that is guaranteed to operate within 5 seconds. When using an instantaneously acting appliance, the minimum instantaneous operating current is taken. If a leakage protector is used. The operating current of the rated sensitivity should be taken.
(4) The leakage protector is preferred as a protection device.
18. Requirements to be met when using the TN-C system
If the TN-C system is used in the low-voltage distribution line, that is, the protection is connected to zero, the following requirements shall be met:
(1) The neutral point of the power supply side must be directly and well grounded, and the working grounding resistance should meet the specified requirements.
(2) The neutral wire should be grounded repeatedly as specified.
(3) Do not allow any of these devices to use protective grounding.
(4) Switches and fuses must not be installed on the zero line.
(5) The selection of the neutral section, in addition to the mechanical strength required, must also ensure that when a short-circuit fault occurs, the short-circuit current can reach the level at which the protective device operates.
(6) The protective connection of all electrical equipment shall be connected to the zero mains in parallel, and shall not be connected in series.
19. TN-C system to prevent electric shock and what measures should be taken to eliminate existing defects
In the TN-C system, if a single-machine short-circuit occurs in the electrical equipment, a single-phase short-circuit loop is formed, and the short-circuit current is larger than the short-circuit current of the TT system, so that the protection device can automatically and automatically cut off the power. This system is widely used by various factories and mines in China because it has certain advantages. If the system structure is simple, since the short-circuit current is large, the protection device can be quickly operated, and thus has certain safety and reliability.
However, the experience of companies using the TN-C system indicates that such systems have deficiencies, such as:
(1) When the three-phase load is unbalanced, in normal operation, a current flows through the neutral line, thus generating a voltage drop, causing a voltage to appear on the metal casing of the zero-connecting device.
(2) With this system, it is required to quickly cut off the faulty circuit when a single-phase grounding short circuit occurs. However, in some cases (such as the large capacity, the motor far from the power supply, the choice of its protection device, often can not meet the requirements of quickly cut off the fault circuit), the single-phase short-circuit current is not enough to break the fault loop, resulting in There is a long-standing dangerous voltage on the equipment casing.
(3) When the neutral line is disconnected (for example, the front end of the device is disconnected from the front), the voltage to the ground of the neutral line and the zeroing device rises, posing a risk of electric shock.
(4) It is easy to connect the phase line to the neutral line, or the housing is charged due to the interchange.
(5) In the same system, it is easy to have the situation of zero connection and grounding protection at the same time, which is not allowed by China's specifications. Because in the event of a single-phase short circuit, the short-circuit current is insufficient to operate the protection device, and as a result, dangerous voltages can also appear on the housing of the zero-crossing device.
For the defects of the TN-C system, the following measures and countermeasures can be taken:
(1) For the defect that the neutral line is charged during normal operation, in the case of a significant imbalance of the three-phase load, the TN-S system (the system that separates the working neutral line from the protective ground line) can be used, that is, the single phase is adopted. Three-wire or three-phase five-wire system.
(2) In order to ensure that the faulty circuit is quickly cut off, the requirements for automatic power cut protection must be met.
(3) Measures to prevent the broken wire from being broken: 1) Add repeated grounding; 2 Select the zero wire cross section according to the requirements of mechanical strength.
(4) Separate the working neutral line and the protective ground line from the phase line, such as insulated wires of different colors.
(5) It is not allowed to have zero and ground protection at the same time in the same system of the low-voltage power grid where the neutral point is directly grounded. The national standard also stipulates: "The same generator, the same transformer or the low-voltage line powered by the same busbar should not be connected to zero or ground." This should be prevented.
Twenty, comparison of TN and TT systems with IT system and neutral grounding
Under the premise that all other conditions are the same, if the electrical network is in normal operation (non-faulty state) and the capacitance of the line to ground is small, the IT system (neutral insulation system) is adopted than the TN and TT systems ( Neutral grounding system) is safer. However, in the event of a fault (eg, a single-phase short-circuit accident), the ground-to-ground voltage of the non-faulty phase of the IT system will be close to or equal to the line voltage. There is a potential danger of double grounding and high ground voltage. In this case, the IT system is also dangerous if the insulation monitoring device is not installed.
