What is the weak equipment grounding technology
Led Street Light,Led Light,Led Solar Street Lamp,Led Lamps China Searun Solar Solution Co., Ltd. , https://www.srsolarlights.com
2.1 Purpose of the working ground Because the power system needs operation and safety, the neutral point (N line) is often grounded. This type of grounding method is called working ground. Working ground has the following purposes:
2.1.1 reduce the voltage of electric shock;
In systems where the neutral point is not grounded, when one phase is grounded and the human body touches one of the other two phases, the voltage of the electric shock is 1.732 times the phase voltage. In a neutral-grounded system, the electric shock voltage is reduced to equal or close to the phase voltage.
2.1.2 quickly cut off faulty equipment;
In a neutral grounded system, when one phase is grounded, the ground current is small (because the capacitance and insulation resistance between the wire and the ground, but also constitute the current path) is not enough to make the protection device action and cut off the power supply. Ground faults are not easily detected and will continue for a long time, making them unsafe to humans. In a neutral-grounded system, the ground current after one phase is grounded is large (close to a single-phase short circuit). The protection device quickly operates and disconnects the fault point.
2.1.3 Reduce the insulation level of electrical equipment to ground;
In systems where the neutral point is not grounded, grounding of one phase will raise the voltages of the other two phases to the line voltage. In the neutral grounding system, it is close to the phase voltage, so it can reduce the insulation level of electrical equipment and transmission lines, saving technical capital. For this reason, this article analyzes TN and TT systems separately. 
2.2 TN System The TN system's neutral point of the power supply is directly grounded. It is connected to the neutral point with the metal shell of the device and the protection neutral wire. This method is referred to as protection zero or zero connection. According to the combination of neutral (working zero line) and protection line (protection zero line) TN system is divided into the following three forms:
2.2.1.TN-C system;
In the TN-C system, since the PNE line functions as both a PE line and an N line, one wire is saved, but a three-phase unbalanced current I is passed through the PEN line, and a voltage drop IZPEN on the PEN line exposes the electrical device to the outside. Part of the voltage on the strip. Three-phase unbalanced loads cause the enclosure to have a very low voltage. It will not cause personal accidents in general places, but it may cause sparks on the ground and is not suitable for hospitals, computer center locations and explosion hazardous locations. The TN-C system is not suitable for residential buildings without electrician management. This system does not have a dedicated PE line, but is combined with a neutral line (N line) into a PEN line. PEN is used in residential buildings due to improper maintenance and management. Line interruption, 220V power supply to ground voltage will be shown in Figure 1 through the phase line and the device internal winding to the device shell, so that the shell presents 220V to ground voltage, the risk of electric shock is very large. In addition, the PEN line must not be disconnected (the device loses its grounding wire after the cut-off) and it cannot be electrically isolated. During the electrical maintenance, the PEN may cause personal electric shock due to the voltage on the ground. In the TN-C system, RCD (residual current action protector) cannot be installed, because when the ground fault occurs, the magnetic field in the current transformer in the fault current of the phase line and the PEN line cancel each other, and the RCD will not detect the fault current. No action, so TN-C systems should not be used in residential buildings.
2.2.2.TN-S system;
In the TN-S system, the working neutral line N and the protection neutral line PE are completely separated from the neutral point of the power supply end. The PE line does not pass current at all times and only passes the fault current when a ground fault occurs. Therefore, the exposed conductive part is usually grounded. Without a complete voltage, but need to add a wire, because the equipment and equipment shell to protect the zero line PE, leakage current switch does not have residual current during normal operation, so when the same short circuit protection sensitivity is not enough, you can install a leakage switch to protect the single-phase grounding . RCD has a high sensitivity to ground fault current. Even when it contacts 220V, it can cut off the fault current in milliamperes in tens of milliseconds and protect people from electric shock, but it can only protect it The ground fault has a function and cannot prevent the electric shock accident caused by the fault voltage transmitted from elsewhere.
2.2.3.TN-C-S;
TN-C-S is a combination of TN-C and TN-S systems, as shown in Figure 2. The first part is a TN-C system, the second part is a TN-S system, and the interface is N line and PE line. The connection point. This system is generally used in places where buildings have power from regional substations. TN-C system is used before entering the customer's line. Repeated grounding is performed at the customer's home and becomes TS-S after entering the home. TN-C-S The system is between the above two.
According to the “Low voltage distribution design code†related provisions, when the TN system is selected for building electrical design, equipotential bonding should be used to eliminate dangerous fault voltages that enter from outside the building along the PEN line or PE line, and at the same time reduce the protection of the electrical equipment. The danger of coming and being conducive to eliminating the interference caused by external electromagnetic fields and improving the electromagnetic compatibility of the device.
