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The so-called grounding means that a certain part of the equipment is closely connected with the earth through a grounding device. Up to now, grounding is still one of the most widely used electrical safety measures, and other methods cannot be replaced. Whether it is electrical equipment or electronic equipment, whether it is production equipment or household equipment, whether it is DC equipment or AC equipment, whether it is fixed equipment or mobile equipment, whether it is high-voltage equipment or low-voltage equipment, whether it is power plant or users, different grounding measures are adopted to ensure the normal operation or safety of the equipment.
First, the role of grounding gas
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.
1. Protect people from electric shock
During normal operation, the uncharged metal conductor of electrical equipment should be connected with the grounding electrode to protect human safety and prevent people from getting an electric shock.
When the insulator in a certain part of electrical equipment is damaged, the shell will be charged. Because the neutral point of the power supply is grounded, even if the equipment is not grounded, there is a capacitance between the line and the earth. At this time, when the human body touches the equipment shell, there will be a current flowing through the human body. Or the insulation somewhere on the line is not good, and the human body touches the shell of electrical equipment with damaged insulation, and the current will pass through the human body and become a path, thus causing the human body to suffer electric shock damage.
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. At this time, the human body is connected in parallel with the grounding electrode, and 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. Usually, the resistance of the human body is several hundred times larger than that of the grounding electrode, so the current flowing through the human body contributes several hundred times to the current flowing through the grounding electrode. When the grounding resistance is extremely small, the current flowing through the human body is almost equal to zero, which is equivalent to the grounding electrode shorting the human body. Therefore, the human body can avoid the danger of electric shock.
Therefore, whether in construction or operation, the grounding resistance should be guaranteed not to be greater than the grounding resistance specified in the design or specification throughout the year to avoid electric shock injury.
2. Ensure the normal operation of the electrical system.
Power system grounding is generally neutral point grounding, and the grounding resistance of neutral point is very small, so the potential difference between neutral point and ground is close to zero. When the phase wire collides with the shell or is grounded, the other two relative ground voltages will increase to the cubic root of the phase voltage in the system with neutral point insulation, but they will be close to the phase voltage in the system with neutral point grounding, so neutral point grounding will be beneficial to the stable operation of the system and prevent the system from oscillating, and the insulation level of electrical equipment and lines in the system only needs to be considered according to the phase voltage, which can reduce the manufacturing cost of electrical equipment and the construction cost of lines. Neutral grounding system can also ensure the reliable action of relay protection.
The communication system generally adopts positive grounding, which can prevent noise from entering and ensure the normal operation of communication equipment.
The electronic circuit needs a stable reference point to run normally, so it also needs grounding.
3. Prevent lightning and electrostatic hazards.
Electrostatic induction and electromagnetic induction will be generated when lightning strikes, and the static electricity generated by friction during the production and transportation of materials may cause the danger of electric shock or fire.
The harm of direct lightning is greater than that of induction lightning, and there are more opportunities. Therefore, in order to prevent direct lightning strike, lightning protection devices must be installed.
Among all lightning protection devices and measures to prevent electrostatic hazards, the most important method is to set up grounding devices.
Second, the classification of electrical grounding
1. According to the function or function of grounding.
The so-called grounding is simply the electrical connection between various devices and the earth. There are all kinds of equipment that need to be grounded, such as power equipment, communication equipment, electronic equipment and lightning protection devices. The purpose of grounding is to facilitate the normal and safe operation of equipment and create conditions for the safety of buildings and people. Commonly used grounding methods can be divided into the following types according to their functions or functions.
(1) System grounding connects a point in the power system with the earth, which is called system grounding or working grounding. Such as transformer neutral grounding, neutral line repeated grounding, etc.
(2) Protection and grounding of equipment: metal casings of various electrical equipment, metal pipes of lines, metal protective layers of cables, metal brackets for installing electrical equipment, etc. Conductor insulation may be charged after damage. In order to prevent these uncharged metal parts from generating excessive voltage to the ground and endangering personal safety, this kind of grounding is called protective grounding.
