Various grounding issues in DCS

Connecting circuits, units, and equipotential points or planes that serve as a common reference for signal potentials in a low-impedance manner is referred to as grounding. The meaning of grounding can be understood as an equipotential point or equipotential surface, which serves as the reference potential for a circuit or system, but it is not necessarily the potential of the earth. The protected area must be at the earth potential. According to the design requirements, the signal can be at earth potential or not. The shield ground is designed to prevent electrostatic induction and magnetic field induction. Depending on the purpose of shielding, the wiring of the shield ground varies, but it is generally better to connect it to the ground. Generally, high-frequency circuits should be grounded at multiple points nearby, while low-frequency circuits should be grounded at a single point. The communication ground and the signal ground cannot be shared.

Grounding Preparation 2001-7-3 (l) The function of grounding Generally speaking, there are only two functions of grounding: to protect people and equipment from damage； Suppress interference ； The interference suppression grounding is also referred to as the working grounding in some books, while the former is called the protective grounding. ①Protective grounding: Protective grounding involves establishing a good electrical connection between the metal parts of the DCS that are not charged under normal conditions (such as the cabinet enclosure, control panel enclosure, etc.) and the ground, in order to protect both the equipment and human safety. The reason is that the DCS is powered by high-voltage electricity (220V or 110V); under normal circumstances, the casing and other components are not electrified. When a fault occurs (such as a problem with the main power supply or some other issue) that causes a short circuit between the live wire supplying power and conductive metal parts like the casing, these metal parts or the casing become electrified. If there is no proper grounding, a high voltage difference is created between these electrified parts and the ground. If a person accidentally touches these electrified parts, an electric current can flow through their body, posing a danger. Therefore, a good connection must be established between the metal enclosure and ground to make the enclosure and ground at the same potential. Furthermore, protective grounding can also prevent the accumulation of static electricity. ②Working ground: The working ground is provided to ensure the reliable operation of the DCS and the instruments connected to it, as well as to maintain accuracy in measurements and control. It is divided into machine logic grounding, signal circuit grounding, and shielding grounding; in petrochemical and other explosion-proof systems, there is also intrinsically safe grounding. •The machine ground, also known as the host power ground, is the common ground for the negative logic levels inside a computer; it is also the output point for power supplies such as +5V. •The signal circuits are grounded, such as the negative terminals of various transmitters being grounded, and the negative terminals of digital signals being grounded as well. •Shield grounding (grounding of the shield layer of the signal). •Intrinsic safety grounding refers to the grounding of intrinsic safety instruments or safety barriers. In addition to suppressing interference, this grounding is also one of the measures that give instruments and systems intrinsically safe properties. The intrinsically safe grounding varies depending on the specific safety measures adopted by the equipment. Below, a Zener-type safety barrier is used as an example to illustrate the principles of its grounding, as shown in Figure 3.413: this figure represents the schematic diagram of the grounding for a Zener-type safety barrier. 　　The function of the safety barrier is to ensure that the hazardous area remains within the range of safe power supply and safe voltage levels at all times. If the field side is short-circuited, the current in the wires will be limited to a safe level due to the current-limiting effect of the load resistance and the safety gate resistor R, preventing the field side from reaching high temperatures that could lead to combustion. In the second scenario, if a fault occurs on the computer side, a high-voltage signal is introduced into the signal circuit; thanks to the clamping action of the Zener diode, the voltage remains within a safe range. 　　It is worth noting that the introduction of Zener safety barriers increases the resistance in the signal circuit significantly. Therefore, when designing the load capacity of the output circuit, in addition to considering the actual load requirements, it is necessary to take into account the resistance of the safety barriers as well and leave some margin. 　　In addition to the aforementioned types of grounding, another grounding system that often causes confusion in many situations is the power supply ground, also known as the AC power supply working ground. It is a type of grounding established within electrical systems for operational purposes (such as neutral point grounding). (l) Grounding requirements and methods: Six types of grounding have been described above: power supply system ground, protection ground, logic ground, shielding ground, safety barrier ground, and signal circuit ground. Regarding these six types of grounding, different organizations have their own requirements. Although most of them emphasize the need for one type of grounding, with the resistance value to be less than 1 ohm, there are significant differences in the specific details. Below are a few examples illustrating the common grounding requirements and methods. ①Power supply system ground: In many enterprises, especially power plants and smelting plants, there is a large grounding network within the plant premises, and usually the ground of the power supply system is connected to this grounding network. Some manufacturers emphasize that all groundings of the computer system must be strictly separated from the grounding of the power supply system as well as from other groundings (such as those for lightning protection), with a distance of at least 15 meters between them. To completely prevent the influence of the power supply system, it is recommended to isolate the power lines using isolation transformers. This is something that units with high electricity loads and those whose loads start and stop frequently should keep in mind. From the perspective of suppressing interference, it is beneficial to separate the ground of the power system from that of the computer system, as the ground wire in a power system is generally not very clean. But from an engineering perspective, in some cases it is difficult to design a computer system such that it is kept at a certain distance from the power supply system. In such situations, it is possible to consider using the same ground for both the computer system and the power supply system; several factors need to be taken into account: • Whether there is significant interference from the power supply system’s ground, such as whether high-current devices are turned on and off frequently, which could cause considerable interference to the ground ； •Is the grounding resistance of the power supply system low enough, and are the