2.3 TT System The neutral point of the TT system's power supply terminal is directly grounded. The metal casing of the power supply device is connected to the grounding electrode that is not related to the power supply grounding point by using a protective ground wire. When the TT system is in normal operation, the potential of the metal shell of the electricity-consuming equipment is zero. When the electrical equipment collides with the shell in one phase, the short-circuit current is smaller than that of the TN system, which is usually insufficient for the action of the phase-to-phase short-circuit protection device. The risk is greater when the human body accidentally touches live parts, and safety can be ensured when RCDs are installed at the head end of main lines and electrical equipment. When the neutral point of the transformer and the grounding resistance of the electrical equipment is 4 ohms, the single-phase short-circuit current is Ld=220/(4+4)=27.5 A (line impedance is not counted). Regardless of the main terminal of the mains or the electrical equipment, when the fuse melting current is large or the setting current of the automatic tripping instantaneous release is large, it cannot be operated. Therefore, the TT system can not often use fuses, low-voltage short-circuit for ground fault protection and the need for leakage protection. Another feature of the TT system is that there is no electrical connection between the neutral line N and the protective earth PE, that is, the neutral point grounding and the PE line grounding are separated, so there is no external danger. The fault voltage enters the building along the PE and the call accident occurs. . Each building in the TT system has its own dedicated grounding electrode and PE wire. The PE lines of each building do not conduct to each other, and the fault voltage is not transmitted from one building to another. However, the TT system uses earth as the path for the fault current to return to the power supply, and the fault current is small. An earth leakage fault-responsive leakage protector must be used to prevent personal electric shock. Each of these systems has its own advantages and disadvantages, which need to be selected according to specific circumstances. If the building is powered by the power supply department at a low voltage, a grounding system should be used as required by the power supply department to harmonize with the grounding system in the area. If the TN-CS system is used, care should be taken to divide the PEN line into PE and neutral lines from the building's incoming power distribution box, so that the PEN line no longer appears in the building. This is because the PEN line is passing load current. With potential, it is easy to produce stray currents and potential differences.
If the power supply department supplies 10KV voltage to the residential building and the 10/0.4KV substation is inside the building, the building can only use the TN-S system. Because the use of TN-CS system will appear in the building within the PEN line; TT system requires separate working ground and protective grounding, but in the same building is difficult to achieve two separate grounding, maintenance work is also difficult of. Regardless of which grounding system is used, the aforementioned equipotential bonding must be made according to the specification.
3. The electronic device system working <br> <br> earth grounding system is provided to enable the electronic device electronics and instrumentation associated therewith can be run * and ground to ensure accuracy of measurement and control of the set. It is divided into machine logic, signal circuit grounding, and shield grounding. There is also intrinsically safe grounding in the explosion protection system.
3.1 Logical Ground Grounding a metal plate of an electronic device as a reference point for a logic signal is called logical grounding. Its role is to ensure that the circuit has a uniform reference potential that does not cause signal errors due to floating.
3.2 Signal grounding Various electronic circuits have a reference potential point, which is the signal ground. Its role is to ensure that the circuit has a uniform reference potential that does not float to cause signal errors. The connection of the signal ground is that the ground of the signal input terminal of the same device cannot be connected to the ground of the signal output terminal, but should be separated; the output ground of the preceding stage (device) is connected only to the input ground of the rear stage (device). Otherwise, the signal may be formed in the feedback through the ground, causing the signal to float. This is especially noticeable in connection with the signal ground during the test of the equipment.
3.3 Protective earthing Protective earthing is to make a good electrical connection between the uncharged metal enclosure (or frame) and the grounding device during normal operation of the device. If you do not make protective grounding, when the electrical equipment - the phase of the insulation damage, resulting in leakage and the metal casing with the phase voltage, people will have a contact with electric shock accident. After the protective grounding is applied, the metal housing of the device has been well connected to the earth. If leakage occurs, as long as the grounding resistance meets the specified requirements, grounding can be an effective measure to protect personal safety and prevent electrical accidents. In addition, the protective ground can also prevent the accumulation of static electricity. 
3.4 Lightning protection grounding Grounding for the purpose of rapidly introducing lightning currents to the earth and preventing lightning damage is referred to as lightning protection grounding.
