A, the classification and function of grounding:
1, safety grounding:
(1) Protective grounding (CG, called PE in electrical specialty) is to form a good conductive connection between metal parts in DCS that should not be charged under normal circumstances (cabinet shell, console shell, junction box, conduit, cable rack and exposed metal parts that are not charged under normal circumstances but may be exposed to dangerous voltage, etc.). ) and the ground to protect equipment and personal safety. Because the power supply of DCS is a high-voltage power supply (generally 220V), the shell is uncharged under normal circumstances. When the high voltage part of the power supply is short-circuited with the conductive metal parts such as the shell, 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. If people accidentally touch these charged bodies, they will form a path in the crowd, which will lead to danger. 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.
(2) Lightning protection and grounding: When the signal line of DCS system enters the room from the outdoor, lightning protection and grounding should be carried out under the condition of introducing overhead lines or possibly suffering from induced lightning. Lightning discharge current can directly flow into the earth from the grounding path, and will not affect the normal work and personal safety of the system. The lightning protection and grounding of DCS system should be matched with the lightning protection and grounding system of electrical specialty, and not with independent lightning protection devices. We can use metal cable tubes or trunking to lay signal wires. A well-grounded cable tube or metal trough can effectively eliminate the huge pulse interference on the signal line caused by lightning near the signal cable, and the lightning is coupled to the signal line through distributed capacitance and inductance.
(3) Anti-static grounding: Anti-static grounding should be considered in the control room where DCS is installed. Electrostatic discharge current can directly flow into the earth from the grounding path without affecting the normal work and personal safety of the system. These indoor static conductive floors, movable floors, workstations, etc. It should be grounded against static electricity. Protective grounding and working grounding of instruments and equipment have been done, and anti-static grounding is not needed.
2. working grounding: working grounding is grounded to ensure the reliable operation of DCS and its connected instruments and ensure the accuracy of measurement and control. Working grounding is divided into logic grounding, signal loop grounding and shielding grounding.
(1) Logic ground, also called DC system ground (PG), is the negative terminal of the logic level in the computer, which requires a reference potential for current transmission and information conversion of electronic equipment at all levels. This "ground" is not necessarily a "geographical ground", but may be a metal shell, base, ground wire on the printed circuit board of electronic equipment, or a general grounding terminal or trunk line in a building; It provides a stable reference potential (generally zero) for all components and links of electronic equipment. This land can be connected with the earth or just a public point. If the system is not connected with the earth, it is in a suspended working state, which is called floating ground.
(2) Signal ground, also called signal loop ground, is the negative end of the field return signal, such as the negative end of each transmitter and the negative end of the switch signal.
(3) Shielding grounding, also called analog grounding (AG, signal cable shielding grounding). Electromagnetic shielding is one of the main measures of electromagnetic compatibility technology. That is, the measures to seal the electromagnetic interference source with metal shielding material to make its external electromagnetic field intensity lower than the allowable value; Or sealing the electromagnetic sensitive circuit with metal shielding material to make its internal electromagnetic field intensity lower than the allowable value. The metal shield is well grounded. For electrostatic shielding, the induced charge outside the shield will flow into the earth and will not produce induced electric field. For alternating electric field shielding, the coupling interference voltage of alternating electric field to sensitive circuit depends on the product of alternating electric field voltage, coupling capacitance and grounding resistance of metal shielding. As long as the metal shield is well grounded, the coupling interference voltage of the alternating electric field to the sensitive circuit can be made very small.
3. Intrinsically safe grounding refers to the grounding of intrinsically safe instruments or safety barriers. Mainly used in explosion-proof systems such as petrochemical industry. In addition to suppressing interference, this grounding is also one of the measures to make instruments and systems intrinsically safe.
4. In addition to the above three grounding methods, there is also a neutral grounding method. It is the grounding of power system (mainly transformer or generator) for normal operation and safety needs. The outgoing line of neutral grounding is called neutral line or zero line, which can be divided into protective zero line (PE line, commonly known as ground line), working zero line (N line, commonly known as zero line), or a combination of the two, called PEN line.