In addition, the current electrical wiring is generally long, and there are many branch lines, which objectively increases the capacitance current between the phase line and the earth, thus increasing the risk of single-phase electric shock in the IT system.
Therefore, from a safety point of view, the choice of a reasonable neutral point system should be measured in combination with the specific conditions of the line, the specific conditions of the electrical equipment and the actual level of the testing means.
21. Neutral grounding of general low-power distribution systems
In the 380/220 volt three-phase four-wire (or five-wire) low-voltage distribution system, the neutral point of the distribution transformer is generally grounded (or system grounded) because:
(1) Under normal power supply conditions, the phase-to-ground voltage can be kept basically stable, so that two voltage supplies can be applied to the load, that is, 380 volts for power load, and 220 volts for civil or industrial loads such as lighting and electric heating.
(2) Compared with neutral ungrounded systems (such as IT systems), it is more in line with the needs of modern industrial power supply and distribution, with fewer restrictions and higher relative security.
(3) It is possible to avoid the danger of high voltage switching to low voltage.
Therefore, China's low-voltage distribution lines generally use a neutral point grounding system. Only in special cases or when there are special requirements (such as mines), neutral point ungrounded systems are used.
22. The two protection methods of protection grounding and protection zeroing in the same distribution system cannot be mixed.
Low-voltage power distribution systems powered by the same distribution transformer can only be protected by one type of protection, that is, all or all of them are protected and grounded, and the two types of protection cannot be mixed.
If two types of protection are used at the same time, that is, some equipment adopts protective grounding, and other equipment adopts protective zero connection, once the equipment that implements grounding protection is short-circuited, the voltage of the neutral line will rise to an unacceptable procedure. This can result in high potentials on the enclosure of the zero-protected device, posing a risk of electric shock to personnel who come into contact with these devices.
When the device with grounding protection is short-circuited by the shell, the voltage of the neutral line will rise to
At this time, if the outer casing of the D motor is connected to the neutral line in the system (that is, the TN-CS system in the IEC standard), the safety requirements can be met. The grounding resistance R<sub>d</sub> of the D motor housing is actually equivalent to the repeated grounding of the system.
Twenty-three, insulation monitoring device should be installed in the ungrounded system
In an ungrounded system, when a single-phase short-circuit fault occurs, the voltage of the other two phases to ground will rise to a level close to the line voltage. This not only damages the insulation of the line and the electrical equipment, but also increases the risk of electric shock. Moreover, the single-phase grounding current is small, which is insufficient for the line protection device to operate, and the failure may last for a long time, thereby increasing the possibility of electric shock. Therefore, in an ungrounded system, the insulation of the system should be monitored frequently. In the event of a single-phase short-circuit to ground or a significant deterioration in ground insulation, the monitoring device signals the electrician to eliminate the fault in time to protect the equipment and the person.
Twenty-four, anti-high pressure into the low-voltage side
The so-called high voltage intrusion into the low voltage side means the insulation damage between the high voltage side and the low voltage side of the transformer, or the high voltage line is broken on the low voltage distribution line, so that the voltage of the whole low voltage system is raised to the equivalent of the high voltage system. The voltage to ground. The high voltage is forced into the low pressure, which seriously threatens the safety of various types of workers in the low-voltage system, so that the probability of electric shock and the dangerous procedure of electric shock are greatly increased.
For low voltage systems where the neutral point is not grounded, the neutral point or a phase should be grounded through the breakdown fuse. With this measure, under normal conditions, the low-voltage system is still an ungrounded system, but when the high voltage breaks into the low-voltage system, the breakdown fuse is broken down, and the fault current flows into the ground through the grounding device. If the fault current is large, the overcurrent protection device on the high voltage side can be caused to operate to cut off the power supply; if the fault current is small enough to cause the high voltage protection device to operate, the voltage of the low voltage system can be made by the shunting of the grounding resistor. Raise up to 120 volts for a certain protection purpose.
