Viewing the principles and purposes of instrument grounding from the perspective of standard SH/T3081-2019

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SH/T3081-2019 \"Code for Grounding Design of Petrochemical Instruments\" specifies in detail what and how to carry out grounding for instruments and control systems. Since it is necessary to use language and structure that comply with the requirements of these standards, the rationale and background behind the provisions in the code cannot be explained, which makes it tedious to read and sometimes even difficult to understand. This article focuses on discussing the principles and purposes of instrument grounding from nine aspects, and can serve as supplementary material to relevant standards for readers' reference. Instrument grounding: yunrun.com.cn/tech/3336.html. The purposes of grounding: There are mainly two purposes for grounding instruments and control systems. One is to ensure personal safety and the safe operation of electrical equipment, including protective grounding, grounding for intrinsically safe systems, anti-static grounding, and lightning protection grounding – these are referred to as safety grounding or protective grounding ； The second is grounding for signal transmission and interference reduction, known as working ground or reference ground. The purposes of these two types of grounding are different, and their grounding connection methods also vary; however, they are related to each other and cannot be completely separated. The safety grounding or protective grounding of instruments and control systems, referred to herein as protective grounding, is the grounding required for the operation of instruments. The power supply for instruments comes from 220V AC electricity used in industrial or domestic applications. Therefore, the protective grounding for instruments is the same as that used in electrical systems; it falls under the category of grounding in low-voltage electrical power distribution systems. As such, it should be implemented in accordance with the relevant standards, specifications, and methods established for electrical systems, and it should be connected to the grounding system of those low-voltage electrical power distribution systems. Protective grounding is related to the power supply configuration of electrical low-voltage supply and distribution systems, and there are various forms of it. Based on the nature and characteristics of alternating current used in instrumentation and control systems, the TNS configuration is widely adopted. It features a separate protective earthing wire PE; this is a relatively safe way of electrical power supply. The TNS power supply configuration is shown in Figure 1. The instrument protection grounding is the same as the PE wire from the electrical department; it represents redundant grounding. Figure 1 TN-S wiring diagram. The working ground or reference ground of the instrumentation and control system, referred to as working ground in this text, is the ground of the DC power supply system or the common point ground; it represents a connection to a common voltage reference point, and it is not necessary for this ground to be physically connected to the earth. Different documents use various terms, definitions, and classifications for the instrument working ground, but the essence is the same. Functions of grounding: 1. Grounding serves three purposes: ① It creates a potential on electrical equipment that is close to the ground potential; thus, when the insulation of such equipment is damaged and leakage occurs, it prevents harmful contact voltages from being generated for those who are standing on the ground and in contact with the metal parts of the equipment ； ②It generates a leakage circuit current, causing the leakage protection device to activate and thus providing protection ； ③It is used for discharging surge currents, which may originate from the power supply or from lightning. 2. The work grounding serves three purposes: ① It provides a reference point for the electrical circuits of DC-powered devices; using a common busbar for these circuits helps to reduce wiring requirements. ②Provides a reference point for electronic circuits, forming a working circuit or eliminating noise. ③The shield is grounded so that it forms a Faraday cage, thereby providing shielding against static electricity, electric fields, or electromagnetic fields within a certain range. Principles of grounding: In physics and electricity, the term “voltage” refers to the potential difference between two points, or the potential of a point relative to a reference point. Depending on the reference point used, the voltage value may vary. Electric current flows from one point through a current path to another; in power supply and load circuits, it flows back to the current source, thus a current loop must be formed. However, this is different in electrostatics and electric fields; current is the result of accumulated static charge suddenly flowing when the insulation is broken down, and there is no current circuit. Examples include discharge due to friction, static discharge from objects during dry seasons, and lightning discharge. Based on the principles and functions of the aforementioned voltage and current