Electrical AC: [Weekly Question] Week 19 of 2011 – How is the ground resistance value determined?

5 Grounding connection methods and requirements for grounding resistance　　5.0. The protective grounding of electronic equipment such as instruments, PLCs, DCSs, and computer systems shall be connected to the grounding network of the plant’s electrical system, with a grounding resistance of less than 4 Ω. 　　5.0.2 The working grounding of electronic devices such as instruments, PLCs, DCS systems, and computer systems (signal circuit grounding and shielding grounding) can be carried out in one of the following two ways: 5.0.2.1 When the grounding resistance value of the plant’s electrical system grounding grid is less than 4Ω, and it meets the requirements of the instrument system, with no special requirements from the instrument manufacturer, these devices can be directly connected to the plant’s electrical system grounding grid ； 　　5.0.2.2 When the grounding resistance of the grounding grid in the plant’s electrical system is high, or when the instrument manufacturer has specific requirements, an independent instrument grounding system should be established, with a grounding resistance of less than 4Ω (or as specified by the instrument manufacturer). 　　5.0.3 Under normal circumstances, instrument circuits and systems should have only one grounding point for the signal circuit. When using a transformer-coupled isolator or an optocoupled isolator, signal circuit ground points can also be provided on each side of the isolator. 　　5.0.4 The shielding layer of the wires used for transmitting signals shall be grounded at the grounding terminals or grounding busbars of the instrument panel (cabinet); it shall not be left ungrounded or grounded multiple times. 　　5.0.5 The grounding system for the Zener-type safety barriers in intrinsically safe instrumented systems should be installed separately, with a grounding resistance of less than 1Ω. The grounding electrodes of intrinsically safe instrument systems should remain independent; the distance between them and the grounding grid of the plant’s electrical system or that of other instrument systems should be no less than 5.0 m. 　　5.0.6 Lightning protection (surge protectors) for the instrument system on the control room side shall be grounded; if the grounding resistance value of the existing instrument system is 1Ω or less, it is possible to share the grounding electrode with the protective grounding and working grounding of the instruments ； Otherwise, a separate grounding system should be installed to ensure that the grounding resistance value for lightning protection (surge protectors) of the instrumentation system is no greater than 1Ω. 　　5.0.7 For the grounding of lightning protection (surge protection) devices for field transmitters, this can be achieved by connecting the instrument itself to a grounded metal cable conduit or similar method.

Agree with the opinion above! It should be added that flexible configuration is possible based on the various grounding methods of the electrical system, as well as the local soil conditions

Reply 1# czywqa 1. Correctly understanding the meaning of grounding 1.1 Definition of “ground” “Ground” is a potential reference point in electrical engineering (often used as zero potential). Electrical engineering includes power engineering and electronic engineering. ““The ground” can refer to the Earth, and the scale of the “point” is that of a three-dimensional Earth ； ““Ground” can also be a certain point in a circuit; its size is that of a finite conductive surface, line, or point. The potential reference point is the benchmark for voltage; it can be a certain point in the power system, such as the neutral point of a transformer ； It can also be the positive and negative poles of a DC power supply, or some point in between. 1.2 The function of grounding Grounding is generally divided into system grounding and protective grounding. Some are divided into functional grounding and protective grounding. Protection is also a function, so the previous classification is more accurate. System grounding is implemented to ensure the stable operation of the system; examples include grounding the neutral point of transformers and providing a common potential reference point for AC signals. Protective grounding includes power supply ground fault protection, electrostatic grounding, shielding grounding, lightning protection grounding, etc. There are also cases where grounding serves both of these purposes. Grounding is an essential measure in electrical engineering. 1.3 Grounding methods: Grounding can involve direct connection to the earth, such as lightning protection down conductors and grounding of the transformer neutral point ； Some are connected to the ground via impedance components, such as resistors, reactors, or arc-suppression coils. In other cases, the “ground” is not connected to the earth at all; for example, the signal grounds of certain electronic devices (primarily for analog signals) often remain unconnected to the earth. In such cases, it is referred to as a floating ground. 2. Ground resistance is an engineering term; the claim that it is \"unmeasurable\" is incorrect. 2.1 The value of ground resistance serves as a basic quantitative indicator of the performance of a grounding system. Grounding is an important technical measure in electrical engineering, and it is necessary to test grounding systems. Ground resistance is the most commonly used and primary basic quantitative indicator of the performance of a grounding system. Using ground impedance is more accurate in non-DC systems, but its measurement is rather difficult. 2.2 Definition of ground resistance There are various definitions for ground resistance. For example: The grounding resistance is the sum of the resistance of the grounding electrode to the ground and the resistance of the grounding wires; numerically, it equals the ratio of the voltage across the grounding device to the current flowing into the ground through the grounding electrode (Cihai) ； The power frequency grounding resistance is the resistance between the grounding electrode and the distant ground when power frequency current flows through the grounding system. Its value equals the ratio of the voltage of the grounding system with respect to the distant ground to the current flowing into the ground through the grounding electrode (GB/T19663-2005) ； Ohm’s law between the grounding electrode and a distant grounding electrode at zero potential ； When a grounding electrode is subjected to a grounding current I, its potential rises from its original value to U. The ratio U/I is regarded as the grounding resistance of the electrode (Japan). The conventional grounding impedance is defined as the ratio of the peak grounding voltage to the peak grounding current; however, these two values rarely occur simultaneously. The voltage at the