When the height of the lamp from the ground is less than 2.4m, the exposed conductor that the lamp can touch must be reliably grounded (PE) or grounded (PEN), and there are special grounding bolts and signs.
Grounding means that the neutral point of power system and electrical device, the exposed conductive part of electrical equipment and the conductive part outside the device are connected to the earth through conductors. It can be divided into working grounding, lightning protection grounding and protective grounding.
Working grounding is set for power system operation (such as neutral grounding), so in general, there will be a long-term current flowing through the grounding electrode, but it is only an unbalanced current of several amperes to several tens of amperes. When the grounding fault occurs in the system, thousands of amperes of working current will flow through the grounding electrode, but the current will be cut off by the relay protection device within 0.05~0. 1s, even for backup protection, the action is generally within 1s.
Lightning protection grounding is designed to eliminate the influence of overvoltage, such as grounding of lightning rod, lightning wire and lightning arrester. Lightning protection grounding only under the action of lightning impact will there be current flow. The lightning current flowing through the lightning protection grounding electrode can reach tens to hundreds of amperes, but the duration is very short.
Protective grounding is to prevent the metal shell of power equipment, reinforced concrete poles, metal towers and other equipment from endangering personal safety due to insulation damage and electrification. Only when the insulation of the equipment is damaged, will there be current flowing through the protective grounding, and its value can vary within a wide range.
When the current flows through the above three kinds of grounding electrodes, the potential of the grounding electrodes will rise, which will affect the safety of people and equipment. Therefore, it is necessary to limit the potential rise of grounding electrode or take corresponding safety measures to ensure the safety of equipment and personnel.
Interpretation principle
(1) grounding
1) Electricially, the earth is an object with low resistance and large capacitance, which has the ability to absorb infinite charges and can keep the potential unchanged after absorbing a large amount of charges, so it is used as a reference potential body in electrical systems.
2) Ground potential. A conductor or a group of conductors in close contact with the earth and forming electrical contact is called grounding electrode, which is usually made of round steel or angle steel, and can also be made of copper bars or sheets. When the current flowing into the earth is dispersed into a hemisphere through the grounding electrode, the hemispherical sphere is smaller at the place closer to the grounding electrode and larger at the place farther away from the grounding electrode, so the resistance is larger at the place closer to the grounding electrode and smaller at the place farther away from the grounding electrode. Experiments show that there is actually no resistance at a distance of 20 meters from a single grounding electrode or grounding point, and the potential there is close to zero.
(2) Grounding
Grounding Ji ∑ D ∑.
(1)[ grounding; Grounding; Grounding] [power]
(2)[ touchdown score; Ground contact]
As far as the power supply system in the United States is concerned, in addition to the live wire and neutral wire, the pin inserted in the middle round head is the so-called grounding pin.
Connecting a part of a power system or electrical equipment to a grounding electrode through a grounding wire is called grounding. The wire connected to the ground electrode is called the ground wire. Grounding electrode and grounding wire are collectively referred to as grounding device. Several grounding bodies are connected with each other in the earth to form a grounding network, and grounding wires can be divided into grounding trunk lines and grounding branch lines. According to the regulations, the grounding trunk line should be connected to the grounding network with at least two wires at different positions. The grounding point of power system is generally neutral point. The grounding part of electrical equipment is an exposed conductive part, which can be contacted in electrical equipment. It is not charged under normal conditions, but may be charged under fault conditions. The external conductive part of the equipment is also called the external conductive part, which does not belong to electrical equipment. Generally, it is the metal pipeline of water, heating, gas and air conditioning and the metal structure of buildings.
(3) the role of grounding
The main function of grounding is to prevent electric shock, damage to equipment and lines, fire and lightning, electrostatic damage and normal operation of power system.
Grounding is an electrical safety measure to ensure the normal work and personal safety of electrical equipment, which is realized by connecting metal wires with grounding devices. Common grounding methods include protective grounding, working grounding, lightning protection grounding, shielding grounding and anti-static grounding. Grounding devices introduce leakage current, static charge and lightning current that may be generated on electrical equipment and other production equipment into the ground to avoid personal electric shock and possible accidents such as fire and explosion.
1) Protect people from electric shock.
Under normal circumstances, the uncharged metal part of electrical equipment should be connected with the grounding electrode to protect human safety.