Twenty-five, matters needing attention when using the breakdown fuse
The breakdown fuse is the primary protection against high voltage intrusion in ungrounded low voltage systems. The gap that penetrates the fuse consists of a pair of plate electrodes and a holed mica plate whose discharge voltage is greater than the corresponding rated voltage. Once an overvoltage occurs, reaching the discharge voltage of the fuse, the gap is discharged, and the fault current flows into the ground through the grounding device, so that the overvoltage can be limited to a certain value. Note the following when using the breakdown fuse:
(1) According to the voltage level of the high voltage system, the breakdown fuse of the corresponding specification should be selected so that the breakdown voltage is compatible with the phase voltage of the high voltage system.
(2) Under normal conditions, the fuse must be well insulated to ensure that the low voltage system is not grounded.
(3) In order to ensure reliable operation of the breakdown fuse, the insulation should be checked frequently or frequently monitored by a high internal resistance voltmeter.
Twenty-six, repeated grounding
In the low-voltage distribution system where the neutral point is directly grounded, in order to ensure the safe and reliable operation of the line and prevent the damage caused by the broken line, in addition to the system (work) grounding, the system must also be carried out at other places where the zero line is drawn. Repeat the grounding as necessary. The locations where repeated grounding is required are:
(1) The outdoor overhead line should be grounded repeatedly; the terminal line and branch line length of the overhead line pass through the branch of 200 meters and every 1 km along the line, the neutral line should be grounded repeatedly.
(2) When the high-voltage line and the low-voltage line are erected on the same pole, the low-voltage neutral line at both ends of the same section should also be grounded repeatedly.
(3) If the cable or overhead line is introduced into the entrance of the workshop or large building, if it is more than 50 meters away from the grounding point, the neutral line should be repeatedly grounded; or the neutral line and the power distribution panel (disc) and control panel ( The grounding device of the disk) is connected.
(4) The low-voltage cable with the metal sheath as the neutral wire should be grounded repeatedly.
(5) Loop-type repeated grounding should be implemented inside the workshop; the zero line and the grounding device should be connected at least two points. Except for the point of the incoming line, the farthest point at the diagonal should also be connected; when the circumference of the workshop is more than 400 meters At a time, every 200 meters should be connected to the grounding device.
Twenty-seven, the relationship between zero protection and protection devices on the power supply line
In a power distribution system with zero-connection protection, if the electrical equipment leaks, whether the protection device on the power supply line operates quickly is the key to preventing or reducing the electric shock accident. For the protection device to operate quickly, a large enough single-phase short-circuit current is required, and the magnitude of the short-circuit current depends on the impedance of the phase-line loop. From a safety point of view, it is desirable to increase the cross-section of the phase and neutral lines to reduce the impedance of the phase-zero loop, but this inevitably increases the material consumption, thereby increasing the cost and is also undesirable. On the other hand, when the line is selected, the short-circuit current is also constant, and whether the protection device can operate quickly depends on the magnitude of the operating current of the protection device. The action current is adjusted small, and the protection device moves fast, which is beneficial to ensure safety; however, if the action current is adjusted too small, it will cause unnecessary trip and affect the normal operation of the device. Therefore, when selecting and adjusting the protection device, the issues concerning safety and economy and the magnitude of the operating current of the protection device should be comprehensively measured and comprehensively considered.