circuits, the earth is usually used as a reference point, possessing a relatively stable potential. Since the ground provides a circuit reference point, forms electrical circuits, or is used for discharging abnormal currents, in principle either distributed grounding or common grounding can be employed. However, in order to reduce the voltage or current between dispersed grounding points or dispersed grounding systems, using a common ground is a safer and more reliable approach. It not only minimizes the number of different reference points, but also creates an excellent (low-impedance) path for current flow and a discharge route for stray currents. This helps ensure the safe operation of electrical equipment while reducing or eliminating interference with electronic devices. Under certain specific conditions, it is not convenient to use the earth as a reference point; for example, mobile devices can use the common electrical connection points of the device or system as grounding points, allowing the electrical system of the mobile device to function independently, with similar results. This type of \"grounding\" that is not directly connected to the ground is referred to as a \"grounding connection\"; some literature uses the term \"ground bonding.\" There is disagreement between these terms, but the basic idea is the same: connecting devices, instruments, metal objects that may be charged, and large isolated metal objects using conductors, so as to make the potential of these objects approximately equal. Therefore, it is also called equipotential connection or equipotential bonding. Of course, this “grounding” can either not be connected to the ground or it can be connected to the ground. Grounding classification: The grounding of instruments and control systems can be classified based on their functional purposes or based on their roles, with no strict definition established. Some types of grounding can belong to both this category and another; different literature uses various classifications. The newly revised SH/T 3081 \"Code for Design of Grounding in Petrochemical Instruments\" classifies grounding into protection grounding, operational grounding, grounding for intrinsically safe systems, shielding grounding, anti-static grounding, and lightning protection grounding, among others. 1. Protective grounding: Protective grounding is a type of grounding implemented for the safety of people and electrical equipment. The exposed conductive parts of instruments and control systems are not charged under normal conditions, but may carry hazardous voltages in case of faults, damage, or abnormal situations; protective grounding should be applied to such equipment. Meters powered by a voltage below the safe level do not require protective grounding, except in cases where they may come into contact with electrical equipment operating at a voltage higher than the safe level. Safe voltage refers to a voltage level that poses no danger to the human body. The values of safe voltage specified by various standards and at different times vary. Depending on environmental conditions, individual physical characteristics, the working environment, and frequency, the contact voltages and currents that can cause harm also differ. China once set the safe voltage at 36V, and later specified different safe voltages for various working environments. GB/T3805-2008 \"Limits for extra-low voltage (ELV)\」specifies the voltage levels that will not cause harm to humans under various environmental conditions and in different fault scenarios. When electrical equipment is already naturally grounded, repeated grounding is not necessary. For example, instruments installed on grounded metal dashboards, boxes, cabinets, or frames and in good electrical contact with them may not require protective grounding. 2. Operating grounding: In this document, operating grounding refers to the operating grounding for instruments and control systems, including grounding of instrument signal circuits and grounding of the reference point for DC power supply. The signals of the instrumentation and control systems are DC standard signals or low-frequency communication signals ； Network communication signals and carrier signals are separate matters. Non-isolated signals are typically referenced to the negative terminal of the DC power supply and grounded; the signal distribution in conventional analog instruments is also based on this reference point. The circuit for the isolation signal is insulated from other circuits and also insulated from ground, so it does not need to be grounded. The nature of the isolation signal: ① The circuit system is floating and has no electrical connection to any ground reference point; therefore, the voltage of the circuit with respect to ground is uncertain ； ②The power supply system is another grounding system ； The potential difference between the two grounding systems at both ends of the isolation circuit is indeterminate; it varies depending on the conditions of the grounding systems and the environment. 