For electrical equipment with grounding device, when the insulation is damaged and the shell is charged, grounding current will flow along the grounding electrode and human body at the same time. The current flowing through each path will be inversely proportional to its resistance. The smaller the resistance of the grounding electrode, the smaller the current flowing through the human body. When the grounding resistance is extremely small, the current flowing through the human body approaches zero, and the human body avoids the danger of electric shock. Therefore, in any case, ensure that the grounding resistance is not greater than the grounding resistance specified in the design or regulations.
2) Ensure the normal operation of the electrical system.
Power system grounding is generally neutral point grounding, so the potential between neutral point and ground is close to zero. When the phase wire touches the shell or the ground, the other two relative ground voltages will increase to twice the phase voltage in the neutral insulation system; It is close to the phase voltage in neutral grounding system. Because of the existence of neutral grounding wire, the reliability of relay protection can be guaranteed. The DC power supply in communication system generally adopts positive grounding, which can prevent noise from entering and ensure the normal operation of communication equipment.
Four species
Protective grounding
situation
Grounding system
Safe grounding is to form a good conductive connection between metal parts (cabinet shell, console shell, etc.) that are usually not electrified in the system. ) and the ground to protect equipment and personal safety. The reason is that the power supply of the system is strong electricity (380,220 or 1 10V). Under normal circumstances, the casing is not charged. When the power supply line of the power supply is short-circuited with the conductive metal parts such as the shell due to a fault (such as power failure of the main engine or other faults), these metal parts or the shell form a charged body. If there is no good grounding, there is a high potential difference between charged body and ground. Therefore, it is necessary to make a good connection between the metal shell and the ground to make the shell and the ground equipotential. In addition, protective grounding can also prevent the accumulation of static electricity.
working earthing
Work contact
Schematic diagram of grounding grid
Grounding is to make the system and the instruments connected with it run reliably and ensure the accuracy of measurement and control. It can be divided into mechanical logic grounding, signal loop grounding, shielding grounding, petrochemical and other intrinsically safe grounding in explosion-proof systems.
Machine logic ground, also called host power ground, is the negative terminal of logic level in computer and the output ground of +5V power supply.
The signal loop is grounded, such as the negative terminal of each transmitter and the negative terminal of the switching signal.
Shield grounding (analog signal shield grounding).
Intrinsically safe grounding refers to the grounding of intrinsically safe instruments or safety barriers. In addition to suppressing interference, this grounding is also one of the measures to make instruments and systems intrinsically safe. Due to the different intrinsically safe measures adopted by the equipment, the intrinsically safe grounding will be different. Here, the Zener safety barrier is taken as an example to illustrate its grounding content.
safe
Grounding short circuit device
The function of power grid is to protect the dangerous site from the influence of safe current and safe voltage. If the field terminal is short-circuited, the current on the conductor will be limited in a safe range due to the current limiting effect of the load resistance and the safety barrier resistance R, so that the field terminal will not generate high temperature and cause combustion. In the second case, if there is a fault at one end of the computer, a high-voltage electrical signal is added to the signal loop, and the voltage is within the safe range due to the clamping effect of the Zener diode.
It is worth reminding that due to the introduction of Zener safety barrier, the resistance on the signal circuit has increased a lot. Therefore, when designing the load capacity of the output circuit, in addition to the real load requirements, the resistance of the safety barrier should be fully considered, leaving room.
In addition to the above grounding, there is also a power system grounding, also called AC power working grounding, which is a kind of grounding (such as neutral grounding) that needs to be set in the power system for operation.
grounding for lightening
Lightning protection grounding is a grounding system to prevent lightning damage (direct strike, induction or line introduction). Signal (weak current) lightning protection grounding and power supply (strong current) lightning protection grounding are often different. The reason for the distinction lies not only in the different grounding resistances, but also in the engineering practice, the signal lightning protection grounding is often attached to the signal independent grounding, and is built separately from the power supply lightning protection grounding.