Twenty-eight, in the direct-grounded low-voltage distribution system, the reason why the neutral line often has charging phenomenon
In low-voltage distribution systems, neutral charging is generally more common, for several reasons:
（1） 线路上有电气设备æ¼ç”µï¼Œè€Œä¿æŠ¤è£…置未动作。
（2） 线路上有一相接地，而系统ä¸çš„总ä¿æŠ¤è£…置未动作。
（3） 零线æ–开，æ–开处åŽé¢çš„电气设备æ¼ç”µï¼Œæˆ–者接有å•ç›¸è´Ÿè·ã€‚
（4） 在接零ä¿æŠ¤ç³»ç»Ÿï¼ˆTN-C系统）ä¸ï¼Œä¸ªåˆ«é‡‡å–ä¿æŠ¤æŽ¥åœ°çš„设备æ¼ç”µæˆ–碰壳。
（5） 在采å–接零ä¿æŠ¤çš„系统ä¸ï¼Œæœ‰ä¸ªåˆ«å•ç›¸è®¾å¤‡é‡‡ç”¨ä¸€ç›¸ä¸€åœ°ï¼ˆä¸ç”¨å·¥ä½œé›¶çº¿ï¼‰æ–¹å¼ï¼Œä½¿é›¶çº¿å¸¦ç”µã€‚
（6） 系统ä¸æœ‰äº›ç”µæ°”设备的ç»ç¼˜ç”µé˜»æŸåï¼Œå› è€Œçˆ¬ç”µã€‚
（7） 系统接地ä¸è‰¯ï¼ŒæŽ¥åœ°ç”µé˜»è¾ƒå¤§ï¼Œä¸‰ç›¸è´Ÿè·ä¸¥é‡ä¸å¹³è¡¡ã€‚
（8） 采用二线一地è¿è¡Œæ–¹å¼æ—¶ï¼Œå¦‚果接地体é 近低压工作接地或é‡å¤æŽ¥åœ°ï¼Œé›¶çº¿ä¹Ÿå¾€å¾€å¸¦ç”µã€‚
（9） ç£åœºæ„Ÿåº”或é™ç”µæ„Ÿåº”使零线带电。
（10） 由于ç»ç¼˜ç”µé˜»å’Œå¯¹åœ°ç”µå®¹çš„分压作用，å¯èƒ½å¯¼è‡´ç”µæ°”设备外壳带电。
二åä¹ã€ 对零线的安全è¦æ±‚
从安全ç€çœ¼ï¼Œå¯¹é›¶çº¿æœ‰ä»¥ä¸‹è¦æ±‚：
（1） 对其截é¢ç§¯çš„è¦æ±‚从安全并兼顾节约的观点考虑，其最大截é¢ï¼Œé’¢çº¿ä¸å¤§äºŽ800毫米＜sup＞2＜/sup＞，é“线ä¸å¤§äºŽ70毫米＜sup＞2＜/sup＞，铜线ä¸å¤§äºŽ50毫米＜sup＞2＜/sup＞。为ä¿è¯é›¶çº¿å…·æœ‰è¶³å¤Ÿçš„机械强度，其最å°æˆªé¢ä¸å¾—å°äºŽä¸‹åˆ—值：裸铜线为4毫米＜sup＞2＜/sup＞；裸é“线为6毫米＜sup＞2＜/sup＞；ç»ç¼˜é“œçº¿ä¸º1.5毫米＜sup＞2＜/sup＞；ç»ç¼˜é“线为2.5毫米＜sup＞2＜/sup＞；é“线为12毫米＜sup＞2＜/sup＞。
（2） 对其连接的è¦æ±‚é›¶çº¿è¿žæŽ¥çº¿ä¸Žè®¾å¤‡çš„è¿žæŽ¥åº”ä½¿ç”¨èžºæ “åŽ‹æŽ¥ï¼Œå¿…è¦æ—¶è¦åŠ 弹簧垫圈。钢质零线或零线连接线本身的连接应采用焊接。利用自然导体作为零线时，在连接ä¸å¯é 的地点，应å¦åŠ 跨接线。所有电气设备的接零线，å‡åº”以并è”æ–¹å¼æŽ¥åœ¨é›¶å¹²çº¿ä¸Šï¼Œä¸å¾—串è”。
