【Daily Question 20090303】Which devices should be grounded for operational purposes?

First of all, what the landlord is asking about is working ground, not protective ground, right? The two of them must be different! PLCs, DCS, computer systems, etc., should be provided with a working ground. Work grounding includes: signal circuit grounding, shielding grounding, and intrinsically safe instrument system grounding!

The shield grounding of the shielded cables entering the DCS is of the type related to operational grounding, while the ground wire of the cabinet belongs to the operational grounding category； The ground wire of the enclosure should be connected to the protective ground. Finally, the protection grounding and the working grounding are connected together, and then a single ground wire is used to lead them to a single grounding point.

Working ground of the instrumentation system: The grounding of the instrumentation system is divided into protective grounding and working grounding. 1. Protective grounding: Automatic control devices that typically require grounding include instrument panels, instrument cabinets, instrument boxes, the cabinets and operation stations of DCS/PLC/EDS systems, instrument power supply equipment, cable trays, conduit pipes, junction boxes, as well as the shielding layer of armored cables; in addition, anti-static floors in control rooms also fall under this category. Generally speaking, field instruments, transmitters, etc. that use DC24V as power supply and have no special requirements do not need to be protected by grounding. Methods for protective grounding: On-site instrument trays and conduit pipes should be connected to grounded metal components using grounding wires every 30 meters. It should be specifically noted that on-site grounding must never be carried out using metal equipment and pipelines used for storing or transporting flammable media, as well as the metal components connected to them. The instrument automation equipment, cabinets, dashboards, etc. in the control room should be equipped with separate protective grounding busbars. Its grounding electrode can be shared with the grounding electrode of the power system. The identification color for the instrument protection grounding connection wire is green. II. Working Grounding Working grounding includes signal circuit grounding, shielding grounding, and intrinsically safe grounding. 1. Grounding of the signal circuit: In non-isolated signal systems, a unified signal reference point should be established. That is, ground the signal circuit. It is usually grounded to the negative pole of the DC power supply. Using a non-isolated signal system is generally my preferred approach in design. During operation, it is extremely rare for the system to be disrupted. In isolated signal systems, the isolated signals can be ungrounded. The isolation referred to here means that the circuit for each input/output signal is insulated from other input/output signals. Ensure independent power supplies, mutual isolation, and a floating reference point. I think this method should not be used lightly in systems with many loops. Signal and shield grounding busses should be provided within the control cabinet. The ground wire is identified by yellow/green wires. 2. Shield grounding: The shield layer of the cable and the shielding wires should be grounded for shielding purposes. In areas with severe lightning strikes, for ordinary multi-core cables installed outdoors without shielding, the spare cores should be shielded and grounded. It is mainly to prevent lightning from inducing high voltages in the signal lines. Inside the on-site junction box, the cable shielding wires on both sides of the terminals should be bridged within the box. For the same signal circuit, the same shielding layer should be grounded at a single point. Generally, the shielding ground should be connected to ground on the control room side. Signal and shield grounding busses should be provided within the control cabinet. The ground wire is identified by yellow/green wires. 3. Intrinsic safety grounding: The busbar of the zener safety barrier must be connected to the common terminal of the DC power supply (primarily to ensure protection of hazardous areas in the event of a power failure). Its bus bar or guide rail is grounded intrinsically safe. An intrinsically safe grounding busbar should be installed in the control cabinet. The ground wire is identified by blue/green wires. Method of working ground connection: The signal and shielding ground busses, as well as the intrinsically safe ground bus, are connected to the working ground bus through their respective grounding wires. Since the 1990s, various relevant regulations have specified that once the electrical engineering team has grounded all the metal supports, steel structures, metal pipes, roof frameworks, etc. of a building or installation, the working ground for instruments can share the same grounding system as that used by the electrical engineering team. This can reduce damage from lightning strikes and minimize interference. When the electrical discipline does not provide such a grounding connection, the instrument working ground should be grounded using a separate grounding electrode. The grounding electrode should be at a distance of not less than 5 meters from the electrical grounding electrode. The ground resistance should not exceed 4 ohms.

HG/T20513-2000 \"Specifications for Grounding Design of Instrument Systems\": The aspects of working grounding include grounding of signal circuits, shielding grounding, and grounding of intrinsically safe instruments.

Grounding of signal circuits, shielding grounding, and grounding of intrinsically safe instruments.

Grounding of signal circuits, shielding grounding, intrinsically safe instrument systems for PLCs, DCSs, computer systems, etc

All moving and stationary equipment in the factory must be grounded

Working ground connection should include: signal circuit grounding, shielding grounding, and intrinsically safe grounding.