As a part of lightning protection measures, lightning protection grounding is used to introduce lightning current into the earth. The lightning protection of buildings and electrical equipment mainly uses one end of lightning arrester (including lightning rod, lightning belt, lightning protection network and lightning eliminator, etc.). ) is connected with the protected equipment, and the other end is connected with the grounding device. When lightning strikes directly, the lightning arrester leads the lightning to itself, and the lightning current enters the earth through its downlead and grounding device. In addition, due to the side effects of electrostatic induction caused by lightning, in order to prevent indirect injuries, such as fire or electric shock in buildings, metal equipment, metal pipes and steel structures in buildings are usually grounded; Lightning waves will invade the house along low-voltage overhead lines and TV antennas, causing insulation breakdown of electrical equipment in the house, thus causing fire or personal electric shock casualties. Therefore, the iron feet of insulated porcelain bottles on the line and in front of the house should be grounded.
cohere
It is an effective measure to eliminate the harm of electromagnetic field to human body and prevent electromagnetic interference. High frequency technology has been widely used in electric heating, medical treatment, broadcasting, communication, TV station, navigation, radar and other fields. Under the action of electromagnetic field, the radiation energy absorbed by human body will produce biological effects, which will cause harm to human body, such as slight trembling of fingers, skin scratch, decreased vision and so on. Setting a shielding device on the equipment shell that generates magnetic field and grounding the shielding body can not only reduce the intensity of electromagnetic field outside the shielding body and achieve the purpose of reducing or eliminating the harm of electromagnetic field to human body, but also protect the equipment in the shielding grounding body from the interference of external electromagnetic field.
Anti-static grounding
Preventing the influence of electrostatic hazards and releasing it is the most important part of electrostatic protection.
5 mode
brief introduction
Modern power system itself is a strong source of electromagnetic interference, which mainly interferes with communication equipment in this frequency band through radiation. In order to suppress the interference of external high voltage transmission lines, grounding measures are adopted. There are two common grounding methods, which are discussed as follows:
Decentralized grounding mode
scatter
Shell grounding circuit
Grounding is to connect the lightning protection grounding of communication building, power supply system grounding, various grounding of communication equipment and grounding of other equipment into a separate grounding system. Due to the increasing number of ground wire systems, the potential coupling effect between ground wires is often difficult to avoid, but decentralized grounding is easy to cause interference. At the same time, the height of the main building is getting higher and higher, and the unsafe factors brought by its grounding mode are also increasing. When the facility is struck by lightning, it is easy to form an underground counterattack and damage other equipment.
Common grounding mode
Common grounding mode is also called single point grounding mode, that is, all grounding systems use the same "ground". Public grounding has the following characteristics:
(1) The grounding system of the whole building constitutes a cage-type equalizing body, and for direct lightning, all points on the same floor in the building are relatively uniform; For induced lightning, the cage-type equalizing body and frame structure of the building can also provide 10-40dB shielding effect against external electromagnetic field interference;
(2) The grounding resistance of common grounding method is generally very small, and there is no coupling effect between various grounding bodies, which is beneficial to reduce interference;
(3) Yes
No thunder in dry days
Save metal materials and occupy less land.
It is not difficult to see from the above that the interference of external high-voltage transmission lines can be effectively suppressed by using the common ground method.
The grounding wire for anti-static grounding shall be connected in series with a current-limiting resistor of 1 megohm, that is, connected to the grounding device through the current-limiting resistor. Isn't the grounding resistance as small as possible? Why series resistors?
Computer grounding aims at the easy flow of grounding current, and the smaller the grounding resistance, the better. The grounding of the calculation center should minimize the potential change caused by noise, and at the same time, it should be noted that the signal loop and the power loop, the high-level loop and the low-level loop cannot use the same * * * grounding loop. For network wiring with high transmission bandwidth requirements, isolation and shielding should be adopted to prevent interference caused by electrostatic induction. Try to be simple, economical and effective in design. If grounding energy and shielding are effectively combined, interference will be better solved and noise will be suppressed.
six
7 safety protection
Exchange workplace
In the computer system, besides DC electrical equipment, a large number of AC electrical equipment are equipped and used. AC working grounding is to make the equipment using alternating current in the computer system grounded twice or make metal connection with the earth through special equipment, and its function is to ensure personal and equipment safety. The implementation of AC working grounding can be divided into two situations: AC equipment used in computer system and AC equipment matched with computer system, which should be independently grounded according to electrical standards to prevent the risk of electric shock due to insulation damage.