（3） 对其防è…çš„è¦æ±‚在有è…蚀性物质的环境ä¸ï¼Œä¸ºé˜²æ¢é›¶çº¿è…蚀，其表é¢åº”涂以防è…涂料。
（4） å¯¹åŠ å¼ºæ£€æŸ¥çš„è¦æ±‚对临时设备ã€ç§»åŠ¨å¼è®¾å¤‡ã€æºå¸¦å¼è®¾å¤‡å’Œæ‰‹æŒç”µåŠ¨å·¥å…·çš„零线，è¦åŠ 强检查，以防错接ã€æ–线和万一å‘ç”Ÿç¢°å£³æ—¶é€ æˆè§¦ç”µäº‹æ•…。
（5） 对ç¦æ¢å®‰è£…æ–æµè®¾å¤‡çš„è¦æ±‚ç¦æ¢åœ¨é›¶çº¿ä¸Šå®‰è£…熔æ–器或å•ç‹¬çš„æ–æµå¼€å…³ï¼Œå¦åˆ™ï¼Œä¸€æ—¦äº§ç”Ÿç¢°å£³çŸè·¯ç”µæµï¼Œç†”体熔æ–或开关动作时，零线将被切æ–。æ¤æ—¶å¦‚果相线没有åŒæ—¶æ–å¼€ï¼Œä¼šé€ æˆä¸¥é‡çš„触电事故。
三å〠设备的基本ç»ç¼˜ã€é™„åŠ ç»ç¼˜ã€åŒé‡ç»ç¼˜å’ŒåŠ 强ç»ç¼˜
基本ç»ç¼˜ï¼Œæ˜¯æŒ‡ç”¨äºŽå¸¦ç”µéƒ¨åˆ†ï¼Œæ供防触电基本ä¿æŠ¤çš„ç»ç¼˜ã€‚
é™„åŠ ç»ç¼˜ï¼Œæ˜¯ä¸ºäº†åœ¨åŸºæœ¬ç»ç¼˜å¤±æ•ˆåŽæ供防触ä¿æŠ¤ï¼Œè€Œåœ¨åŸºæœ¬ç»ç¼˜ä»¥å¤–å¦åŠ çš„å•ç‹¬ç»ç¼˜ã€‚
åŒé‡ç»ç¼˜ï¼Œæ˜¯ç”±åŸºæœ¬ç»ç¼˜å’Œé™„åŠ ç»ç¼˜ç»„åˆè€Œæˆçš„ç»ç¼˜ã€‚
åŠ å¼ºç»ç¼˜ï¼Œæ˜¯ç”¨äºŽå¸¦ç”µéƒ¨åˆ†çš„一ç§å•ä¸€ç»ç¼˜ç³»ç»Ÿï¼Œå…¶é˜²è§¦ç”µä¿æŠ¤ç‰çº§ç›¸å½“于åŒé‡ç»ç¼˜ã€‚
三å一〠åŒé‡ç»ç¼˜ç»“æž„
通常，具有åŒé‡ç»ç¼˜ç»“构的电气设备，ä¸éœ€é‡‡å–ä¿æŠ¤æŽ¥åœ°æˆ–å…¶ä»–ç‰¹æ®Šçš„å®‰å…¨æŽªæ–½ï¼Œå°±å…·å¤‡ä¸€å®šçš„é¢„é˜²é—´æŽ¥æŽ¥è§¦è§¦ç”µçš„åŠŸèƒ½ã€‚å› æ¤ï¼Œè¿™ç§è®¾å¤‡çš„应用范围日益广泛。
æŸäº›åœºæ‰€ï¼Œå¦‚果采用普通电气设备，难以采å–其他安全措施，或者ä¸è¶³ä»¥ä¿è¯å®‰å…¨ï¼Œåˆ™å¯é‡‡ç”¨åŒé‡ç»ç¼˜ç»“构的电气设备；手æŒç”µåŠ¨å·¥å…·å’Œç§»åŠ¨å¼ç”µæ°”设备，由于使用地点ä¸å›ºå®šï¼Œå…¶ç»“æž„å¯ä»¥é‡‡ç”¨åŒé‡ç»ç¼˜ï¼›ç‰¹åˆ«æ½®æ¹¿æˆ–有è…蚀性介质的场所，所使用的电动机多为åŒé‡ç»ç¼˜ç»“构。æ¤å¤–，æŸäº›å®¶ç”¨ç”µå™¨æˆ–器械的外壳和手柄，也采用åŒé‡ç»ç¼˜ã€‚
