Knowledge||DCS data fluctuations? The DCS grounding should be checked!

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I. Basic requirements for grounding of DCS systems. The grounding of DCS systems is intended to ensure that, in the event of problems with the signals entering the DCS system, the power supply, or the DCS system equipment itself, an effective grounding system can handle the overload current and quickly direct it into the ground. The grounding system can provide a shielding layer for the DCS, eliminate electronic noise interference, and offer a common signal reference point (i.e., a reference zero potential) for the entire control system. When there are problems with the grounding system (such as excessive grounding resistance, multiple grounding points, broken grounding wires, or contact between grounding wires and high-voltage or high-current equipment), it can cause electric shock to people and damage to equipment. It is known that some DCS systems often experience \"crashes\" (or unexplained malfunctions), and this is mostly due to poor or defective grounding systems. Therefore, a proper, reliable, and correct grounding is key to the safe, reliable, and proper operation of a DCS system. II. DCS Grounding Classification Under normal circumstances, a DCS control system requires two types of grounding: protective grounding and working grounding (logical grounding, shielding grounding, etc.). For systems equipped with safety barrier explosion protection measures, such as those used in the chemical industry, intrinsically safe design is also required. 2.1 Cabinet Grounding (CG) is a protective measure taken to prevent the accumulation of static charge on the equipment’s enclosure and avoid potential harm to people. All operator cabinets, field control station cabinets, printers, terminal cabinets, etc., in the DCS system must be connected to the protective ground. The protected area should be connected to the plant’s electrical grounding grid, with a grounding resistance of less than 4Ω. 2.2 Logic, also known as machine logic or host power ground, is the common ground for the negative logic levels inside a computer; it is also the ground point for power outputs such as +5V. Such as the negative terminals of 5 volts and 12 volts positive and negative for the CPU. It is necessary to connect to a common ground electrode. 2.3 Shielded ground (AG, Analog Grounding), also known as analog ground, can block the interference encountered during the transmission of field signals, thereby improving signal accuracy. The shielding layer of the signal cables in the DCS system should be grounded for shielding. The cable shield must be grounded at one end to prevent the formation of a closed loop that could cause interference. The metal armor of armored cables shall not be used as a grounding conductor for shielding protection; instead, a copper mesh or aluminum-coated shielding layer must be grounded. Connected to a common ground electrode. 2.4 A separate grounding system shall be provided for intrinsically safe systems, with a grounding resistance of ≤4Ω. The ground system of the Inert Gas system should remain independent, with a distance of at least 5 meters from the plant’s electrical ground grid or the ground grids of other instrumentation systems. III. Grounding method of DCS system: 1. Use the electrical grounding grid as the DCS grounding grid, that is, share the same ground with the electrical grounding grid ； 2. Set up a dedicated independent grounding network for the DCS system ； 3. A dedicated DCS grounding grid is provided, which is connected to the electrical grounding grid via grounding wires. Since the third grounding method shares many similarities with the second, dedicated grounding grids were previously widely used in computer or DCS systems. However, the disadvantages of this grounding method are: it requires a very large area of land, involves high investment, consumes a great amount of steel for cables and grounding grids; it is located at a considerable distance from the plant building (as it’s difficult to find a suitable location within the building); it is inconvenient to manage, maintain, measure, and locate the grounding electrodes and grounding wires; and its effectiveness is not particularly good. Actual operation has shown that establishing a dedicated DCS grounding grid is both difficult and unsafe. IV. Requirements for the common grounding electrode (network) 1. When the resistance of the electrical grounding network in the plant area to ground is ≤ 4Ω, this electrical grounding network can be used as the common grounding electrode (network) for the DCS system. 2 When the grounding resistance of the electrical grounding grid in the plant area is high or chaotic, a separate grounding system should be established, namely the common grounding electrode (grid) for the DCS system. 3 The earth distribution resistance with respect to the ground of public grounding electrodes (grids) that are not connected in intrinsically safe manner is less than 4Ω ； There is a intrinsically safe value of less than 1Ω. The line impedance of the main grounding conductor is less than 0.1Ω. There are no connection points for lightning protection systems within 15 meters of the 4 grounding electrodes, and no connection points for the enclosures of high- or low-voltage electrical equipment with a power rating of over 30 KW within 8 meters. When the site cannot meet this condition, the lightning protection system is connected to the main line of the common grounding electrode through a lightning arrester/shock wave suppressor. The welding electrode must not be connected to the public grounding electrode or its grounding grid; there should be a distance of more than 10 meters between them. V. Grounding Principles of the DCS System 1. Grounding devices installed in the DCS system 1.1 Control panels, printing stations, and server cabinets: equipped with protective ground screws. 