Safety protection place
Good grounding between the metal shell or frame insulated from the live part of electrical equipment and the ground is called safety protection grounding. If the casing is not grounded, the casing has a high potential, and there is a danger of electric shock after human contact. When insulation breaks down, the short-circuit current will flow into the earth along the ground wire and the human body at the same time. The casings of AC equipment commonly used in computer rooms (such as air conditioners, frequency and voltage stabilizing devices, transformers, UPS standby power supplies, etc.). ) should also be grounded according to relevant electrical codes.
grounding for lightening
Lightning protection grounding is mainly used to lead lightning current to the earth to protect the safety of people and buildings. Lightning protection grounding is related to the lightning protection measures taken by the computer center building. Due to the electromagnetic induction caused by lightning current, a huge electromagnetic field is generated, which has a great destructive effect on computer centers and related equipment. It is required to keep a sufficient distance between the lightning protection grounding device and all other electrical equipment. Therefore, lightning protection measures can not be ignored. During a thunderstorm in the summer of 1997, Air China Inner Mongolia Branch failed to improve the lightning protection device, resulting in network paralysis and equipment damage, resulting in direct economic losses. Therefore, the lightning protection facilities of buildings must be built in strict accordance with the provisions of lightning protection facilities and standards, and the good degree of lightning protection grounding piles should be tested at least once a year.
Relationship between grounding of various devices
The grounding relationship of all kinds of equipment in computer center is actually the relationship between DC ground and other places. The size of DC ground, the connection mode and the relationship with other computer ground depend on different design requirements.
(1) Relationship between DC grounding suspension and grounding
In DC floating system, other places can be individually grounded or connected together.
(2) the relationship between 2)DC and the earth.
In the DC grounding system, due to the different requirements of each computer system, the relationship between DC grounding and other places is very different, basically there are the following connections.
A) DC grounding, safety grounding, AC grounding and lightning protection grounding are respectively connected to different piles. This connection seems to be irrelevant, and the cost of a single pile is cheap. But in fact, this method is not only complicated and expensive, but also difficult to meet the requirements of relative isolation between sites, which is easy to impact DC system and affect the reliability of equipment.
B) DC grounding, lightning protection grounding, safety grounding and AC grounding are respectively the same pile. The situation is basically the same as a).
C) DC grounding, AC grounding and safety grounding of the machine room are all systems, and each has a grounding bus connected to the center line of the power distribution cabinet. This method is convenient for construction, and can keep the required distance from lightning protection and grounding, with a large amount of calculation.
Grounding mode adopted by mechanical system.
D) If all grounding is connected with lightning protection grounding, it is required to keep enough distance between lightning protection grounding device and all electrical equipment in order to prevent lightning voltage from counterattack. However, it is difficult to ensure that this condition is met, especially when reinforced concrete building structures use steel bars as lightning protection networks, and this distance is actually impossible to guarantee. In this case, these places should be connected together and the same grounding method should be adopted. In order to prevent the grounding potential from rising when lightning strikes, it is best to limit the * * * resistance to below 1ω.
E) AC grounding, DC grounding and safety grounding of all equipment in the machine room should be in the same pile. Using the same pile in all places brings great insecurity to people, especially DC ground and lightning protection ground, and grounding facilities will be damaged by years.
Years of disrepair lead to the increase of grounding resistance, which poses a great threat to the safety and reliability of computers. Therefore, the author suggests that DC, AC and safety should be connected together and then connected to the same pile instead of this grounding method.
Problems needing attention in handling grounding wire
(1) grounding resistance-* * The grounding resistance of grounding piles shall meet the requirements of minimum grounding resistance among all kinds of grounding.
(2) In order to prevent the mutual interference of grounding systems and ensure the insulation of buildings, the grounding bus bar should be shielded wire with insulating sheath, and one end of the shielding sheath should be grounded.
(3) Although DC ground, AC ground and safety ground are finally connected to the ground pile, it does not mean that they can be connected at will. Only after strict insulation can they be connected to the same ground pile according to the above requirements.
(4) In computer equipment, the DC ground and the chassis are safely grounded separately, because the DC ground and the chassis are strictly insulated and connected to the system ground piles respectively, but some computer chassis and DC ground are electrically connected together, and the workplace of AC equipment is strictly insulated from the chassis.