3. Grounding of intrinsically safe systems: An intrinsically safe system using an isolated safety barrier is employed, with the input and output ends being isolated from each other; there is no path for current to flow, so no special grounding is required ； A intrinsically safe system using a Zener-type safety barrier has its input and output terminals connected in a circuit; to limit the fault voltage at the field side, it is necessary to establish a grounding connection system to the reference point. In a Zener-type safety barrier, the grounding of the intrinsically safe system and the grounding of the instrument signal circuit should not be separated; in practice, they cannot be separated either. 4. Shield grounding: According to the Faraday cage principle, a metal shield needs to be grounded in order to be effective. Shield grounding comes in two types: line shielding and equipment shielding. The purpose of shielding is to protect the lines or equipment from external influences, so it serves a protective function. The function of shielding is to reduce interference from static electricity, external electric fields, or certain electromagnetic fields on the shielded objects. Therefore, it is neither a protective grounding for electrical safety purposes nor the grounding required for the operation of internal circuits in instrumentation systems. Under normal circumstances, the current resulting from electrostatic discharge and eddy currents in the electric field through the shielding layer is relatively small; therefore, the shield grounding can be connected either to protective grounding or to working grounding. The shielding layer of the signal shielding cable is grounded at one end in order to prevent currents generated by different ground potentials on the shielding layer from interfering with the signal conductors. To facilitate installation without compromising the shielding effect, the shielding layer of the signal shielding cable is usually grounded at the instrument end in the control room. In lightning protection applications, shield grounding serves to discharge lightning current; therefore, it is more appropriate to connect it to the protective ground. In some cases, the shielding current generated by external electromagnetic fields is specifically utilized to reduce interference; this approach of using ground loop currents at different locations requires grounding both ends of the shielding layer. There are two ways to resolve this contradiction: one is to use a single-layer shield, where one end of the shield is directly grounded, and the other end is grounded through a capacitor ； Secondly, a dual-layer shielding approach is used: the inner shielding layer is grounded at one end, while the outer shielding layer is grounded at both ends. The outer shielding can be implemented using metal protection tubes, metal cable trays, etc.; it does not have to be the shielding layer of the cable itself. Therefore, the metal protective conduits and metal cable trays for instrument cables should be grounded at both ends. For longer conduits or cable trays, grounding should also be repeated at regular intervals ； The metal armor protective layer of the armored cable used for instrument signals must also be connected to protective grounding at both ends. Since the shield current is not a current that affects electrical safety, and due to certain engineering conventions, it is often connected to the instrument’s working ground; this is what leads to uncertainty in the implementation of shield grounding projects. The function of grounding field instruments is primarily to achieve simple shielding through the metal casing, while the circuits inside the instruments are not grounded; any claims in some literature that assume grounding of the internal circuits as a premise for analysis are unfounded. 5. Anti-static grounding: The main methods to prevent static electricity from damaging electronic devices are to prevent the accumulation of static electricity and to provide conductive pathways for its discharge. Generally, in control rooms, cabinet rooms, and server rooms for process control computers where equipment such as DCS, PLC, SIS, and PCs are installed, anti-static measures should be considered. The static-dissipative flooring, metal raised floors, workbenches, cabinets, and other components in these rooms must be grounded. The accumulation and discharge of static electricity are closely related to air humidity. Static electricity is relatively easy to discharge; a discharge resistance to ground of less than 100Ω yields good results. Therefore, instruments and equipment that already have protective grounding and working grounding do not require additional anti-static grounding. In recent years, control devices such as DCS, PLC, SIS, and PCs have been equipped with anti-static features as well as connectors that prevent voltage or current surges, which enables effective protection against damage caused by static electricity. As a result, the requirements for protecting against static electricity in external environments are not as stringent anymore. 