grounding terminal represents the potential difference between the grounding system and the distant earth (IEC 62305-3), etc. From a purely physical perspective, none of the above definitions are rigorous. The definitions are all related to “far away”. Where is the distance? Moreover, the value defined should be the ground impedance, not just resistance. Nevertheless, the term “ground resistance” has been used around the world for over 100 years and is still in use. The reason is that ground resistance is not a purely physical concept; it is an engineering term. 2.3 Ground resistance falls under the category of engineering terminology. In many situations within electrical engineering, the effects of grounding inductance and capacitance can be disregarded, with ground resistance being used as a substitute for grounding impedance. Generally, at a distance of 20–40 meters from the grounding electrode, this point can be regarded as a remote zero-potential point; the resulting calculation error remains within acceptable limits for engineering purposes. For example, ground resistance is often used in the calculation of power supply ground fault protection, and this has been proven in practice. The measurement of power frequency grounding resistance is convenient ； Within the main spectrum of lightning discharge, the term impulse resistance is used, and it can also be approximately calculated from the power-frequency grounding resistance. The imprecision in the definition of earth resistance is acceptable in engineering. It is indisputable that the term “ground resistance” is widely used in electrical engineering. 2.4 The “unmeasurability” of grounding resistance is detrimental to engineering construction. In engineering practice, some problems have arisen regarding the measurement of grounding resistance. First, using different types of instruments yields varying test results for the same location ； Second, the results obtained by the same instrument when measuring in different directions vary. The possible reasons for the first scenario are: whether all the instruments are certified; and whether the operations were performed correctly ； Differences in the matching between the design of measurement principles for different instruments and the characteristics of the objects being measured ； Different instruments have varying degrees of sensitivity to stray currents in the soil ； After the first measurement, reversible or short-term irreversible changes occurred in the soil’s chemical and physical properties, and the soil conditions had already changed by the time another instrument was used for measurement. The reasons for the second scenario may be that the current and voltage terminals of the instrument are affected in different ways by the layout of underground pipelines ； The physical and chemical structures of the soil may not be exactly the same in all directions ； The effects of each measurement on the soil chemical structure also vary, among other things. Differences that occur during measurement can be addressed through instrument calibration and proper statistical methods. Ground resistance can be measured, and its accuracy meets engineering requirements. This has been verified by countless engineering practices at home and abroad. The “unmeasurability theory” of grounding resistance can lead to confusion and helplessness, as well as a loss of confidence in identifying the causes; this is detrimental to construction projects. The \"unmeasurability\" of ground resistance should be abandoned. 3. The determination of the grounding resistance value must be well-founded and consider economic efficiency. The quantitative requirements for the grounding resistance value should be based on quantitative calculation formulas. 3.1 The value of the grounding resistance is closely related to the grounding current; the grounding impedance depends on the magnitude and frequency of the grounding current. At lower frequencies, resistance is the main component of the impedance. 3.2 Ground resistance of power frequency power supply systems
The ground resistance R of low-voltage distribution systems depends on the grounding current of the power supply. It should be such that the touch voltage generated on the exposed conductive parts of equipment due to the grounding current remains less than 50 V (in normal environments). When the inductive reactance is ignored in the TN system, R must be satisfied≤

In items 2# and 4#, no mention is made of the grounding of equipment and facilities; this type of grounding resistance falls under the category of static electricity grounding for flammable and explosive environments. In accordance with Article 6.10.2 of GB50028-93 \"Code for Design of Urban Gas Systems\", the impulse grounding resistance of the lightning protection grounding system shall be less than 10Ω. Article 6.10.3: The grounding resistance of the static electricity grounding electrode shall be less than 100Ω. Article 7.2.31: When a building is located outside a lightning protection zone, the lead of the vent pipe shall be grounded, with the grounding resistance to be less than 10Ω. The common terms are as follows: 1) Ground 　　The earth or conductive material whose potential is conventionally taken as zero at any given point. 　　2) Grounding　Electrical connection to an object capable of supplying or receiving a large amount of electric charge (such as the ground, a ship, or the metal hull of a vehicle, etc.). 　　3) (Electrostatic) Grounding A measure to electrically connect a metal conductor to the ground (through a grounding electrode), thereby bringing the potential of the metal conductor close to that of the ground. 　　3) Grounding a. Direct grounding or connection to ground through a low-impedance path. 　　b. Connected to the ground through a wire or other conductor with very low or almost zero resistance (impedance). 　　5) Soft grounding　Grounding is achieved through sufficient impedance, thereby limiting the current to a level that is safe for humans (typically 5 mA). The impedance required for a soft ground depends on the voltage levels that people near the grounding point may come into contact with. 　　6) Direct grounding A type of grounding that establishes a conductive connection between a metal object and the ground. 　　7) Indirect grounding: A type of grounding method in which, in order to achieve electrostatic grounding for objects other than metals, all or part of their surface is brought into close contact with a grounded metal object. 　　8) Static discharge grounding device A common device that grounds the devices without components connected to it within the static discharge protection area. 　　9) Ground reference plane A flat conductive surface whose potential is used as a common reference potential. 　　10) Static grounding connection system An external conductive path through which charges on charged objects are leaked to and dissipated in the ground.