3.5 Shield Grounding The grounding of shielding measures taken to achieve the electromagnetic compatibility of equipment or wiring is called shield grounding. It is very important for the EMC design of weak electrical equipment. In order to avoid the functional obstacles of the equipment used and to avoid the equipment damage that will occur, the equipment that constitutes the wiring system should be able to prevent internal conduction and external interference. Therefore, protective shielding measures must be taken to protect these devices and their wiring from all kinds of interference.
3.6 Anti-static Grounding An object with static electricity or an object that may generate static electricity (a non-insulator) that is grounded through the conductive body and the earth and forms an electrical circuit is called an anti-static ground. In a clean, dry room, people's walking and moving equipment will generate large amounts of static electricity due to their friction. For example, in a relative humidity of 10%-20%, a person's walking can accumulate 35,000 volts of electrostatic voltage. Without good grounding, it will not only cause interference to electronic devices, but will even destroy the device's chips.
3.7 Intrinsically safe grounding is the grounding of an intrinsically safe instrument or safety barrier. In addition to suppressing interference, this type of grounding also has one of the measures that make the instrument and system inherently safe. Intrinsically safe grounding differs depending on the actual equipment used.
The function of the safety barrier is to protect the hazardous field end from being within the safe power supply and safe voltage range. If the field end is short-circuited, the current on the wire will be limited to a safe range due to the current limiting effect of the load resistance and the safety barrier resistance R, so that the on-site end will not produce a very high temperature, causing combustion. In the second case, if a fault occurs on one end of the computer, a high-voltage electrical signal is added to the signal loop. Because of the clamping action of Zener secondary voltage, the voltage is also placed in a safe range.
It is worth noting that, due to the introduction of the Zener Barrier, the resistance on the signal circuit has increased a lot. Therefore, when designing the load capacity of the output circuit, in addition to considering the actual load requirements, it is necessary to fully consider the safety barrier. The resistance, leaving room.
4. Grounding principles and technical requirements
In a system, a large number of electronic devices are installed. These devices belong to different special systems. Since these devices have different operating frequencies, anti-jamming capabilities and functions, the requirements for grounding are also different. In practical engineering design and construction, the signal ground, logic ground, anti-static grounding, shield grounding, and protective grounding of electronic equipment are generally combined with one grounding pole, and the grounding resistance is not greater than 4Ω; when the grounding of the electronic equipment is connected to the AC frequency grounding When lightning protection is combined with a grounding electrode, the grounding resistance is not more than 1Ω. If shield ground is set separately, the ground resistance is generally 300Ω. Equipment with poor anti-interference ability should be grounded separately from lightning protection ground. The distance between the two should be within 20m. For electronic equipment with strong anti-interference ability, the distance between the two can be reduced as appropriate, but should not be less than 5m. When the grounding and lightning protection of the electronic equipment adopts a common grounding device, the two are protected against lightning strikes and ensure the safety of the equipment. They shall be powered by buried armored cables. The cable shield must be grounded. To avoid interference currents, ground the signal cable and low-frequency cables of 1 MHz or less at one point. For cables of more than 1 MHz, use multiple-point grounding to ensure that the shield is at ground potential. Both CCTV and industrial TV must be grounded.
The above-mentioned several grounding equipment manufacturers have their own specific technical requirements. Although most of them emphasize grounding, the grounding resistance must be less than 1 ohm, but the specific content varies greatly. Combining with the technical requirements of the industrial control system for grounding, elaborate the grounding principle and grounding method.
4.1 Power supply system In many enterprises, especially power plants, smelters, etc., there is a large ground network in the plant area, and usually the power supply system is connected to the ground network. Some manufacturers emphasize that all grounding of the computer system must be strictly separated from the power supply system and other (such as lightning protection grounds), and at least a distance of at least 15 m should be maintained between them. In order to completely prevent the influence of the power supply system, it is recommended that the power supply line be separated by an isolation transformer. From the standpoint of suppressing interference, it is advantageous to separate the power system from all of the computer system because the ground wire of the general power system is not very clean. However, from an engineering point of view, it is difficult to set up a computer system and ensure that it is separated from the power supply system by a certain distance. In this case, it may be considered whether the computer system can be shared with the power supply system. There are several factors to consider:
4.1.1 Whether the power supply system is greatly disturbed on the ground, such as whether high-current devices are started or stopped frequently, whether the interference to the ground is large or not; 4.1.2 The grounding resistance of the power supply system is sufficiently small, and the potential difference of the entire ground network Is it small, that is, whether the resistance between the various parts of the ground network is small?
4.1.3 The anti-interference ability of microelectronic devices and the anti-interference ability of the transmitted signals used, such as the direct transmission of small signals (couples, thermal resistance).