为确ä¿åŒé‡ç»ç¼˜çš„电气设备安全å¯é 地è¿è¡Œï¼ŒåŒé‡ç»ç¼˜ç»“æž„ä¸çš„零部件应满足以下基本è¦æ±‚：
（1） 带电零件与ä¸å¯è§¦åŠçš„金属零件之间，必须用工作（基本）ç»ç¼˜éš”开。
（2） ä¸å¯è§¦åŠçš„金属零件与å¯è§¦åŠçš„金属零件之间，应使用ä¿æŠ¤ï¼ˆé™„åŠ ï¼‰ç»ç¼˜éš”开。
（3） 带电零件与å¯è§¦åŠçš„金属零件之间，必须用åŒé‡ç»ç¼˜æˆ–åŠ å¼ºç»ç¼˜éš”开。
（4） 上述å„零件之间，应有必è¦çš„爬电è·ç¦»å’Œç”µæ°”è·ç¦»ã€‚
三å二〠电气隔离
所谓电气隔离，就是将电æºä¸Žç”¨ç”µå›žè·¯ä½œç”µæ°”上的隔离，å³å°†ç”¨ç”µçš„分支电路与整个电气系统隔离，使之æˆä¸ºä¸€ä¸ªåœ¨ç”µæ°”上被隔离的ã€ç‹¬ç«‹çš„ä¸æŽ¥åœ°å®‰å…¨ç³»ç»Ÿï¼Œä»¥é˜²æ¢åœ¨è£¸éœ²å¯¼ä½“故障带电情况下å‘生间接触电å±é™©ã€‚è¦å®žè¡Œç”µæ°”隔离，必须满足以下æ¡ä»¶ï¼š
（1） æ¯ä¸€åˆ†æ”¯ç”µè·¯ä½¿ç”¨ä¸€å°éš”离å˜åŽ‹å™¨ï¼Œè¿™ç§å˜åŽ‹å™¨çš„è€åŽ‹è¯•éªŒç”µåŽ‹ï¼Œæ¯”普通å˜åŽ‹å™¨é«˜ï¼Œåº”符åˆâ…¡çº§ç”µå·¥äº§å“（åŒé‡ç»ç¼˜æˆ–åŠ å¼ºç»ç¼˜ï¼‰çš„è¦æ±‚，也å¯ä½¿ç”¨ä¸Žéš”离å˜åŽ‹å™¨çš„ç»ç¼˜æ€§èƒ½ç›¸ç‰çš„绕线型å‘电机。
（2） 被隔离的电路，其电压ä¸è¶…过500ä¼ï¼Œçº¿è·¯é•¿åº¦ä¸è¶…过500米，以防æ¢ç”µå®¹ç”µæµè¿‡å¤§ã€‚
（3） 被隔离的电路与其他电路ä¸å¾—有任何连接，尤其与大地必须ç»ç¼˜ã€‚
（4） 在被隔离的电路ä¸ï¼ŒåŽŸåˆ™ä¸Šä¸€å°éš”离å˜åŽ‹å™¨åªå‘一个用电设备供电。如果å‘多个用电设备供电，则所有用电设备的外露导电部分è¦ä½œç‰ç”µä½è¿žæŽ¥ï¼Œå¹¶ä¸”ä¸å¾—接地。æ¤å¤–，所有软电缆都必须有用作ç‰ç”µä½è¿žæŽ¥çš„ä¿æŠ¤å¯¼ä½“。
在些电气隔离系统ä¸ï¼Œå¦‚æžœä¸æ˜¯æŽ¥è§¦ä¸¤ç›¸ï¼Œè€Œæ˜¯åªæŽ¥è§¦å•ç›¸ï¼Œç”±äºŽä¸Žå¦ä¸€ç›¸ä¸èƒ½å½¢æˆé€šè·¯ï¼ˆé€šè¿‡ç»ç¼˜ç”µé˜»æˆ–电容），所以是能够防æ¢é—´æŽ¥æŽ¥è§¦è§¦ç”µçš„。
三å三〠ä¿è¯æºå¸¦å¼ç”µæ°”设备安全è¿è¡Œåº”采å–的措施