(3) Lightning protection grounding In order to safely discharge lightning current to the earth and protect buildings or power equipment that have been struck by lightning, it is called lightning protection grounding.
(4) Shielding grounding is to prevent the interference and invasion of external electromagnetic waves, resulting in the misoperation of electronic equipment or the decline of communication quality, and to prevent the high-frequency energy generated by electronic equipment from being released to the outside world. The grounding of the electrostatic shielding layer of the line filter, transformer and the metal shielding layer of the cable is called shielding grounding. In order to reduce the induced potential caused by lightning current induction in the vertical pipe of high-rise building, grounding the steel bar in the concrete wall of the vertical pipe also belongs to shielding grounding.
(5) Anti-static grounding static electricity is the charge accumulated due to friction and other reasons. In order to prevent accidents caused by static electricity or affect the normal work of electronic equipment, it is necessary to have a grounding, so that static electricity can be quickly discharged to the earth. This kind of grounding is called antistatic grounding.
(6) Equipotential grounding In some special examination rooms, treatment rooms, operating rooms and wards of hospitals, there should be no dangerous potential difference in the metal parts that patients can come into contact with (such as bedsteads, bed lamps, medical instruments, etc.). ), so these metal parts should be connected to each other to become equipotential bodies and grounded. This grounding method is called equipotential grounding. In order to reduce the potential difference caused by lightning current in high-rise buildings, the steel mesh of each floor is connected with large metal objects to be grounded, which is equipotential grounding.
(7) Signal grounding of electronic equipment and power grounding Signal grounding (or logic grounding) of electronic equipment refers to the grounding of amplifiers, mixers, scanning circuits, logic circuits, etc. In a signal loop with a unified reference potential. The purpose of grounding is not to cause the error of semaphore. Power grounding is the unified grounding of all relays, motors, power devices, high-current devices, indicator lights and other circuits to ensure that the interference signals in these circuits are discharged into the ground and will not interfere with the normal work of other sensitive signal circuits.
Generally speaking, grounding is classified as follows.
According to the formation of grounding, it can be divided into normal grounding and fault grounding. The former is artificial grounding gas for a certain need, and the latter is grounding gas naturally formed by various external or self factors.
According to different grounding functions, normal grounding can be divided into working grounding and safety grounding.
Working grounding usually has the following three situations.
① Use the earth as the grounding of the loop. In general, this kind of grounding also has current passing through the earth, such as DC working grounding, weak current working grounding, and one of the "two wires and one ground" power supply modes.
② Grounding to maintain the safe operation of the system. Under normal circumstances, there is no current or only a small unbalanced current passing through the earth. For example, the neutral point of the system above 1 10kv is grounded, and the transformer neutral point of the low-voltage three-phase four-wire system is grounded.
(3) In order to prevent lightning and overvoltage from harming equipment and personnel, grounding is overvoltage protection grounding, also known as lightning protection grounding.
Safe grounding mainly includes: protective grounding to prevent personal safety from being endangered after insulation damage of power facilities or electrical equipment; Anti-static grounding, eliminating the accumulation of static electricity in the production process, causing electric shock or explosion; The shielding of the metal shell, shielding cover or shielding wire sheath of the equipment is grounded to prevent electromagnetic action; In order to prevent the pipeline from being corroded after electrochemistry, cathodic protection or sacrificial anode electrical protection is used to protect the grounding.
