Commonly used protective grounding, work grounding, lightning grounding, shielding grounding, anti-static grounding.
Protective grounding
Lightning grounding is subject to lightning strikes (direct, induction or line introduction), in order to prevent damage to the grounding system. Often signal (weak) lightning protection ground and power (strong) lightning protection ground points, the reason for the distinction is not only because of the requirements of the grounding resistance is different, but also in engineering practice, the signal lightning protection ground is often attached to the signal on an independent ground, and the power supply lightning protection ground separate construction. Enclosure grounding system
Safety grounding is the system is usually not charged metal parts (cabinet shell, console shell, etc.) and the ground to form a good conductive connection between the protection of equipment and personal safety. The reason is that the power supply system is a strong power supply (380, 220 or 11OV), usually the case of the enclosure and so on is not electrically charged, when a fault occurs (such as a host power failure or other faults) caused by the power supply of the power supply line of fire with the enclosure and other conductive metal parts of the short-circuit, these metal parts or the enclosure is formed by the charged body, if there is no good grounding, then the charged body and ground between the high potential difference, the ground is a good conductive connection. If a person accidentally touches these charged bodies, then a pathway will be formed through the person, creating a hazard. Therefore, it is necessary to make a good connection between the metal enclosure and the ground, so that the enclosure and the ground are equipotential. In addition, protective grounding can also prevent the buildup of static electricity.
Working ground
Working ground network schematic
Ground is in order to make the system as well as the instrumentation connected to it can operate reliably and to ensure the accuracy of measurement and control of the ground. It is divided into machine logic ground, signal circuit ground, shielding ground, in the petrochemical and other explosion-proof systems and intrinsically safe ground. Machine logic ground, also known as the host power supply ground, is the logic level of the computer within the negative end of the public **** ground, but also +5V and other power supply output ground. Signal circuit ground, such as the negative end of each transmitter ground, the negative end of the switching signal ground. Shield grounding (grounding of the shield of analog signals). Intrinsically safe grounding, the grounding of intrinsically safe instruments or safety grids. In addition to suppressing interference, this type of grounding is one of the measures to make the instrument and system intrinsically safe. Intrinsically safe grounding varies depending on the intrinsic safety measures of the equipment used, and the following is an example of the grounding contents of a Zener-type safety barrier. Safety grounding short
The purpose of the grid is to protect the hazardous site end from always being within the safe power supply and safe voltage range. If the field end is short-circuited, the current on the wires will be limited to a safe range due to the current limiting effect of the load resistance and the safety grid resistance R, so that the field end does not generate a very high temperature and cause combustion. In the second case, if a fault is generated at one end of the computer, the high voltage electrical signal is added to the signal loop, then the voltage is also kept in the safe range due to the embedding effect of the Zener secondary. It is worth reminding that, due to the introduction of the Zener safety grid, so that the signal loop resistance increases a lot, therefore, in the design of the output circuit load capacity, in addition to consider the real load requirements, but also to fully consider the resistance of the safety grid, to leave room for manoeuvre. In addition to the above grounding, in many cases easy to cause confusion there is a power supply system ground, also known as the AC power supply ground, it is the power system for the operation of the grounding needs to be set up (such as neutral grounding).
Lightning protection ground
Composed of a part of lightning protection measures. Its role is to introduce the lightning current into the earth. Lightning protection of buildings and electrical equipment is mainly with lightning arrester (including lightning rods, lightning straps, lightning nets and lightning protection devices, etc.). One end of the lightning arrester is connected to the protected equipment, and the other end is connected to the grounding device. When the occurrence of direct lightning, lightning arrester will lead the lightning to itself, lightning current through its lead line and grounding device into the earth. In addition, due to lightning caused by electrostatic induction side effects, in order to prevent indirect damage, such as house fires or electrocution, etc., usually also need to be grounded inside the building of metal equipment, metal piping and reinforced structures; lightning waves will be invaded along the low-voltage overhead line, TV antennae into the house, caused by insulation breakdown of electrical equipment inside the house, resulting in fires or personal injury or death from electrocution, so it is also necessary to be on the line and into the house before the Insulated porcelain vase iron foot grounding.
Shield grounding
It is an effective measure to eliminate the hazards of electromagnetic fields on the human body, but also to prevent electromagnetic interference. High-frequency technology has been widely used in electric heating, medical, radio broadcasting, communications, television and navigation, radar and other aspects. The human body under the action of electromagnetic fields, absorbed radiation energy will occur biological role, causing harm to the human body, such as slight trembling fingers, skin scratches, vision loss. Generate magnetic field of the equipment shell shielding device, and the shielding body grounding, not only can reduce the shielding body outside the electromagnetic field strength, to reduce or eliminate the harm of electromagnetic fields on the human body, but also to protect the shielding grounding body of the equipment from the outside world from the impact of electromagnetic field interference.
Anti-static grounding
To prevent the effects of electrostatic hazards and discharge, is the most important part of electrostatic protection.