8 electrostatic protection
Static electricity is one of the important factors that cause the failure of electronic equipment such as computers, which is mainly reflected in the accumulation of static electricity on the computer shell. When the accumulated energy of charge reaches a certain level, it will give people the feeling of electric shock. When the electrostatic charged body touches the computer, it will discharge the computer, which may cause the logic element to send out wrong signals, cause computer operation errors, even cause program disorder, and even burn the equipment. How to prevent the harm caused by static electricity, analyze the influence of static electricity on computer equipment, find out the root cause of static electricity, and reduce or even eliminate static electricity is a subject that can not be ignored.
In order to reduce the influence of static electricity on computer equipment, in addition to anti-static floors and partitions, grounding shielding is generally adopted. The grounding of the equipment shell is the most basic anti-static measure, which requires the computer itself to have a reasonable grounding and shielding system, so that when the electrostatic charged body contacts the computer shell to discharge, static electricity can leak into the ground through the grounding wire without causing system operation failure. Usually, the instantaneous electrostatic potential is too high, which is easy to cause ground potential fluctuation. Secondly, it is necessary to cut off the intrusion of electrostatic noise into audio channels as much as possible, and try to use shielded wires when jumping audio and digital wires. The outer insulation of the shielded wire should be well grounded, so as to leak the charges accumulated around it.
In view of the fact that grounding system is an important means to improve the reliability of computer network, suppress noise and ensure the safety of computer room equipment, the grounding of computer equipment should be taken seriously. If we don't pay enough attention to it or the grounding system is not handled properly, it will affect the stable work of the computer and even cause failures, and even burn the interfaces and equipment, which will seriously endanger personal safety.
The requirements of computer application technology for "ground wire" are getting higher and higher, which requires us to constantly explore and sum up experience in our work.
9 related terms
Common grounding symbols in circuit diagrams
1)
The potential at any point of a grounded or conductive substance is usually zero.
2) Grounding
Electrically connected to an object that can provide or receive a large amount of charge (such as the ground, the metal shell of a ship or vehicle, etc.). ).
3) (electrostatic) grounding
The measure of electrically connecting the metal conductor (through the grounding electrode) with the earth makes the potential of the metal conductor close to the earth potential.
4) Grounding
A. Grounding directly or through low impedance.
B. Grounding through wires or other conductors with little or almost zero resistance (impedance).
5) Soft grounding
By grounding with sufficient impedance, the current is limited to the level of personal safety (usually 5mA). The impedance required for soft grounding depends on the voltage level that people near the grounding point may be exposed to.
6) Direct grounding
A grounding method for electrically connecting a metal body with the earth.
7) Indirect grounding
In order to make an object other than metal electrostatically grounded, all or part of its surface is closely connected with the grounded metal body.
8) Electrostatic discharge grounding device
A public equipment, and the equipment not connected to the equipment in the ESD protection work area is grounded.
9) Ground reference plane
A flat conductive surface whose potential is used as a common reference potential.
10) electrostatic grounding connection system
The charge on the charged body leaks to the earth and dissipates to the outside.
1 1) Human body grounding
Measures to keep the human body and the earth in a conductive state by using conductive pads, conductive floors, conductive shoes or other grounding devices.
12) grounding rod
A conductor or a combination of several conductors buried in the ground to make good contact with the earth.
13) voltage to ground
The potential difference between a charged body and the earth (making the earth potential zero).
14) (electrostatic) connection
Measures to connect objects without good conductive paths to each other so that they are at the same potential.
15) lap joint
1. Arbitrarily and fixedly combine two conductive objects. This combination can be a direct contact between the conductive surfaces of two objects, or a firm electrical connection between two objects.
B, in electrical engineering, a method of connecting metal parts together, so that they present low-resistance electrical contact to direct current and low-frequency alternating current.
16) Single point grounding
Each circuit or shield has only one ground connection point. Ideally, a subsystem is only connected to the same grounding point, which can prevent the return current from flowing in the structure.
17) connecting line [strip, piece]
A, a kind of metal braided wire or metal strip [sheet] for lap joint.
B. A metal braided wire or metal strip [sheet] that provides necessary conductivity between components and structures when sufficient electrical contact cannot be maintained by other methods.
18) leakage resistance
Equivalent resistance between the measured point and the grounding system when the object is uncharged.
19) leakage current
Refers to the current that the charge on the charged body leaks to the earth through various leakage paths.
20) Electrostatic leakage channel
The way that the static charge in the charged area passes through the inside and surface of the charged body and makes it leak.