6. Lightning protection grounding: The function of lightning protection grounding is to discharge lightning current into the ground. Lightning current includes direct lightning current and surge currents generated by lightning electromagnetic induction. Due to the extremely high intensity of lightning currents, even the induced surge currents have magnitudes ranging from tens to hundreds of amperes. However, the duration of lightning currents is very short; therefore, they are essentially high-frequency, high-intensity current pulses. To discharge lightning surge currents, a as short as possible discharge path and a as low as possible path impedance are required. Since the vast majority of instrument circuits are not directly exposed in areas such as open spaces that are prone to lightning strikes, the majority of lightning currents that damage instruments are surge currents rather than direct lightning currents. When dealing with lightning protection grounding, attention must be paid to the potential difference that occurs along the path for the discharge of lightning surge currents, as well as the backflow voltage of the ground potential. For the lightning protection grounding of instrumentation and control systems, there are certain considerations and methods related to the dissipation of surge currents; moreover, they should share the grounding device with the electrical professional’s grounding system. The grounding standard SH/T3081-2019, \"Code for Grounding Design of Petrochemical Instruments\", specifies the specific methods and details for protective grounding, operational grounding, grounding for intrinsically safe systems, shielding grounding, and anti-static grounding. SH/T3164-2012 \"Code for Lightning Protection Design of Petrochemical Instrumentation Systems\" specifies the grounding methods in instrument lightning protection projects. The grounding design for instrument systems specified in this standard is aimed at protecting instruments from lightning strikes, and it serves as a supplement and expansion to the \"Code for Grounding Design of Petrochemical Instruments\". The grounding system implemented in accordance with the \"Code for Lightning Protection Design of Petrochemical Instrumentation Systems\" can fulfill the grounding functions specified in the \"Code for Grounding Design of Petrochemical Instruments\", and the grounding methods prescribed in the former can completely replace those specified in the latter. The anti-interference effect of grounding: 1. Sources of signal interference. Interferences in instrument signals include differential mode interference and common mode interference, which can cause signal distortion and errors. Differential mode interference is the voltage or current that acts directly between the signal lines, while common mode interference is the voltage or current between the signal lines and a common reference point, or between the signal lines and the ground. The nature of such interfering voltages or currents can be DC, AC, pulsed, or irregular. The main sources of signal interference are electric fields (static electricity) and electromagnetic fields, with electromagnetic fields being further divided into high-frequency and low-frequency types. The principle of interference is electric field induction and electromagnetic field induction. The main measures to prevent or reduce interference with instruments are shielding and the cancellation and attenuation of interfering electromagnetic fields by counteracting electromagnetic fields. For example: twisted pair cables, multiple-point grounding with external shielding, etc. The causes of accidents and abnormal disturbances are usually signal errors or even instrument damage resulting from short circuits or open circuits in the lines. 2. The anti-interference effect of grounding – Shielding: Shielding is one of the effective means of combating interference. Depending on the source of the interference, it can be divided into electric field shielding, magnetic field shielding, and electromagnetic field shielding. Since all three conditions exist in the instrument’s operating environment, shielding should be considered comprehensively. The principle of electric field shielding is to use a conductor as a shield, achieving electrostatic equilibrium under grounded conditions, thereby eliminating the capacitive coupling from sources of electric field and low-frequency electromagnetic field interference and realizing electric field shielding. Magnetic field shielding is used to prevent external static magnetic fields and those generated by low-frequency currents from entering the area that needs protection. A magnetic medium must be used as the enclosure; the thicker the enclosure and the higher its magnetic permeability, the better the shielding effect will be. Iron materials have a very high magnetic permeability; therefore, their shielding effect is much better than that of materials with lower magnetic permeability. Electromagnetic shielding also utilizes a shield made of conductive materials. By exploiting the phenomenon of electromagnetic induction, eddy currents are generated on the surface of the shield; these produce a reverse magnetic field that cancels out or weakens the original interfering magnetic field, thereby achieving the purpose of shielding. Ordinary carbon steel and iron materials have high electrical and magnetic conductivities; therefore, they exhibit good effects in shielding against electric fields, magnetic fields, and electromagnetic fields. The grounding of the shield is one of the important conditions for achieving shielding effects. 