Modernized power system itself is a strong source of electromagnetic interference, mainly through radiation interference in the frequency band of communication equipment. In order to suppress the impact of interference from external high-voltage transmission lines, the use of grounding measures, commonly used grounding methods there are two, are discussed below:
Dispersion of grounding methods
Dispersion of Shell Grounding Circuit
Grounding is the communications building lightning grounding, power supply system grounding, grounding of various types of communications equipment and other equipment grounding access to each other separated from each other. Grounding system, due to the increasing number of ground systems, ground between the potential coupling effect is often difficult to avoid, decentralized grounding but easy to cause interference. At the same time the height of the main building is increasing, the grounding method of the insecurity factor is also growing. When a facility is struck by lightning, it is easy to form an underground counterattack, damaging other equipment.
Joint grounding method
Joint grounding method, also known as the single-point grounding method, that is, all grounding systems **** with a **** the same "ground". Joint grounding has the following characteristics: (1) the entire building grounding system consists of a cage-type equalizer, for direct lightning, the same floor of the building at various points is relatively uniform; for induced lightning, cage-type equalizer and the building's frame structure of the external electromagnetic field interference can also provide a shielding effect of 10-40dB; (2) the general joint grounding method of grounding resistance is very small, there is no coupling between the various grounding body. There is a variety of coupling effects between the grounding body, is conducive to reducing interference; (3) can Dry weather grounding mines
Save metal materials, occupies less land. From the above it is not difficult to see, the use of joint grounding can effectively inhibit the interference of external high-voltage transmission lines. Antistatic grounding of the ground wire should be connected in series with a 1 megohm current-limiting resistor, that is, through the current-limiting resistor connected to the grounding device. Isn't the grounding resistance as small as possible? Why string resistance? Computer grounding is based on the goal of easy flow of grounding current, which requires the smaller the grounding resistance, the better. The grounding of the computing center should minimize noise-induced potential shifts, and it should be noted that signal circuits and power supply circuits, high-level circuits and low-level circuits can not use the same **** ground loop. For network wiring with high transmission bandwidth requirements, isolated shielded grounding should be used to prevent interference from electrostatic induction. Strive for simplicity, economy and effectiveness in the design of the grounding if you can effectively combine with shielding, will be able to better solve the interference, noise suppression.
AC working ground
In the computer system, in addition to the use of DC electrical equipment, but also a large number of equipment and use of AC electrical equipment. AC grounding is the use of AC power in the computer system equipment to do 2 grounding or by special equipment and the earth for metal connection, its role is to ensure personal and equipment safety. The implementation of AC working ground can be divided into computer systems using AC equipment and computer systems supporting the two cases of AC equipment, should be independent of each other according to the electrical standard grounding, in order to prevent the risk of electric shock due to insulation damage.
Safety ground
The electrical equipment and the charged part of the metal casing or rack insulated with the ground between the ground to do a good job of grounding is called safety ground. If the chassis is not grounded, the chassis with a higher potential, the human body after contact with the risk of electric shock, when the insulation is penetrated, grounding short-circuit current will flow along the ground wire and the human body two paths at the same time into the earth. Usually used in computer rooms AC equipment chassis (such as: air conditioners, frequency and voltage regulators, transformers, UPS backup power supply and other equipment shell) should also be grounded according to the relevant electrical specifications.
Lightning ground
Lightning protection ground is mainly used to the earth to drain the lightning current, the purpose is to protect personnel and building safety. Lightning protection ground and the computer center buildings using lightning protection measures related to the phenomenon of electromagnetic induction due to lightning current, resulting in a huge electromagnetic field, the computer center and related equipment has a great destructive effect, the requirements of lightning protection ground device and all other electrical equipment to maintain a sufficient distance between. Therefore lightning protection measures cannot be ignored. In the summer of 1997, a thunderstorm, Air China Inner Mongolia Branch due to the failure to improve the lightning protection device, resulting in network paralysis, equipment damage, resulting in direct economic losses. Therefore, the building lightning protection facilities must strictly follow the provisions of the lightning protection facilities, according to the standard construction, at least once a year to test the good degree of lightning grounding piles.
Problems to be noted when dealing with the ground of the **** ground
(1) grounding resistance - **** with grounding stakes grounding resistance should meet the requirements of the minimum grounding resistance of the various grounding. (2) In order to prevent mutual interference of the grounding system and to ensure insulation to the building, the grounding busbar should use a shielded wire with an insulating jacket, and one end of the shielding jacket should be grounded. (3) Although DC ground, AC ground and safety ground are finally connected to the ground stake, it doesn't mean that all kinds of ground can be connected to each other at will, and they should be strictly insulated from each other according to the above requirements before they are connected to the same ground stake. (4) in the dc ground and chassis safety ground in separate computer equipment, because of its dc ground and the rack is strictly insulated, each separately connected to the system ground stake, but some computer chassis and dc ground is connected together electrically, and the working ground of the ac equipment and the chassis is strictly insulated.