potential differences between various parts of the grounding network minimal? In other words, are the resistance values between different parts of the grounding network small (＜1Ω)? What is the interference resistance of the DCS, as well as that of the transmission signals used? For example, is there any direct transmission of weak signals (such as thermocouples or thermal resistors)? ②All computer wiring uses a single-point grounding scheme, and there is also much controversy regarding this approach. Some manufacturers’ systems propose using several types of grounds: logical ground, shield ground (also known as analog ground), signal ground, and protection ground, with each of these being connected to the ground through separate grounding devices. However, most systems require that these various grounds be connected separately inside the cabinet, at a single point, after which thicker conductors (copper) are used to link these connection points together and connect them to a common grounding electrode. There are a few points to note here: DCS itself is composed of multiple devices; in addition to the control station, it also includes many peripherals, and there are more than one data source as well. This leads to issues related to grounding for multiple devices. Furthermore, in conventional DCS systems, each station (control station, operation station, etc.) is powered separately by a dedicated line, meaning they do not supply power to one another. Figure 3.4.14 is a commonly used multi-station connection diagram. 　　Protective ground: All devices in the DCS have a protective ground connection, which is usually installed internally during the design and manufacturing of the cabinets and other equipment. In some systems, this protective ground connection is linked internally to the protective ground of the power supply cable (the middle pin of the three-pin plug); in other systems, it is not allowed to connect the protective ground to this cable. Users must carefully read the grounding installation instructions provided by the manufacturer. In either case, the CG connections of all peripherals or systems on a single device (such as a control station or operator station) must be connected together, and then the CG connections of all these stations should be linked using thicker insulated copper wires, before being connected to the earth grounding system at one point. It is also worth noting that all peripherals of the DCS must be powered from a single power supply line. The power for a device – such as all the peripherals connected to the operator station and the main system components (CRT, printer, copier) – must come from that device’s power distributor; it is not allowed to draw power from other sources, as this could damage the interfaces or even the devices themselves. In cases where long cables are necessary, it is necessary to use thicker wires for power supply or to implement communication isolation measures. 　　The CGs of various stations can be connected using either a radial connection method or a serial connection method. The power supply logic (P) is shown in Figure 3.4.14. First, the logic within each station must be located at a single point PG; then, the thick insulated wires are connected radially to that point and subsequently to the ground wire. In some systems, all inputs and outputs are isolated, making the internal logic an independent unit with no electrical connections to other parts. In such systems, it is often not necessary to ground the PG; instead, the internal circuit remains floating. Therefore, when designing and installing grounding systems, users must carefully read the technical specifications and grounding requirements of the products. •Simulated ground (AG): Simulated ground (also known as shielded ground) is the type of ground that requires the highest standards among all types of grounds. Almost all systems require AG to be grounded at one point, with the grounding resistance being less than IQ. In the design and manufacturing of DCS systems, AG busbars or other devices are installed inside the cabinets. During wiring, the users connect the shield wires to the AG busbars; at the bottom of the cabinet, these wires are connected together using insulated copper strands. Then, the connection points from each cabinet are linked to the ground point in a radial pattern, also using insulated copper strands or copper bars. Most DCS requirements stipulate that not only must the AG resistance to ground for each cabinet be < 1 ohm, but the resistance between cabinets as well must be < 1 ohm. •Processing of the signal ground: In principle, it is not allowed for individual transmitters and other sensors to be grounded at the field side; instead, their negative terminals should all be grounded at the computer terminal. However, in some cases, the field side must be grounded. In such situations, it is essential to ensure that the input terminals of the original signal (the upper terminals) have no electrical connection whatsoever with the computer’s grounding wire; moreover, when processing such signals, the computer must employ effective isolation measures at the front end. •Grounding of the safety barrier: Let’s take another look at the safety barrier circuit diagram shown in Figure 3.4.13. As can be seen from the diagram, there are three grounding points: B, E, and D; usually, both B and E are on the computer side. They can be connected together to form a single point of grounding. Point D represents the grounding of the transmitter housing at the site; if there is a potential difference between the grounding points at the site and in the control room, then the potentials at points D and E will be different. Assuming we use E as the reference point, and suppose a potential of 10V appears at point D, then the potentials at points A and E remain at 24V. In this case, there could be a potential difference of 34V between A and D, which exceeds the safe limit for potential differences. However, the Zener diode will not break down, as the potential difference between A and E remains unchanged, and thus it cannot provide any protection. If, at this point, the signal wires in the area accidentally come into contact with the casing, sparks may be generated, which could ignite any flammable gases present. In such a case, the system loses its intrinsically safe properties. Therefore, when designing and implementing the grounding system for safety barriers, it is essential to ensure that the potentials at points D and B (E) are approximately equal. In practical applications, this problem can be solved by using a thicker wire to connect point D to point B, thereby ensuring that the potentials at points D and B are relatively close to each other. Another approach is to use a unified grounding grid to connect them to this grid; in this way, if the resistance of the grounding grid itself is low, and good connections are used, it is still possible to ensure that the potentials at points D and B are approximately equal. But note that this grounding must not conflict with the aforementioned types of grounding. 　　The above discusses several grounding methods and precautions. In different systems, the requirements for these various grounding configurations vary, but in most systems, the grounding resistance of AG is required to be below 1 ohm, while the grounding resistance of safety barriers should be < 4 ohms; ideally, even less