1.2 Relay cabinets, UPS cabinets, and power distribution cabinets: equipped with protective earth screws. 1.3 I/O cabinets of the DCS: Equipped with a shielded grounding busbar, protective grounding screws, and floating system ground (+24V ground). 1.4 Instrument cabinets and manual control panels: equipped with shielded earth grounding busbars and protective earth screws. 1.5 Safety barrier cabinet: equipped with a shielded ground busbar, an intrinsically safe ground busbar, and protective ground screws. 2. Signal shielding and grounding: In accordance with relevant technical regulations, the shield of signal cables used in computers or DCS systems must not be left floating; it must be grounded. The grounding method shall comply with the following provisions: 2.1 When the signal source is floating, the shield shall be grounded on the computer side ； 2.2 When the signal source is grounded, the shielding layer should be grounded on the side of the signal source ； 2.3 When the amplifier is floating, one end of the shielding layer should be connected to the shield, while the other end should be connected to the common-mode ground (or to the signal ground when the signal source is grounded). Connect to the site ground when the signal source is floating) ； 2.4 When a shielded cable is interrupted or combined at a junction box, the shielding layers of the cables at both ends should be connected inside the junction box. VI. Methods to reduce soil resistivity for DCS system grounding: 1. Modify the soil structure around the grounding electrode. Within a range of 2 to 3 meters around the grounding electrode, water-impermeable materials with good water absorption properties such as charcoal, coke cinder, or slag are mixed into the soil; this method can reduce the soil resistivity to 1/5 to 1/10 of its original value. 2 Reduce soil resistivity using salt and charcoal; compact them in layers using salt and charcoal. Charcoal and fine material are mixed together to form a layer about 10–15 cm thick; then 2–3 cm of table salt is added on top, for a total of 5–8 layers. After laying, drive in the grounding electrode. This method can reduce the resistivity to 1/3–1/5 of its original value. However, table salt gradually loses itself through running water, and it generally needs to be replenished every two years or so. 3 Use long-acting chemical resistivity reducers. The use of long-acting chemical resistivity reducers can reduce soil resistivity to 40% of its original value. VII. Grounding materials and requirements for DCS systems 1. Material requirements for grounding electrodes and the main lines of the grounding network: The steel grades suitable for grounding electrodes and the main lines of the grounding network can be selected as indicated in the table below; if the required grounding resistance cannot be achieved, copper materials can also be used. If the grounding electrodes and the main conductors of the grounding grid are installed in areas with high corrosion, anti-corrosion measures such as hot-dip galvanizing or hot-dip tin plating should be adopted according to the nature of the corrosion, or the cross-sectional area should be increased appropriately. 2. Grounding connection requirements: The protective ground and shielding ground of the DCS system shall be connected to the plant’s dedicated electrical grounding grid or grounding electrode using copper-core insulated wires or cables. When the length of the grounding conductor is relatively long, when the DCS system has high requirements for grounding resistance, or when there are many branch circuits branching off from the main grounding conductor, it is advisable to select wires and cables with a larger cross-section as listed in the table. VIII. Common precautions for on-site grounding 1. Due to the rubber layer between the cabinet body and the base, which provides insulation, as well as the insulation between the shielding ground busbar and the base, the on-site control station must be properly grounded in accordance with regulations. That is, they are connected to the grounding busbar at the field control station respectively. The power ground of the I/O cabinet and the power ground of the UPS must be connected to the same ground to ensure equipotentiality. 2. The operator stations, engineer stations, network switches, server hosts, system monitors, and other hardware devices are either equipped with enclosures for grounding, or their power ground wires are directly connected to the electrical grounding grid. 3. The negative terminal of the 40-pin analog module of the I/O components, which is the 24-volt DC negative terminal, is connected to the logic ground bus; this logic ground bus is then connected to the shielded ground, and from there to the main grounding bus. 4. Shield grounding: The protective ground of the field control station should be connected from the grounding screw beneath the cabinet to the ground branch line; the shield grounding of the field control station should be connected from the grounding busbar to the common connection plate. 5. Ground resistance testing: The resistance of the grounding system must be tested to ensure that it meets the requirements set by the controller manufacturer. IX. Construction specifications for grounding: 1. The normally non-electrified metal parts of electrical instruments—such as enclosures, panels, cabinets, boxes, cable trays, protective conduits, supports, and bases—that may become energized at hazardous voltages due to insulation failure must be provided with protective grounding. For local instruments, switches, etc. with a supply voltage not exceeding 36V, protective grounding may be omitted if there are no special requirements in the design documents. 2 The metal enclosures of small low-voltage electrical devices such as buttons, signal lights, and relays installed on metal panels in areas where there is no explosion hazard can be exempt from protective grounding, provided that they are in good contact with grounded metal panels. 