Grounding of signal circuits in PLCs, DCS systems, computer systems, etc.; shielding grounding; intrinsically safe instrument systems. Automation equipment such as dashboards, instrument cabinets, instrument boxes, cabinets and operation stations for DCS/PLC/EDS systems; instrument power supply equipment; cable trays, conduit pipes, junction boxes, and the shielding layer of armored cables; as well as anti-static floors in control rooms

1 Basic requirements for grounding of DCS systems. The grounding of DCS systems is intended to ensure that, in the event of problems with 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 power plant DCS systems often experience \"crashes\" (or unexplained malfunctions), and this is mostly due to poor or faulty grounding systems. Therefore, a proper, reliable, and accurate grounding is key to the safe, reliable, and proper operation of a DCS system. 1.1 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. 　　1.1.1 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 shall be connected to the protective ground. The protective area should be connected to the plant’s electrical grounding grid, with a grounding resistance of less than 4Ω. 　　1.1.2 Logically: also known as machine ground or host power ground, it is the common ground for the negative logic levels inside a computer, as well as the output ground for power supplies 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. 　　1.1.3 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 shielding layer must be grounded at one end to prevent the formation of a closed circuit that could cause interference. The metal armor of armored cables should not be used as a shielding protection ground; it must be grounded through a copper wire mesh or an aluminum-plated shielding layer. Connected to a common ground electrode. 　　1.1.4 A separate grounding system shall be provided for intrinsically safe systems, with a grounding resistance of ≤4Ω. The ground system of the Intrinsic Safety area should remain independent, with a distance of at least 5 meters from the plant’s electrical grounding grid or the grounding grids of other instrumentation systems. 1.2 Grounding method of DCS system – General grounding methods for DCS systems 　　1.2.1 Using the electrical grounding grid as the DCS grounding grid, that is, sharing the same ground connection with the electrical grounding grid ； 　　1.2.2 A dedicated and independent grounding grid shall be provided for the DCS system ； 　　1.2.3 A dedicated DCS grounding grid shall be 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 large amount of space, involves high costs, consumes a lot of cable and steel for the grounding grid, needs to be placed at a considerable distance from the factory building (as it is difficult to find a suitable location within the building), and makes it inconvenient to manage, maintain, measure, and locate the grounding electrodes and wires; moreover, its effectiveness is not very good. Actual operation has shown that establishing a dedicated DCS grounding grid is both difficult and unsafe. For example, a power plant once experienced the unit tripping dozens of times due to grounding issues. According to investigations, many power plants later switched to using the electrical grounding grid for grounding, achieving good results. 1.3 Requirements for the common grounding electrode (network) 1.3.1 When the resistance of the electrical grounding network in the plant to ground is ≤ 4Ω, this electrical grounding network can be used as the common grounding electrode (network) for the DCS system. 　　1.3.2 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. 　　1.3.3 The earth distribution resistance of public grounding electrodes (grids) that are not connected in intrinsically safe manner is less than 4 ohms ； There is a low value of less than 1 ohm in milliamps. The line impedance of the main grounding conductor is less than 0.1 ohms. 　　1.3.4 There shall be no connection points for lightning protection grounds within 15 meters of the grounding electrode, 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. 2. Grounding principles for DCS systems 2.1 Grounding devices installed in DCS systems 2.2.1 Control panels, printing stations, server cabinets: Equipped with protective ground screws. 　　2.2.2 Relay cabinets, UPS cabinets, and power distribution cabinets: equipped with protective earth screws. 　　2.2.3 DCS I/O cabinet: equipped with a shielded grounding busbar and protective ground screws. Systematically suspended at (+24V ground). 　　2.2.4 Instrument cabinets and manual control panels: equipped with shielded earth grounding busbars and protective earth screws. 　　2.2.5 Safety barrier cabinet: equipped with a shielded ground busbar, an intrinsically safe ground busbar, and protection ground screws. 2.2 Signal Shielding and Grounding 　　2.2.1 In accordance with relevant technical specifications, the shielding layer of signal cables in computer or DCS systems must not be left ungrounded; it must be grounded, and the method of grounding shall comply with the following requirements: 　　2.2.1.1 When the signal source is ungrounded, the shielding layer should be grounded on the computer side ； 　　2.2.1.2 When the signal source is grounded, the shielding layer should be grounded on the side of the signal source ； 　　2.2.1.3 When the amplifier is floating, one end of the shielding layer should be connected to the shield, while the other end should preferably 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). 　　2.2.1.4 When a shielded cable is interrupted or combined at a junction box, the shield layers of the cables at both ends should be connected within the junction box. 　　2.2.2 The selection and installation of signal cables in DCS systems must be carried out in strict accordance with relevant regulations. The shielding layer of shielded cables shall be grounded in accordance with the above requirements. To improve the interference resistance of DCS systems, it is appropriate to use flame-retardant twisted copper mesh shielded computer cables for the digital input/output signals of DCS systems. 