3. Current diversion effect: The grounding of the shield directs static charges into the ground, causing the electric field to terminate at the metal surface of the shield. This achieves electrostatic equilibrium, that is, electric field shielding. When surge current flows along the signal line, due to the skin effect of alternating current, part of the surge current is dissipated through the grounding of the shielding layer, while the remaining surge current is dispersed into the ground under the action of the surge protector’s current-diverging mechanism; this is the function of the grounding system in guiding current. Grounding Methods 1. Overview Grounding methods include single-point grounding and multi-point repeated grounding. In principle, both of these methods can meet the requirements for various types of grounding, but in actual engineering applications, the effectiveness of grounding varies depending on factors such as the size of the area, the length of the grounding wires, and the nature of abnormal or interference currents. DC signals, low-frequency signals, DC power supply, power supply and distribution in industrial and residential buildings, power supply and distribution for industrial equipment, power supply and distribution failures, lightning, and so on can all generate currents in the grounding circuits or grounding points. However, the operating frequencies of components, buses, and communication systems in electronic devices are not the frequencies of ground currents under normal or abnormal conditions; for example, computers operate at frequencies in the GHz range, but chips and circuit boards do not need to be connected to ground – they serve only as reference points for the relative electric potential of local circuits. What is actually grounded is the power supply and distribution grounding, that is, the protective grounding, or the grounding required for DC and low-frequency signals. 2. Single-point grounding: The signals in instruments and control systems are DC signals or low-frequency signals, with frequencies below 10 kHz. To prevent ground potentials at different grounding points from interfering with these signals, and to eliminate the effects caused by the distributed capacitance in the wiring, single-point grounding is commonly used for the working grounding of instruments. Ground loops should be avoided in signal circuits. The single-point grounding for the instruments is shown in Figure 2, while the separate grounding of the signal source and the receiving instruments is shown in Figure 3. Figure 2 Schematic of single-point grounding for the instrument work order (Note: U=US ； U is the signal received by the instrument ； USμ signal source output signal) http://yunrun.com.cn/upload/202009/16/202009160006151804.png Figure 3 Schematic showing the grounding of the signal source and the receiving instrument respectively (Note: U = US + Ug) ； (The ground potential difference between different grounding points of Ugwei) As can be seen from Figures 2 and 3, when the signal source and the receiving instrument are grounded separately, a ground potential difference Ug is generated between these different grounding points. To prevent the ground potential from affecting the signal, the signal should not form a circuit through the ground. If both the signal source and the receiving instrument on a circuit must be grounded, an isolator should be used to separate these two grounding points in order to eliminate the effect of the ground potential difference Ug. The separation of the two grounding points by an isolator is shown in Figure 4. http://yunrun.com.cn/upload/202009/16/202009160021071447.png Figure 4 shows a schematic illustration of isolating two points from ground using an isolator. In a single-point grounding system, all grounding wires are typically gathered at one common plate – the grounding point – which is then connected to the grounding grid. The grounding of this common plate is shown in Figure 5, where the dashed lines represent the grounding wires. http://yunrun.com.cn/upload/202009/16/202009160025450354.png Figure 5: Schematic diagram of grounding for the distribution panel. 3. Multiple grounding points: Multiple grounding, also known as repeated grounding, is used to discharge large currents in case of abnormalities in the same device or circuit. It is also employed for creating an equipotential connection in large-scale conductive devices as a safety measure. It is primarily used for grounding purposes related to power supply and distribution as well as lightning protection. For a single small device, the exposed conductive area is relatively small, and the path for abnormal currents or the grounding connection is short; thus, no potential difference is generated. Therefore, one grounding wire is sufficient. However, for large or long equipment, grounding must be carried out at regular intervals, that is, repeated grounding, such as the grounding of cable trays. 