2 1) stray current
Any current that does not flow according to the specified path can be the earth, pipes connected with the earth and other metal objects or structures.
10 grounding system
brief introduction
Grounding system is divided into TT system, TN(TN-C, TN-S, TN-C-S) system and IT system.
The first letter indicates the relationship between the power supply (power) system and the ground. T means that the neutral point is directly grounded, and I means that all points are partially insulated (ungrounded). The second letter indicates the relationship between the exposed metal part of electrical equipment and the ground. For example, T means that the equipment shell is grounded, which is not directly related to any other grounding point in the system, and N means that the load is connected to zero protection. The third letter stands for work.
tt system
Do a good job in the combination of zero line and protection line. For example, C indicates that the working zero line and protection line are integrated, such as TN-C, and S indicates that the working zero line and protection line are strictly separated, such as TN-S..
tt system
TT mode refers to the protective system that the metal shell of electrical equipment is directly grounded, which is called protective grounding system, also known as TT system.
TN system
TN system refers to a low-voltage distribution system in which the power supply system is directly grounded at a point (the neutral point in the power supply transformer in the building industry), and the exposed conductive part of the load equipment (such as the metal shell) is connected to this point through a protection line, which is called another protection system.
TN power supply system is divided into TN-C, TN-S and TN-C-S systems according to whether the protection line PE is separated from the working zero line N.
TN-C system
The protection line PE and the working zero line N are combined into a PEN line, and all exposed conductive parts of load equipment are connected by PEN line (only used when the three-phase load is basically balanced).
TN-C system
TN-S system
TN-S is a power supply system that strictly separates the working zero line N from the special protection line PE. TN-S is safe and reliable, and is used in low-voltage power supply systems such as industrial and civil buildings.
TN-S system
TN-C-S system
The front end is TN-C system, and the back end is TN-S system. TN-C-S system with independent transformer
TN-C-S system
Equipment is widely used in living quarters.
Information technology system
There is no working grounding on the power supply side of IT system, or it is grounded through high impedance, and the electrical equipment on the load side is grounded. When the power supply distance of IT system is not very long, the power supply reliability is high and the security is good. It is generally used in places where power failure is not allowed, or places that require strict and continuous power supply, such as electric power, steelmaking, operating rooms in large hospitals, places, etc.
Information technology system
Down the mine, etc.
1 1 grounding safety
1) The working grounding resistance value of a single power transformer or generator with a capacity greater than 100 kva or a power transformer or generator operating in parallel with the same grounding device with a total capacity greater than 100 kva should not be greater than 400. Or the working grounding resistance value of power transformer or generator with the same grounding device running in parallel and total capacity less than 100 kva should not be greater than 654. 38+00 120 In the area where the soil resistivity is greater than 1000 f ~ m, when it is difficult to reach the above grounding resistance value, the working grounding resistance value can be increased to 3 ⅸ 2.
2) The protective zero line in the IN system must be repeatedly grounded at the middle and end of the distribution system, except in the distribution room or the main distribution box. In TN system, the grounding resistance value of each repeated grounding device protecting the zero line should not be greater than 10Q. In the power system that allows the working grounding resistance to reach 10Q, the equivalent resistance of all repeated grounding should not be greater than 10Q.
3) In the 1N system, it is forbidden to repeatedly ground the working zero line laid separately.
4) The grounding wire of each grounding device should use two or more conductors, and the grounding body should be electrically connected at different points. Aluminum conductor shall not be used as grounding body or underground grounding wire. The vertical grounding body shall be angle steel, steel pipe or smooth round steel, and rebar shall not be used. Natural grounding body can be used for grounding, but its electrical connection and thermal stability should be guaranteed.
5) Metal shell of electrical equipment powered by mobile generator. Or the base should have reliable electrical connection with the grounding device of the generator power supply.
6) The grounding of mobile generator system shall meet the requirements of grounding of power transformer system. In the following cases, zero connection can be performed without additional protection:
(1) The mobile generator and electrical equipment are fixed on the same metal bracket and do not supply power to other equipment.
(2) When there are no more than two electrical equipment powered by special mobile generators, the distance between the power supply and the electrical equipment is not more than 50m, and there is a reliable electrical connection between the power supply and the metal shell of the electrical equipment.