2. Grounding mode of DCS system: All kinds of safety grounding and working grounding of DCS system should be connected to the grounding wire of this cabinet respectively. After the corresponding grounding wires of each cabinet are connected, lead them to the general grounding plate with copper core cables. According to the different connection modes between the general grounding plate and the grounding electrode, there are the following grounding modes:
1. Equipotential grounding is adopted for grounding. When equipotential grounding is used, it is required that the metal structure, foundation reinforcement, metal equipment, pipeline, PE (protective grounding wire) bus of incoming distribution box and lightning arrester downlead of the building (or device) form equipotential connection. The safety grounding and working grounding of the control system should be classified and summarized into the general grounding plate to realize equipotential connection, and the grounding device and the electrical device are shared and grounded. However, the access point of the control system on the grounding network should keep a distance of not less than 5 meters from the access point of lightning protection, high current or high voltage equipment. Unless explicitly stated by the manufacturer, the cabinet of computer control system is not allowed to be directly connected with the steel bars of the building.
2. Use the electrical grounding grid as the DCS grounding grid, that is, with the electrical grounding grid. Under normal circumstances, the electrical grounding network is equipotentially connected; When the equipotential grounding cannot be met, the working grounding of the system is allowed to be grounded separately, and the protective grounding of the system is connected with the electrical grounding. In the case that system grounding and protection grounding cannot be separated, system protection grounding and working grounding can be separated. When sharing a grounding grid with the power system of a power plant, the connection between the grounding line of the control system and the electrical grounding grid must use low-voltage insulated power cables, and only one connection point is allowed, and the grounding resistance should be less than 0.5? . It should be noted that the access point should be as far away from the access point of the large motor as possible, and the distance from the lightning protection grounding access point should be more than 20m.
3. Set a dedicated and independent grounding grid for DCS system; When an independent grounding grid is used, it should be as far away from the lightning protection grounding grid as possible. Unless otherwise specified by the manufacturer, the grounding resistance (including the grounding lead resistance) shall not be greater than 2Ω. In addition, the distance between the independent grounding body and other electrical professional grounding bodies should be greater than 5m, and the distance between the independent grounding body and lightning protection grounding body should be greater than 20m. The specific form of one-point grounding can be selected under the following circumstances according to the site conditions. (The following is the connection of working grounding. Under normal circumstances, the protective grounding should be connected to the electrical grounding. If working grounding cannot be separated from protective grounding, protective grounding and working grounding can be connected to the same single grounding body. ).
4. Set a special grounding grid for DCS, and then connect it to the electrical grounding grid through the grounding wire.
3. Grounding requirements and principles: Because the grounding wire and grounding resistance of the system cannot be zero, and when a large current is injected into the earth from the grounding electrode, the ground potential of the grounding electrode and its vicinity will rise. If there are multiple points, there will be potential difference between grounding points, which will cause interference. Even all kinds of grounding wires in the same equipment should follow the principle of one-point grounding, otherwise a grounding loop will be formed, and the ground potential difference between each point will interfere and be introduced into other circuits.
Therefore, unless specified by the manufacturer, all kinds of grounding in the whole computer control system should be led to the general grounding plate through insulated cables or wires to ensure "one-point grounding". If the equipment spacing is large, one point in series should be used for grounding. All kinds of ground should be collected separately in the cabinet and finally collected at one point. Then connect the central grounding point of each equipment cabinet with a large section copper bus (or insulated cable), and then directly connect the grounding bus to the grounding electrode. The grounding wire shall be made of copper wire with a cross section greater than 22mm2, and the bus bar with a cross section greater than 60mm2 shall be used. The grounding resistance of grounding electrode is less than1Ω, and it is best to bury it at a distance of/kloc-0 ~15m from the building. The grounding point of DCS system must be more than 10m from the grounding point of high-voltage equipment. The sum of the resistances between the grounding terminal of the instrument or equipment and the grounding electrode and connection point is called the grounding connection resistance. The grounding connection resistance should not be greater than 1 ohm. The sum of grounding resistance and grounding connection resistance is called grounding resistance. Grounding resistance, should not be greater than 4 ohms. When the length of the grounding trunk line exceeds10m or there is strong magnetic field equipment around it, shielding measures should be taken to protect the grounding trunk line through steel pipes, which are connected into a whole. Or use shielded cable, and the shielding layer of steel pipe or shielded cable should be grounded at one end. When the grounding trunk line runs outdoors, when the distance exceeds 10 meter, double-layer shielding should be adopted, with the inner layer grounded at one point and the outer layer grounded at both ends to prevent electromagnetic pulse interference.