4. Grounding network: For a grounding network with good conductivity, if there is no faulty grounding from high-power electrical equipment in the area where the instrumentation devices are located, it can be regarded as a grounding plane. All types of grounding in that area can be connected to this grounding network, thereby achieving an excellent grounding effect in a simple and straightforward manner. This is what is known as the mesh grounding method. Grounding schemes: 1. Star grounding. The star grounding configuration is commonly used for grounding in areas with a relatively small size; it represents a typical single-point grounding approach, such as for the grounding of control rooms and cabinet rooms. The star grounding is illustrated in Figure 6. http://yunrun.com.cn/upload/202009/15/202009151740504771.png Figure 6: Schematic of star grounding. The implementation of the star grounding configuration involves two types of connections: ① Groundings of the same type are connected together. The instrument protection grounding wires are connected to the protection grounding assembly board ； The instrument working ground wires are gathered on the working ground collection board; in situations where there are many instrument ground wires, it is possible to collect them separately first and then combine those collections together. ②Bring together different buses. For example: the work ground collection plate and the protective ground collection plate are connected to the main ground plate, which is then connected to the grounding system. In the star section that adopts star grounding or a combined star-network grounding configuration, the working ground shall not be mixed with the protective ground before it is connected to the working ground collection plate. The connections for working ground, including various grounding wires, grounding main lines, and grounding busbars, should be insulated at all points other than the normal connection points before being connected to the main ground plate. Although there is insufficient evidence and rationale for this regulation, it is primarily intended to avoid the adverse effects caused by abnormal current circuits resulting from errors in construction work. 2. Mesh grounding: The mesh grounding configuration is equivalent to a local grounding grid and offers excellent grounding performance; it is suitable for various locations and different grounding requirements, and can be used in both single-point and multi-point grounding systems. Its drawback is that it is somewhat more expensive. The schematic of the mesh grounding is shown in Figure 7. http://yunrun.com.cn/upload/202009/15/202009151815316806.png Figure 7: Schematic diagram of mesh grounding. Mesh grounding strips use multiple grounding circuits to connect to the grounding device, with the aim of providing more paths for current dispersion and thereby improving the efficiency of grounding, especially in situations where large grounding currents are present. This approach also facilitates the proper discharge of lightning surges from instruments. The \"Code for Lightning Protection Design of Petrochemical Instrumentation Systems\" adopts this grounding connection method. The lightning protection grounding for the instrument system in the control room should adopt a mesh grounding system, with mesh grounding bars installed beneath the cabinets. The various busbars inside the cabinet can be directly connected to the grid-shaped grounding bar at the bottom of the cabinet, resulting in a shorter grounding path. At the same time, the grounding wire should be as short as possible, and bending during installation should be avoided. Mesh grounding bars can be made of copper plates with a thickness and width of 4 mm × 40 mm, hot-dip galvanized flat steel, or stainless steel, and are suitable for all equal potential connection applications. 3. Star-network composite grounding: The star-network composite grounding configuration is shown in Figure 8. http://yunrun.com.cn/upload/202009/15/202009152345102050.png Figure 8: Schematic diagram of star-network composite grounding. Grounding principles: 1. Shared grounding system. The “Code for Grounding Design of Petrochemical Instrumentation Systems” stipulates that the grounding of instrumentation and control systems should be integrated with the low-voltage power supply and distribution system used in electrical engineering. It also specifies that the protective grounding, operational grounding, grounding for intrinsically safe systems, shielding grounding, anti-static grounding, and lightning protection grounding for instrumentation and control systems shall all use a shared grounding system. The use of a common grounding system is the accepted practice for electrical grounding systems as well as for the grounding of instruments and control systems. Most current national and international standards stipulate the principle of using a common grounding system; in particular, GB 50057-2010 \"Code for Design of Lightning Protection of Buildings\" requires that the grounding system used to protect against lightning induction share the same infrastructure as the grounding system for electrical equipment. Based on the working principle of the instrument and the design of the electronic circuit, there are no special requirements regarding the grounding resistance of the instrument circuit. Whether it is for instrument protection grounding, instrument operation grounding, intrinsically safe grounding, shielding grounding, anti-static grounding, or lightning protection grounding, all such aspects must comply with the electrical industry’s regulations on grounding; therefore, there is no reason to impose any special requirements on the grounding resistance. A thorough search of relevant literature reveals no principles or basis for any specific requirements regarding the grounding resistance of electronic devices such as instruments, communication equipment, electronic products, and computers. 