1) ground The potential at any point is conventionally taken to be zero earth or conductive material. 2) ground Electrical connection to an object that can supply or receive a large electrical charge (such as on the ground, the metal casing of a ship or carrier, etc.). 3) (electrostatic) grounding A measure to electrically connect a metal conductor (via a grounding electrode) to the earth so that the potential of the metal conductor is close to the earth's potential. 3) Grounding a. Directly grounded or connected to earth through a low impedance. b. Connected to earth by a wire or other conductor having a small or almost zero resistance (impedance). 5) Soft Grounding Grounding with sufficient impedance to limit the current to a level below which it is safe for human beings (usually 5mA). The impedance required for soft earthing depends on the voltage level to which persons close to the earthing point may be exposed. 6) Direct grounding A type of grounding that electrically connects a metallic body to the earth. 7)Indirect grounding A grounding method in which all or part of the surface of an object other than metal is connected to the grounded metal body in close proximity for the purpose of electrostatic grounding. 8)Electrostatic discharge grounding device A public **** device, in the electrostatic discharge protection work area with the device connected to the device without device is grounded. 9) Grounding Reference Plane A flat conductive surface whose potential is used as a common **** reference potential. 10) electrostatic grounding connection system Charged body on the charge to the earth leakage, dissipation of the external export channel. 11) human body grounding measures to keep the human body and the earth conductive through the use of conductive mats, conductive ground, conductive shoes or various other grounding appliances. 12) grounding (electric) pole buried in the earth in order to make good contact with the earth conductor or a combination of several conductors. 13) voltage to ground the potential difference between a charged body and the earth (so that the earth's potential is zero). 14) (electrostatic) connection the measure of connecting objects that do not have a good conductive path between them electrically so that they are at approximately the same potential as each other. 15) lap a. Any fixed bond that makes a conductive connection between two objects. This bond can be a direct contact between the conductive surfaces of the two objects, or it can be retrofitted with a firm electrical connection between the two objects. b. In electrical engineering, a method of connecting various metal parts together so that they present a low-resistance electrical contact to direct current and low-frequency alternating current currents. 16) single-point grounding A form of grounding in which each circuit or shield has only one point of connection to ground. Ideally, a subsystem is connected to the same ground point only, and this method prevents return currents from flowing through the structure. 17) LAP WIRE [BAR, PIECE] a. A braided metal wire or metal bar [piece] used for lapping. b. A braided metallic wire or strip [piece] of metal that provides the necessary electrical conductivity between parts and structures when adequate electrical contact between them cannot be maintained by other means. 18) Leakage resistance The equivalent resistance of an object, when not electrically charged, from the point to be measured to the grounding connection system. 19) Leakage current The charge on a charged body through various leakage paths to earth leakage current. 20) electrostatic leakage channels The electrostatic charge of the charged area through the interior and surface of the charged body to make the leakage path. 21) stray current any current that does not flow according to a specified pathway, these non-specified pathways can be the earth, pipelines connected to the earth, and other metal objects or structures.
Grounding systems are divided into TT systems, TN (TN-C, TN-N, TN-C-S) systems, and IT systems. The first letter of the power (power) system to the ground relationship. t indicates that the neutral point is directly grounded, I indicates that all with point part of the insulation (not grounded). The second letter indicates the relationship of the exposed metal part of the power-using device to ground, such as T indicates that the equipment shell is grounded, which has no direct relationship with any other grounding point in the system, and N indicates that the load is protected by zero connection. The third letter indicates the combination of the working TT system
The combination of the working zero line and the protection line, such as C indicates that the working zero line and the protection line are united, such as TN-C, and S indicates that the working zero line and the protection line are strictly separated, such as TN-S.
TT system
TTThe TT method refers to the protection system in which the metal casing of the electric equipment is directly grounded, which is known as the protective grounding system , also known as TT system.
TN system
TN system refers to the power supply system there is a point (in the construction industry usually refers to the neutral point in the transformer of the building power supply) directly grounded, the load equipment of the exposed conductive part (such as metal casing) through the protective wire connected to this point of the low-voltage power distribution system, called the other protection system. TN mode power supply system, according to its protection line PE whether with the work of the zero line N separate and divided into TN-C, TN-S, TN-C-S system.
TN-C system
Protection line PE and the working zero line N into a PEN line, all the load equipment of the exposed conductive part of the PEN line is connected to a form (only used in the three-phase load basic balance). TN-C system
TN-S system
TN-S is a kind of power supply system that strictly separates the working zero line N and the special protection line PE. TN-S is safe and reliable, and is used in low-voltage power supply system for 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 is more commonly used in living quarters with independent transformer TN-C-S system
The device.
IT system
IT system power supply side does not have a working ground, or through the high impedance ground, the load side of the electrical equipment for ground protection. IT system in the power supply distance is not very long, the power supply is highly reliable, good security. Generally used in places where power outages are not allowed, or places that require strictly continuous power supply, such as electric power, steelmaking, operating rooms in large hospitals, underground IT systems
under the mine and so on.