3. Grounding methods for DCS systems 3.1 Grounding methods for DCS devices arranged in a centralized manner 3.2 Grounding methods for DCS devices arranged in a decentralized manner. The connections between devices in a decentralized DCS system are generally network (communication) cables; for example, the field control stations are located at various sites on-site, while the operator stations are situated in different control rooms. The distance between these sites ranges up to 500 meters, and they are connected using multi-mode fiber, Category 5 twisted pair cables, or DP shielded twisted pair cables. 　　3.2.1 Sites connected via optical fiber: The grounding method within each site is the same as that of the centrally located DCS equipment. 　　3.2.2 Stations connected using Category 5 twisted pair or DP shielded twisted pair: 3.2.2.1 All types of ground wires in the control room are first connected to a common connection plate, which is in turn connected to the common grounding electrode via a grounding main line. Seen from the common ground electrode, the entire grounding network has a star topology. 　　3.2.2.2 Use Category 5 twisted pair or DP shielded twisted pair, with both ends connected to the DCS’S SWITCHES, HUBS, REPEATERS, or other network devices via network surge protection devices (signal lightning arresters with a current rating of not less than 5 KA). The stations on both sides have their own common grounding electrodes; there is no need for a metallic connection between them. The grounding method for each station is the same as that used for the DCS equipment installed in a centralized manner. Category 5 twisted pair or DP shielded twisted pair must be laid in galvanized steel pipes or metal trays, and the pipes or trays must be reliably grounded. When lightning strikes or electrical accidents cause an excessive potential difference between the two sides, signal surge protectors can protect the equipment on both sides. 3.3 Grounding Installation of DCS Equipment 　3.3.1 Ground electrode: A good conductor that is driven into the ground; the current coming from the main grounding wire is conducted into the earth through this ground electrode. Copper welding is used between the grounding electrode and the main grounding conductor, and anti-corrosion treatment should be applied after welding. Multiple grounding electrodes can be connected together to form a network using grounding grid main lines; the grounding grid must meet the requirements regarding the grounding resistance of the DCS system. When lap welding is used for the connection between the grounding grid main conductor and the grounding electrode, the lap length must be twice the width of the flat steel or six times the diameter of the round steel. Figure 3-2 shows a typical multi-grounding installation diagram. 3.4 Methods for reducing soil resistivity in DCS system grounding 　　3.4.1 Changing 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 cinders, or slag are mixed into the soil; this method can reduce the soil resistivity to 1/5 to 1/10 of its original value. 　　3.4.2 Reducing soil resistivity using table salt and charcoal: Compact the layers using table 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 it out, drive in the grounding electrode. This method can reduce the resistivity to 1/3 to 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. 　　3.4.3 Use long-acting chemical resistivity reducers. The use of long-acting chemical resistivity reducers can reduce soil resistivity to 40% of its original value. 3.5 Grounding materials and requirements for DCS systems 　3.5.1 Material requirements for grounding electrodes and main grounding wires 　The steel grades suitable for grounding electrodes and main grounding wires 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. 　　3.5.2 Requirements for grounding connections The protective and shielding grounds of the DCS system shall be connected to the plant’s dedicated electrical grounding grid or grounding electrodes using copper-core insulated wires or cables. The table lists the available specifications for various types of grounding cables. When the distance of the grounding connection is long, the DCS system has high requirements for grounding resistance, or there are many branches branching off from the main grounding conductor, it is advisable to use wires and cables with a larger cross-sectional area as listed in the table. 4. Common precautions for on-site grounding 　　4.1 On-site control station: Since rubber provides insulation between the cabinet itself and its base, as well as between the shielding ground busbar and the base, the on-site control station must be properly grounded in accordance with relevant 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 equal potential. 　　4.2 Field control station: The operator station, engineer station, network switches, server hosts, system monitors, etc. have their enclosures grounded, or the power ground wire is connected directly to the electrical grounding grid. 　　4.3 I/O Module: The 40th terminal of the analog module, which is the negative terminal of the 24-volt DC supply, 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. 　　4.4. The protective ground of the field control station shall be connected from the grounding screw under the cabinet to the grounding busbar, while the shielding ground of the field control station shall be connected from the grounding busbar to the common connection plate. 4.5 The resistance of the grounding system must be tested to ensure that the grounding meets the requirements of the control system manufacturer. Just for fun, posting a long post. Think it might be helpful to everyone

For reference: Working ground includes signal circuit grounding, shielding grounding, and intrinsically safe instrument grounding. Its purposes are to provide a potential reference point, suppress interference, and ensure explosion protection, among others.