由于æºå¸¦å¼ç”µæ°”设备在使用ä¸éœ€è¦ç»å¸¸ç§»åŠ¨ï¼Œä¸”有些æºå¸¦å¼ç”µæ°”è®¾å¤‡çš„æŒ¯åŠ¨å¾€å¾€è¾ƒå¤§ï¼Œå› æ¤ï¼Œå¯¼çº¿æˆ–电缆容易æŸå而产生碰壳çŸè·¯äº‹æ•…。æ¤å¤–，这ç§è®¾å¤‡éƒ½æ˜¯åœ¨å·¥ä½œäººå‘˜ç´§æ¡ä¹‹ä¸‹ä½¿ç”¨å’Œè¿è¡Œçš„ï¼Œæ›´å¢žåŠ äº†è§¦ç”µçš„å±é™©æ€§ï¼Œæ‰€ä»¥å¿…须采å–å¯é 的措施，以ä¿éšœå…¶å®‰å…¨è¿è¡Œã€‚这些措施包括：
（1） 对æºå¸¦å¼ç”µæ°”设备进行接零或接地ä¿æŠ¤ï¼Œä¸Žè¿™äº›è®¾å¤‡ç›¸è¿žçš„软电缆或橡套软线ä¸åº”有专用于接零或接地的芯线，芯线截é¢åº”ä¸å°äºŽè§„定值，以ä¿è¯å•ç›¸ç¢°å£³æ—¶åŠæ—¶åˆ‡æ–电æºã€‚
（2） 在特别å±é™©çš„场所，应采用安全电压，且应由安全å˜åŽ‹å™¨æˆ–隔离å˜åŽ‹å™¨ä¾›ç”µï¼Œä¸å…许采用自耦å˜åŽ‹å™¨ä½œä¸ºç”µæºã€‚
（3） 采用åŒé‡ç»ç¼˜çš„设备，并且移动电缆和软线应ä¿è¯ä¸è‡´å› 拉ã€ç£¨ã€ç¢¾ç‰æœºæ¢°ä½œç”¨è€Œç ´æŸï¼Œå› æ¤ï¼Œåº”ç»å¸¸è¿›è¡Œæ£€æŸ¥ã€‚
（4） 采用防护用具，如ç»ç¼˜é´ã€ç»ç¼˜åž«ã€ç»ç¼˜æ‰‹å¥—ç‰ï¼Œä½¿äººä¸Žå¤§åœ°æˆ–与å•ç›¸è®¾å¤‡çš„外壳隔ç»ã€‚
三åå››ã€ æ€Žæ ·ä½¿ç”¨ä¸‰çœ¼æ’座
通常，å•ç›¸ç”¨ç”µè®¾å¤‡ï¼Œç‰¹åˆ«æ˜¯ç§»åŠ¨å¼ç”¨ç”µè®¾å¤‡ï¼Œéƒ½åº”使用三芯æ’头和与之é…套的三眼æ’座。三眼æ’座上有专用的ä¿æŠ¤æŽ¥é›¶ï¼ˆåœ°ï¼‰æ’å”，在采用接零ä¿æŠ¤æ—¶ï¼Œæœ‰äººå¸¸å¸¸ä»…在æ’座底内将æ¤å”接线柱头与引入æ’åº§å†…çš„é‚£æ ¹é›¶çº¿ç›´æŽ¥ç›¸è¿žã€‚è¿™æ˜¯æžä¸ºå±é™©çš„ã€‚å› ä¸ºä¸‡ä¸€ç”µæºçš„零线æ–开，或者电æºçš„ç«ï¼ˆç›¸ï¼‰çº¿ä¸ºé›¶çº¿æŽ¥å，其外壳ç‰é‡‘属部分也将带有与电æºç›¸åŒçš„电压，这就会导致触电。上述错误接线方法，ä¸ä½†åœ¨æ•…障情况下ä¸èƒ½èµ·ä¿å®‰ä½œç”¨ï¼Œç›¸å还å¯èƒ½åœ¨æ£å¸¸æƒ…å†µä¸‹ä¹Ÿæ˜“é€ æˆè§¦ç”µäº‹æ•…。
å› æ¤ï¼ŒæŽ¥çº¿æ—¶ä¸“用接地æ’å”应与专用的ä¿æŠ¤åœ°çº¿ç›¸æŽ¥ã€‚ When using zero-connection protection, the zero line should be specifically drawn from the power supply terminal, and should not be used near the neutral line of the socket.