2. Equipotential bonding: The factors that can affect the proper operation of instruments or cause damage due to overvoltage include circuit short circuits, ground potential effects, and surge currents. Good conductivity of the grounding wires, equipotential bonding, and shared grounding systems can effectively address these issues. The functions of the protective grounding and operating grounding for instruments can be summarized as follows: first, to reduce the potential difference generated by ground currents in the circuits ； Second, establish a public reference potential. Equipotential bonding achieves these two objectives very well; either star grounding or mesh grounding can be used, with mesh grounding offering better results and being simpler to implement ； But note that grounding by connecting cabinets in series or instruments in series should not be used. 3. Ground resistance: Due to the use of equipotential bonding and a common grounding system, the ground resistance of the instruments and control systems is equivalent to that of the grounding system in the low-voltage electrical power supply and distribution system. The grounding resistance of low-voltage power supply and distribution systems is calculated and specified based on factors such as electric shock scenarios and personal safety; Chinese electrical industry standards stipulate that the grounding resistance under normal conditions should not exceed 4Ω. The defined value of the grounding resistance of a grounding device is equal to the ratio of the voltage of the grounding device with respect to the ground to the current flowing into the ground through the grounding electrode. Since the frequency of industrial and domestic alternating current in China is 50Hz, the measurement of grounding resistance also requires the use of 50Hz alternating current; therefore, the resistance measured and calculated based on the power-frequency alternating current passing through the grounding electrode is referred to as the power-frequency grounding resistance. Based on the principles of instrument grounding and its functions, a good equipotential connection is more important than the ground resistance value. ISARP12.6 \"Implementation of Grounding for Instruments in Hazardous Areas – Part 1: Intrinsic Safety\" specifies that the connection resistance of the grounding circuit should be less than 1Ω; it does not specify a specific value for the grounding resistance. These are two different concepts, and they should not be confused. The grounding method specified in ISARP12.6 proposes a method of repeatedly connecting two grounding wires, so that the resistance of the grounding path can be measured by disconnecting one wire from the circuit, rather than measuring the resistance of the ground connection itself. In summary, whether it is for the protective grounding of instruments or their operational grounding, there are no specific requirements regarding the grounding resistance. Articles, discussions, and materials that specify or discuss the grounding resistance of instrument signal circuits do not explain the rationale behind this, and thus such requirements have no basis. Based on the purpose and working principle of grounding in instrument and control systems, it is a consensus among professionals that both protective grounding and operational grounding for instruments should share the same grounding system as the electrical low-voltage power supply and distribution system, a fact that has been fully proven through years of practical engineering experience. The functions of the instrument protection ground and the working ground are to establish a common reference potential and reduce the potential difference generated by ground currents in the circuit; equipotential bonding effectively achieves these two objectives. Based on the basic principles of grounding, the earth is usually used as a reference point, possessing a relatively stable potential. Under certain specific conditions, when it is not convenient to use the earth as a reference point, the common electrical connection point of the equipment or system can be used as the ground point, with the same effect. There is no need for electronic circuits to be connected to ground; there is no sufficient theoretical basis or experimental evidence to show that electronic circuits must be grounded, nor are there any theories or evidence regarding the resistance value when such circuits are grounded. The engineering specifications for the grounding of instruments and control systems should not only meet the purposes and functions of grounding but also take into account the feasibility, convenience of implementation, and engineering practices. Author: Ye Xiangdong, male, Deputy Chief Engineer at Sinopec Beijing Design Institute and Sinopec Engineering Construction Company, professor-level senior engineer, chief editor of standard SH/T3081-2019