Different types of grounding lead to different grounding methods and requirements. The following is a statement of grounding methods and requirements according to different grounding types.
1. protective grounding: all DCS equipment has a protective grounding, which is generally connected internally during the design and processing of cabinets and other equipment. In some systems, the protective ground has been connected with the protective ground of the power supply incoming line (the middle head of the three-pole plug), and some are not allowed to be connected with the line. Users must carefully read the grounding installation instructions provided by the manufacturer, no matter which way. CG must be connected with CG of all peripheral devices or systems on the equipment (control station, operator station, etc.). ), then connect CG of each station with thick insulated copper wire, and finally connect to the earth grounding system at one point. In addition, it is worth reminding that all peripherals of DCS must be powered by a power supply line, and the power supply of a device (such as all peripherals connected to the operator station and the host system (CRT, printer, copier host system) must take power from the power distributor of the device, and it is not allowed to take power from other places, otherwise it may burn out the interface and even the device. For occasions where long lines must be connected, thick wires are used for power supply or communication isolation measures are taken. The CG of each station can be connected by radial connection or series connection. Field instruments with power supply voltage lower than 36V can be grounded without protection, except those that may be in contact with equipment with voltage higher than 36V. When the instruments installed on the metal instrument panel, box, cabinet and rack are in good electrical contact with the grounded metal instrument panel, box, cabinet and rack, protective grounding is not required. The protective grounding PE of electrical equipment shall be connected to the equipotential connection network (grid or star mode).
2. working grounding: The principle of working grounding is single point grounding, and the grounding circuit should be avoided in the signal circuit. If the signal source and receiving instrument on a line are inevitably grounded, it is necessary to isolate the two grounding points with isolators.
(1) Logically (PG). The logical ground in each station must be located at PG point, then connected to a point radially with thick insulated wires, and then connected to the earth ground wire. As long as the grounding is good, this method has strong anti-interference ability. However, the grounding process is complicated, and once the grounding is poor, it will cause unnecessary interference. In some systems, all inputs and outputs are isolated, so its interior is an independent unit logically and has no electrical connection with other parts. In this system, PG is often ungrounded, but it remains floating internally. The floating grounding mode is simple, but the insulation resistance to the ground is higher, generally greater than 50MΩ, otherwise the insulation will be reduced, leading to interference. In addition, floating is easy to cause electrostatic interference.
(2) Shielding ground (analog ground AG) is the most demanding of all grounding. The shielding layer of the whole line should have reliable electrical continuity. When shielded cables are separated or merged through junction box or intermediate terminal cabinet, the shielding layers at both ends shall be connected through terminals in the intermediate terminal cabinet of junction box. Only one grounding point is allowed in the same signal loop or the same line shielding layer, and the grounding resistance should be less than 65438 0 ohms, and the resistance between cabinets should also be less than 65438 0 ohms. The total shielding layer and twisted pair shielding layer of shielded cable, shielded conductor and shielded compensation conductor shall be grounded. The cable shield should be grounded at one end of the cabinet side. When the signal source is grounded, the shielding layer should be grounded on the signal side. When there is a connector in the middle of the signal line, the shielding layer should be firmly connected and insulated, and multi-point grounding must be avoided; When the shielded twisted pair of multiple measuring point signals is connected with the main shielded cable of multi-core twisted pair, the shielding layers should be connected and insulated.
(3) Handling of signal grounding: In principle, all sensors such as transmitters are not allowed to be grounded at the field end, but their negative ends should be grounded at the computer end. However, in some cases, the field terminal must be grounded. At this time, it must be noted that the input end of the original signal (upper double end) is absolutely not allowed to have any electrical connection with the grounding wire of the computer, and the computer must take effective isolation measures at the front end when processing such signals.
(4) Safety fence grounding: Intrinsic safety system with isolation safety fence is adopted without special grounding. Intrinsic safety system with Zener safety barrier shall be equipped with grounding connection system. The grounding of Zener safety barrier intrinsic safety system should not be separated from the grounding of instrument signal loop.
In different systems, the requirements for these types of grounding are different, but most systems generally require that the grounding resistance of AG is lower than 1 ohm, and the grounding resistance of safety barrier is better than 4 ohms.