三å五〠三相移动å¼ç”µæ°”设备è¦ä½¿ç”¨å››çœ¼æ’座
三相移动å¼ç”µæ°”设备之所以è¦ä½¿ç”¨å››çœ¼æ’座，主è¦æ˜¯ä»Žå®‰å…¨æ–¹é¢è€ƒè™‘ã€‚å› ä¸ºå››èŠ¯ï¼ˆå”）æ’头æ’座有专用的ä¿æŠ¤æŽ¥é›¶ï¼ˆåœ°ï¼‰æŸ±å¤´ï¼Œæ’座上接零（地）的å”比其他æ’å”大，其相应的æ’å¤´ä¹Ÿå¤§ä¸€äº›ä¹Ÿé•¿ä¸€äº›ã€‚è¿™æ ·ï¼Œä¸€æ–¹é¢å¯ä»¥ä¿è¯ä¸Žè®¾å¤‡å¤–壳直接相连的接零（地）æ’头åªèƒ½æ’入接零（地）æ’å”，而ä¸èƒ½æ’入其他导电æ’å”ï¼›å¦ä¸€æ–¹é¢ï¼Œç”±äºŽæŽ¥é›¶ï¼ˆåœ°ï¼‰çš„æ’头比其他接相线的æ’头长一些，å¯ä»¥ä¿è¯æ’座和æ’头的接零（地）触头在导电触头接触之å‰å°±å…ˆè¡Œè¿žé€šï¼Œè€Œåœ¨å¯¼ç”µè§¦å¤´è„±ç¦»ä»¥åŽï¼Œæ‰ä¼šæ–开，从而能有效地起到ä¿å®‰ä½œç”¨ã€‚
三åå…〠电工产å“按防æ¢äººèº«è§¦ç”µçš„程度如何分级
电工产å“按防æ¢äººèº«è§¦ç”µçš„程度å¯åˆ†ä¸ºäº”级：
0级——仅有基本ç»ç¼˜ï¼Œæ— æŽ¥åœ°å…ƒä»¶ï¼ˆèžºæ “ã€ç«¯åç‰ï¼‰æˆ–其他防æ¢è§¦ç”µçš„ä¿æŠ¤å…ƒä»¶ã€‚
0â… çº§â€”â€”æœ‰åŸºæœ¬ç»ç¼˜ã€æŽ¥åœ°å…ƒä»¶å’Œè¿žæŽ¥ç”µæºçš„å¯¼çº¿ï¼Œä½†æ— æŽ¥åœ°èŠ¯çº¿ã€‚
â… çº§â€”â€”æœ‰å·¥ä½œç»ç¼˜å’ŒæŽ¥åœ°å…ƒä»¶ã€‚å¦‚æžœâ… çº§äº§å“有连接电æºçš„导线，则这ç§å¯¼çº¿åº”有接地芯线和带接地æžçš„æ’头（使用时æ’在有ä¿æŠ¤æŽ¥åœ°æ’å”的专用æ’座上）。
Ⅱ级——有åŒé‡ç»ç¼˜æˆ–åŠ å¼ºç»ç¼˜ä½†æ— 接地元件。
Ⅲ级——低电压产å“，内部电路和外部电路å‡æ— 45ä¼ä»¥ä¸Šçš„电压。
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The RJ-45 interface can be used to connect the RJ-45 connector. It is suitable for the network constructed by twisted pair. This port is the most common port, which is generally provided by Ethernet hub. The number of hubs we usually talk about is the number of RJ-45 ports. The RJ-45 port of the hub can be directly connected to terminal devices such as computers and network printers, and can also be connected with other hub equipment and routers such as switches and hubs.
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