2. Classification according to grounding form
Grounding electrode (body) can be divided into external grounding electrode and loop grounding electrode according to its arrangement. According to the shape of grounding electrode, there are several basic forms: tubular, strip and ring. According to the structure of grounding electrode, there are natural grounding electrode and artificial grounding electrode. Can be used as natural grounding electrodes are: metal water supply and drainage pipelines; Metal structures of buildings and structures reliably connected with the earth; There shall be no less than two metal sheaths of cables laid underground, except for all kinds of metal pipelines laid underground, flammable liquid and flammable and explosive gas pipelines. Generally, steel pipe, angle steel, flat steel and round steel can be used as artificial grounding electrodes. If it is in chemically corrosive soil, galvanized steel or copper grounding electrode can be used. Schematic diagram of grounding device is shown in Figure 4- 1.
Figure 4- 1 Schematic diagram of grounding device
If the grounding device is installed in a single grounding electrode or an external grounding electrode, there is still the danger of electric shock due to uneven potential distribution. In addition, the reliability of single grounding electrode or external grounding electrode is also poor. As can be seen from Figure 4- 1, there are only two main lines connecting outdoor grounding electrode and indoor grounding trunk line. If these two trunk lines fail, the whole indoor grounding trunk line will be disconnected from the outdoor grounding network, and the indoor related equipment is equivalent to no grounding at this time. Of course, it is unlikely that the two trunk lines will fail at the same time.
In order to eliminate the shortcomings of single grounding electrode or external grounding electrode, ring grounding electrode can be used, as shown in Figure 4-2. The potential distribution of the loop grounding electrode is relatively uniform, and the human contact voltage UC and the step voltage Ub are relatively small. But the potential distribution outside the grounding electrode is still uneven, and its step voltage is still very high. In order to avoid this shortcoming, some flat steel without electrical connection with the grounding electrode can be buried outside the loop grounding electrode. In this way, the potential distribution outside the grounding electrode, as shown in Figure 4-3, drops gently. Therefore, in general, the loop grounding electrode should be given priority. Only when it is difficult or expensive to use the loop grounding electrode will the external grounding electrode be considered.
Figure 4-2 Layout of Circuit Grounding Electrode
Fig. 4-3 potential change after flat steel is buried near the grounding electrode of the loop.
Third, the scope of electrical grounding
1. AC system
(1) AC lines below 50V. AC lines below 50 V are usually not grounded, but one of the following circumstances must be grounded.
(1) is powered by a transformer, and the neutral point of the transformer is not grounded.
(2) the power supply is provided by the transformer, and the voltage of the transformer to the ground exceeds 150V. ..
(3) The isolation transformer should not be grounded, but the iron core must be grounded.
④ Overhead lines installed outside buildings.
(2) AC system of 50 ~ LKV The AC system of 50 ~ LKV may not be grounded when the following conditions are met.
(1) The rectifier of the independent drive system is an industrial speed control drive system.
(2) Electrical system similar to smelting, refining, heating or power supply of industrial electric furnace.
(3) Independent transmission system powered by transformer; The special control system with rated voltage of transformer primary side lower than 1kV has continuous power supply, and the control system is equipped with grounding detector to ensure that only full-time personnel can monitor and maintain the situation.
(3) AC system of1~1okv The AC system of L ~ LOKV can be grounded through arc suppression coils or resistors as required, but the AC system of L ~ 1 OKV for mobile equipment should be grounded.
2.DC system
(1) Two-wire DC system The DC two-wire distribution system shall be grounded.
The following conditions can be ungrounded.
(1) Power supply system for industrial equipment with grounding detector only in a limited range.
② The line-to-line voltage is ≤50V or > > 300V, and the system with insulation to the ground is adopted.
(3) The DC system is powered by the rectified grounding AC system.
(4) DC fire signal line with maximum current of 0.03a..
(2) Three-wire DC system The neutral wire of the three-wire DC system should be directly grounded.
3. Electrical equipment
(1) Grounding of exposed conductive part of electrical equipment The exposed conductive part of electrical equipment shall be grounded under the following conditions.
① Metal bases and housings of motors, transformers, general appliances, portable and mobile appliances, etc.
(2) Generator neutral point cabinet shell and outlet cabinet shell.