2 Protective grounding 2.0.1 The metal enclosures of electrical instruments and automatic control equipment, as well as their normally non-electrified metal parts, should all be provided with protective grounding in cases where they might develop dangerous voltages due to insulation failure. They include: dashboards, instrument cabinets, instrument boxes, PLC and DCS cabinets, operation stations and auxiliary equipment, power distribution panels, power supply boxes, junction boxes, cable trays, cable channels, conduit pipes, and the shielding layers of armored cables. 2.0.2 Field instruments, transmitters, local switches, etc., powered by 24V or below 24V, do not require protective grounding unless there are specific requirements. 2.0.3 The metal enclosures of small low-voltage electrical devices such as buttons, signal lights, and relays installed on metal panels in areas free from explosion hazards need not be provided with protective grounding, provided that they are in good electrical contact with the grounded frame of the metal panel. 3 Working Grounding 3.0.1 Instruments, PLCs, DCS systems, computer systems, etc., should be provided with a working ground. Working ground includes: signal circuit grounding, shielding grounding, and intrinsically safe instrument system grounding. 3.0.2 When electronic devices such as instruments, PLCs, DCS systems, and computer systems require a unified reference potential, the signal circuit grounding should be carried out. 3.0.3 When PLCs, DCSs, computer systems are used in conjunction with analog instruments, a common signal loop ground point should be provided for both the analog and digital systems. 3.0.4 Components in the instrumentation system used to reduce electromagnetic interference (such as cable shielding layers, shielded twisted pairs, and shielding grounding terminals on instruments) should be grounded for shielding. Except for signal sources that are grounded themselves, shielding grounding should be carried out on the control room side. 3.0.5 The intrinsically safe associated equipment that must be grounded in an intrinsically safe instrument system shall be reliably grounded in accordance with the requirements of the instrument manufacturer. 3.0.6 The signal circuit ground and shielding ground of the intrinsically safe instrument system can be connected to the intrinsically safe ground through a grounding bus. 4 Lightning Protection and Grounding for Instrumentation Systems 4.0.1 In petrochemical plants located in areas with frequent or severe lightning strikes, if surge protectors are already installed at the points where the PLC, DCS, and computer system cables enter the control room, as well as at the field instruments, then those surge protectors shall be used for the lightning protection and grounding of the instrumentation systems. 4.0.2 For multi-core cables laid overhead outdoors in areas prone to severe lightning strikes, and not installed in metal cable trays or within conduits, their spare cores should be used for lightning protection grounding. 5 Grounding connection methods and requirements for grounding resistance 5.0. 1 The protective grounding of electronic devices such as instruments, PLCs, DCS systems, and computer systems should be connected to the plant’s electrical system grounding grid, with a grounding resistance of less than 4Ω. 5.0.2 The working grounding (signal circuit grounding, shielding grounding) for electronic equipment such as instruments, PLCs, DCSs, and computer systems can be implemented in two ways: 5.0.2.1 When the grounding resistance value of the plant’s electrical system grounding network is less than 4Ω, it meets the requirements of the instrument system, and the instrument manufacturer has no special requirements, it may be directly connected to the plant’s electrical system grounding network ； 5.0.2.2 When the grounding resistance of the grounding grid in the plant’s electrical system is high, or when the instrument manufacturer has specific requirements, an independent instrument grounding system should be established, with a grounding resistance of less than 4Ω (or as specified by the instrument manufacturer). 5.0.3 Under normal circumstances, instrument circuits and systems should have only one grounding point for the signal circuit. When using a transformer-coupled isolator or an optocoupled isolator, signal circuit ground points can also be provided on each side of the isolator. 5.0.4 The shielding layer of the wires used for transmitting signals shall be grounded at the grounding terminals or grounding busbars of the instrument panel (cabinet); it shall not be left ungrounded or grounded multiple times. 