4.2 There is a lot of controversy on the grounding of the grounding involved in all the computer wiring of the microelectronics working place. Some manufacturers system proposes several grounds: logic ground, shielding ground (also called analog ground), signal ground, and protection ground are grounded on the ground, and most systems indicate that each ground is grounded on the inside of the cabinet. , Converge at one point, and then use the thicker conductor (copper) to place the points. Here are a few things to note:
The control system itself is composed of multiple devices. Apart from the control station, it also includes many peripheral devices, and the number of peripheral devices is also more than one, which involves multiple devices and multiple grounding problems.
4.2.1 Protective Grounding: All weak current devices have a protective ground. The protection is usually connected internally when cabinets and other equipment are designed and processed. In some systems, the protected ground is connected to the power supply internally. The protective ground (the middle of the three-core plug) is connected together, and some do not allow the protection ground to be connected with this line. Users must read the grounding installation instructions provided by the manufacturer carefully. No matter which way, the cabinet ground must be a device. The cabinet grounds of all peripherals or systems (control stations, operator stations, etc.) are connected together, and then the cabinet grounds of the stations are connected together with thick insulated copper conductors, and finally connected to the earth grounding system from one point. . It is also worth noting that all the peripherals of a system must be powered from one power supply line, and one device (such as all the peripheral devices and host systems connected to the operator station (CRT, printer, and copy machine host system). The power supply must be taken from the power distribution device of the device, but not from other places, otherwise the interface or even the device may be burned, and if it is necessary to use a long-line connection, the thicker wire is used to provide power for it, or The communication isolation measures shall be taken and the cabinet grounds of each station may be connected by radiation or serial connection.
4.2.2 Power Supply Ground (P); First, the logic in each station must be located at one point PG. Then, the thick insulated wire is connected to the point in a radial manner and then to the earth ground wire. In some systems, all inputs and outputs are isolated, so that its internal logic is an independent unit, and there is no electrical connection with other parts. In such systems, PG grounding is often not required, but internal floating is maintained. Therefore, when designing and constructing a grounding system, users must carefully read the technical requirements and grounding requirements of the product.
4.2.3 Analog Ground (AG); is the most demanding of all groundings. Almost all systems propose that the AG is grounded at one point and the ground resistance is less than 1 ohm. In the design and manufacture of microelectronic devices, AG bus bars or other facilities are installed inside the cabinet. When users connect the shielded cables to the AG busbars, connect them to the bottom of the cabinet with insulated copper files. Then connect the confluent points of the cabinets to the connections using insulated copper or copper rods. location.
4.2.4 Signal Ground; In principle, it is not permissible for each transmitter and other sensors to be grounded at the field end, and the negative terminal should be grounded at the computer terminals. However, in some cases, the field end must be grounded. At this time, it must be noted that the input terminals (upper double-ended) of the original signal must not be allowed to have any electrical connection with the computer's ground wire. However, when the computer processes such signals, it must be used at the front end. Effective isolation measures.
4.2.5 Safety Grid Grounding: The safety barrier circuit has three grounding points: B, E, D. Usually both B and E are on the computer side. Can be connected together to form a point ground. The D-point is the grounding of the transmitter housing at the site. If there is a potential difference between the two grounding points of the site and the control room, then the potentials of the D-point and the E-point are different. Assume that we use E as a reference point. Assume that the potential of 10V appears at point D. At this time, the potentials at points A and E are still 24V, then there may be a potential difference of 34V between A and D, which exceeds the safe limit potential. Bad, but the Zener tube will not be broken because the potential difference between A and E has not changed and therefore does not provide protection. If accidentally the signal line on the spot touches the casing, it may cause sparks and may ignite the surrounding flammable gas. Such a system will not have intrinsic safety performance. Therefore, when designing and implementing a grounding system involving safety barriers, it must be ensured that the potentials at points D and B(E) are approximately equal. In the specific practice, the following method can be used to solve this problem: Use a thicker wire to connect point D with point B to ensure that the potentials of point D and point B are relatively close. The other is to use a unified grounding grid to connect them to the grounding grid. In this way, if the grounding grid itself has very low resistance, and then a better connection method is used, the potentials at points D and B can be guaranteed to be approximately equal. Note, however, that this grounding must not conflict with the above grounding types.
Several grounding methods and considerations have been discussed above. In different systems, the requirements for these types of grounding are different, but most systems generally require a grounding resistance of 1 ohm or less for AGs, and the grounding resistance of safety barriers should be <4 ohms, preferably <1 ohm, PG and CG. The grounding resistance should be less than 4 ohms. Get up and receive a common grounding body.