③ Secondary winding of voltage transformer and current transformer.
(4) The metal conductive part of the transmission device of electrical equipment.
⑤ Metal frames and bases of power distribution, control and protection screens (cabinets, boxes) and operation desks, and metal shell parts of fully enclosed combined appliances.
⑥ Metal frames of indoor and outdoor power distribution devices and reinforced concrete foundations, as well as metal fences and metal doors near live parts in substations, power plants and other places.
⑦ Exposed conductive parts of lighting fixtures.
(8) Metal casings of junction boxes, junction boxes and expanders of AC and DC power cables, metal protective layers of cables, accessible threading steel pipes, and metal trunking for laying cables and cable trays.
Pet-name ruby installed on the power line tower switchgear, capacitors and other electrical devices exposed conductive parts and their metal brackets.
Attending in residential areas with non-asphalt ground, metal towers and reinforced concrete towers of overhead power lines with grounding wires in ungrounded systems or through arc suppression coils or resistance grounding systems, and towers of overhead power lines with grounding wires.
○ 1 1 is equipped with the metal protective layer of the control cable, and 1 ~ 2 core is idle in the non-installed or non-metal sheathed cable.
○ 12 closed bus metal shell.
○ 13 metal box shell of box-type substation.
(2) The ungrounded exposed conductive part of electrical equipment can be ungrounded if the exposed conductive part has the following conditions.
(1) electrical equipment used in non-conductive places (such as wood, asphalt and other poorly conductive floors and insulating walls).
(2) Metal shells of low-voltage electrical appliances such as electrical measuring instruments and relays installed on distribution panels, control panels and electrical devices.
(3) When insulation damage occurs, the metal base of insulator will not cause dangerous voltage on the bracket.
④ Exposed conductive parts of electrical equipment or devices with AC rated voltage < < 50V and DC rated voltage < < 120V in dry places (except places with explosion danger).
⑤ Equipment installed on a grounded metal frame and having good electrical contact with the frame, such as chassis base (except for places with explosion danger).
⑥ Exposed conductive parts of motors and electrical appliances that have reliable electrical contact with grounding brackets (except for places with explosion hazards).
⑦ Metal bracket in battery room with rated voltage ≤220V.
(3) The outer conductive part of the outer conductive part shall be grounded where electric shock may occur. The parts that usually need to be grounded are as follows.
(1) In order to improve the safety of a large-area metal frame that may be electrified in or on a building, it should be grounded when it may come into contact with people.
(2) Metal frame of elevator with cable.
(3) Crane rails and trusses.
④ If the metal hoisting rope or cable of the elevator car has been connected with the elevator body to form an electrical path, it may not be grounded.
⑤ The exposed metal parts in the mobile house or bus, including the metal structure of the mobile house and the metal frame of the bus, shall be grounded.
⑥ Metal barriers, metal fences and other metal fences around electrical equipment with inter-line voltage exceeding 750V in substation or transformer outdoor.
4. Mobile and vehicle-mounted generators
(1) When the mobile generator only supplies power to the equipment installed on the generator or the socket on the generator only supplies power to the equipment connected by the flexible cord and plug, and the exposed conductive part of the equipment and the grounding electrode on the socket are connected to the generator frame, the frame of the mobile generator can be ungrounded, and the frame can be used as the grounding of the generator power supply system.
(2) When the vehicle-mounted generator meets all the following conditions, the frame of the vehicle used in the generator power supply system installed on the vehicle can be used as the grounding electrode of the system.
① The grounding part of the generator frame has been connected to the vehicle frame.
② The generator only supplies power to the equipment installed on the vehicle or only supplies power to the cord and plug connection equipment through the socket installed on the vehicle or generator.
③ The exposed conductive part of the equipment and the grounding electrode on the socket have been connected to the generator frame.
(3) Connection of neutral wire If the generator is an independent system, the neutral wire should be connected to the frame of the generator.