The entire plant uses equipotential grounding; it does not accept separate grounding from the DCS system, and the grounding resistance must be no less than 10 ohms. Question: Most components of the DCS system are grounded separately, with a grounding resistance of less than 4 ohms. How can the issue mentioned above be explained? I would appreciate your insights

Well, I don’t understand it either: Q. I came to know about these different types of grounding during the ITCC training last time, but I can’t figure out the differences between them. I hope I can get an answer too.

If there is a unified grounding grid for the entire plant, the DCS system can be connected to this grounding grid; if not, it is recommended to provide a separate grounding.

1. Protective grounding: The metal enclosures of electrical instruments and the metal parts of automatic control equipment that are not normally energized must be provided with protective grounding. This includes A) instrument panels, control consoles, instrument cabinets, instrument racks, and instrument boxes; B) BDCS/PLC/ESD cabinets and operation stations; C) computer system cabinets; D) power distribution panels, power supply boxes, cable trays, conduit systems, and junction boxes. Any area that has been given protective grounding can be considered to have static grounding as well. When anti-static flooring is used in control rooms, static grounding must also be provided. 2. Working grounding includes grounding of signal circuits (usually the negative pole of the DC power supply). 3. Shielding grounding refers to the shielding layers of cables. 4. Grounding for intrinsically safe instruments involves using the busbars of zener safety barriers as the grounding point for such instruments

Single-ended grounding is sufficient; modern DCS systems all use a three-ground design

General working ground and protective ground