3 The protective grounding system for instruments shall be connected to the protective grounding network for low-voltage electrical equipment in the electrical engineering works. The connection must be firm and reliable; series grounding is not permitted. 4 The resistance value of the protective grounding connection shall comply with the specifications in the design documents. 5 The cable trays and cable protection pipes installed on buildings can be grounded repeatedly. 6 The instrumentation and control systems shall be provided with a working ground connection. This includes grounding of the signal circuits and shielding, as well as grounding of intrinsically safe circuits when such requirements exist. The connection method of the grounding system and the value of the grounding resistance shall comply with the provisions specified in the design documents. 7 The grounding of the signal circuits of the instrumentation and control systems, as well as the shielding grounding, shall share the same grounding device. 8 Each instrument circuit should have only one signal circuit ground point, unless an isolator is used to isolate the DC signal circuit between the two ground points. The grounding point of the 9-signal circuit should be on the side of the display instrument; when ground-type thermocouples are used and the sensing elements are already grounded, there is no need to ground it again on the side of the display instrument. 10 The shielding layer of instrument cables and wires should be grounded on the side of the instrument panel cabinets in the control room. The shielding layers of circuits belonging to the same circuit must maintain reliable electrical continuity; they should not be left ungrounded or grounded multiple times, nor should they be grounded at the display instruments. The shielding layer of the cable at the field instrument end must not protrude outside the protective layer. 11 When anti-interference requirements apply, the spare conductors in multi-core cables should be grounded at one point; the spare conductors of shielded cables and the cable shielding layer should be grounded on the same side. 12. The various types of grounding for the different circuits within dashboards, cabinets, and boxes should be connected to the grounding busbars or grounding terminal boards via their respective grounding branches. From these busbars or terminal boards, the grounding main wires are led out, which in turn connect to the main grounding wire and the grounding electrodes. The various grounding branches, busbars, or grounding terminal plates shall be insulated from one another at all points where they are not connected. 13 The connections in the grounding system should use copper-core insulated wires or cables, secured with galvanized bolts. The grounding busbars inside instrument panels, cabinets, and boxes should be made of copper and fixed with insulated brackets. Welding should be used between the main grounding conductor and the grounding electrode. 14 Inherently safe circuits shall not be grounded, unless otherwise specified in the design documents. When a diode safety barrier is used, its ground should be connected to the common terminal of the DC power supply. 15 The anti-static grounding shall comply with the provisions of the design documents, and can be carried out simultaneously with the anti-static works for equipment, pipelines, and electrical systems. 16 The grounding conductors shall be made of insulated copper wires or cables with multi-strand stranded cores, connected using galvanized or copper bolts. The grounding busbars shall be made of copper and fixed by insulated brackets. The cross-sectional area of the grounding wires shall comply with the requirements specified in the design documents and by the manufacturer. When no such requirements are stipulated in the design documents or by the manufacturer, the cross-sectional area of grounding connecting wires shall be greater than 2 mm²; that of grounding branch wires shall be greater than 6 mm²; that of grounding main wires shall be greater than 20 mm²; and that of the main grounding conductor shall be greater than 30 mm². 18 Welding shall be used between the main earthing conductor and the earthing electrode. Buried grounding main conductors can be welded using hot-dip galvanized flat steel or round steel with the same effective cross-sectional area. The joints can be made by lapping; for flat steel, the lapping length should be 2 times its width, and for round steel, it should be 6 times its diameter. Good contact is necessary, the connections must be secure, and the welded areas should be treated to prevent corrosion. 19 The grounding resistance value shall comply with the provisions in the design documents and those set by the manufacturer. If no such provisions are given in the design documents or by the manufacturer, the following rules shall apply: a) The protective grounding resistance value for instrument systems is generally 4Ω, with a maximum value not exceeding 10Ω. When a high-sensitivity automatic grounding alarm device or an automatic grounding cut-off device is installed, the grounding resistance value can be greater than 10Ω, but it should remain within 100Ω. When a lightning protection system is in place, the grounding resistance should not exceed 1Ω ； b. The shielding ground and working ground should be within 10Ω ； c. The grounding resistance of the intrinsically safe circuits and that of other instruments shall meet the requirements specified in the product’s technical documentation ； d. The grounding resistance of DCS, PLC, and PCS systems should be less than 4Ω ； e. The distance between grounding plates should be greater than 5 m. 20 During the construction of the grounding system, records of the concealed works should be kept in a timely manner