5.0.5 The grounding system for the Zener-type safety barriers in intrinsically safe instrumented systems should be installed separately, with a grounding resistance of less than 1Ω. The grounding electrodes of intrinsically safe instrument systems should remain independent; the distance between them and the grounding grid of the plant’s electrical system or that of other instrument systems should be no less than 5.0 m. 5.0.6 Lightning protection (surge protectors) for the instrument system on the control room side shall be grounded; if the grounding resistance value of the existing instrument system is 1Ω or less, it is possible to share the grounding electrode with the protective grounding and working grounding of the instruments ； Otherwise, a separate grounding system should be installed to ensure that the grounding resistance value for lightning protection (surge protectors) of the instrumentation system is no greater than 1Ω. 5.0.7 For the grounding of lightning protection (surge protection) devices for field transmitters, this can be achieved by connecting the instrument itself to a grounded metal cable conduit or similar method. Electrical grounding: A safety measure for electrical use aimed at ensuring the proper operation of electrical equipment and protecting human safety. Grounding is achieved by connecting electrical devices to a grounding device through metal wires. This grounding device directs any leakage currents, static charges, or lightning currents that may arise from electrical equipment and other industrial devices into the ground, thereby preventing electric shock as well as potential accidents such as fires and explosions. A grounding device consists of a grounding electrode and grounding wires. The metal conductor that is in direct contact with the soil is called the grounding electrode, while the metal conductor that connects the electrical equipment’s grounding point to this electrode is known as the grounding wire. Grounding electrodes can be divided into natural ones and artificial ones. Natural grounding electrodes include: ① water pipes and other metal pipes buried underground (except those for liquid fuels and flammable or explosive gases); ② metal well pipes; ③ metal structures of buildings and structures that are in contact with the ground or underwater; ④ the reinforced concrete foundations of buildings. Artificial grounding electrodes can be made from vertically buried angle steel, round steel, or steel pipes, as well as horizontally buried round steel or flat steel. When the soil is highly corrosive, the surface of the grounding electrode should be coated with tin or hot-dip galvanized, and its cross-sectional area should be increased appropriately. Horizontal grounding electrodes can generally be made from round steel with a diameter of 8–10 millimeters. Vertical grounding electrodes, such as steel pipes, are usually 2–3 meters long, with an outer diameter of 35–50 millimeters; angle steel typically has dimensions of 40×40×4 or 50×50×4 millimeters. The top end of an artificial grounding electrode should be buried 0.5–1.5 meters below the ground surface. Below this depth, the electrical conductivity of the soil changes little depending on the season, resulting in stable grounding resistance, and it is also less susceptible to external damage. Grounding resistance generally refers to the ratio of the alternating current or direct current voltage applied to the grounding electrode to the current flowing into the ground through it. When dissipating lightning surge currents, grounding resistance refers to the ratio of the peak voltage to the peak current, known as surge grounding resistance. Grounding resistance is mainly determined by the resistance of the soil along the path taken by the current underground. The resistance at the point where the grounding electrode contacts the soil, as well as the resistance of the electrode itself, is negligible. In the event of a grounding short circuit in a power grid, the short-circuit current flows outward from the grounding electrode in a roughly hemispherical pattern (although near the grounding electrode the pattern is not exactly hemispherical, as the distribution of the current depends on the shape of the electrode). The equipotential lines perpendicular to the current are shown in the diagram; the closer these lines are to the grounding electrode, the higher their potential. Since the surface area of a sphere is proportional to the square of its radius, the cross-sectional area through which the current flows increases rapidly as one moves away from the grounding electrode. Since resistance is inversely proportional to the cross-sectional area of the current path, the soil resistance corresponding to the same hemispherical area decreases rapidly as one moves away from the grounding electrode. Under normal circumstances, when a grounding device dissipates current, the potential 20 meters away from a single grounding electrode is essentially zero. Grounding resistance depends on factors such as soil electrical conductivity, the shape, size, and arrangement of the grounding electrodes, and the frequency of the current. Typically, based on the required value of grounding resistance, the shape, size, number, and arrangement of the grounding electrodes are determined. In areas with high soil resistivity, such as mountainous regions, measures can be taken to improve soil electrical conductivity in order to save metal materials. For example, substances with high electrical conductivity can be inserted into the soil around the grounding electrodes, or a layer of resistivity-reducing material (such as a cured resin containing water and strong electrolytes) can be used to lower the grounding resistance. When lightning current flows into a grounding electrode, due to the very high amplitude of this current, the potential on the electrode becomes very high, resulting in intense spark discharges in the soil surrounding it. This increases the electrical conductivity of the soil, effectively reducing the resistance to current flow.