Grounding of instrumentation and control systems is not a new topic; many issues have already been resolved, and correct design methods exist. However, some engineers still have some vague concepts and doubts. The function and classification of grounding have been discussed in many documents, and different classifications based on different methods are all reasonable; this article will not discuss them further. This article mainly discusses how to design grounding and why.
The purpose of grounding for instruments and control systems is mainly twofold: first, for personal safety and the operation of electrical equipment, including protective grounding, intrinsically safe grounding, anti-static grounding, and lightning protection grounding; and second, for signal transmission and interference suppression. However, these two aspects are related and cannot be completely separated.
Currently, my country has not formulated corresponding national standards for instrument system grounding. However, the relevant provisions in the national standards for protective grounding and lightning protection grounding in the electrical engineering field can be referred to.
The relevant standards of international organizations such as IEC and ISA provide excellent references. In particular, the provisions on functional grounding and protective grounding of information technology equipment through equipotential bonding and shared grounding provide designers with authoritative and clear engineering design basis.
01 Protective grounding
Protective grounding is grounding installed for personal safety and the safety of electrical equipment (also known as safety grounding). Like protective grounding in electrical engineering, protective grounding in instrumentation is part of the low-voltage power distribution system and should therefore be carried out in accordance with relevant electrical engineering standards, specifications, and methods. For example, GBJ65-83 "Code for Grounding Design of Industrial and Civil Power Installations," etc.
For grounding of low-voltage power distribution systems, the electrical profession has a series of relatively complete standards and specifications for design, calculation, testing, construction and acceptance. There are relatively complete theories, experiments and methods for each link of the grounding system, and it is by no means something that can be summarized by a certain grounding resistance value.
Instrumentation power typically comes from uninterruptible power supplies (UPS) or building power distribution systems used by electrical engineers. It can be broadly categorized into control room power and field instrument power. Control room power generally uses a TN-S system (where the protective earth and neutral wires are separate) [1]. Field instrument power generally uses a TT system (distributed grounding).
According to the principle of equipotential bonding, the protective grounding for instrument power supply should be an electrical grounding system. Not only is equipotential bonding implemented within buildings, but petrochemical plants generally also employ equipotential bonding across the entire plant.
Grounding engineering should be designed according to electrical engineering standards, specifications, and methods. Some designs separate the protective grounding of the UPS-powered instrument system into a separate grounding system, which is inappropriate. Most UPS systems switch outputs directly without transformer isolation, making a separate grounding system impossible. Furthermore, other power distribution systems within the building (such as lighting and maintenance power distribution) are low-voltage electrical systems, not the instrument power supply from the UPS. Thus, having two grounding systems within the same building, and the possibility of simultaneous contact, violates the electrical engineering standard that "exposed conductive parts that can be touched simultaneously should be connected to the same grounding system." This results in neither complete isolation between the two grounding systems nor equipotential bonding within the building, creating safety hazards.
02 Instrument working grounding
The purpose of grounding for instrumentation and control systems is to suppress interference. This issue has been clearly discussed in numerous documents, and its theoretical, practical, and methodological aspects are all correct and feasible. This article will not repeat it further. From an engineering perspective, grounding for instrumentation and control systems can be divided into shielding grounding and instrument signal grounding, among others.
2.1
There are two types of shielded grounding for instruments. One type is grounding via cable protection pipes, cable troughs, etc. This type of grounding should be connected to the electrical grounding network of the device, belonging to equipotential bonding. The other type is grounding of signal shielded cables, which should adopt different connection methods depending on the different situations of the signal source and receiving instruments. For example, most of the internal circuits of commonly used transmitters are not grounded, so the signal shielded cable is generally grounded on the control room side. The grounding of the signal shielded cable should be a single-point grounding.
Shielding can be categorized by its function into electric field shielding, magnetic field shielding, and electromagnetic field shielding to address interference issues. Electric field shielding, also known as electrostatic shielding, addresses interference caused by distributed capacitance. It uses materials with high conductivity and must be grounded. Magnetic field shielding uses materials with high magnetic permeability, requires a closed magnetic circuit, and may not need grounding at low frequencies. Electromagnetic field shielding protects against various types of electromagnetic radiation interference, uses low-resistance materials, and the shielding body may or may not be grounded.
2.2
Instrument signal grounding is divided into isolated signals and non-isolated signals. Isolated signals generally do not need to be grounded. Here, isolation means that the circuit of each input signal (or output signal) is isolated from the circuit of other input signals (or output signals), is isolated from ground, and its power supply is independent and mutually isolated.
Non-isolated signals typically use the negative terminal of a 24VDC power supply as a reference point and are grounded. Signal distribution uses this as the reference point. The common-mode rejection voltage of this type of circuit is usually very low, and grounding is the primary measure to eliminate such interference. When designing grounding systems, care should be taken to avoid voltage drops on the ground wire during equipment operation, which could interfere with signals.
Different series of conventional instruments have different grounding connection specifications. This is because signal transmission between secondary instruments in conventional instruments is relatively complex. For example, the signal grounding of I-series instruments is described in detail in the "Design Guide for I-series Electronic Instrument Systems".
The grounding of instrument signal common points, the grounding of non-isolated inputs of distributed control systems (DCS) and programmable logic controllers (PLCs), etc., should all be connected to the grounding junction box from the terminal blocks or busbars. This is essentially a form of equipotential bonding. EK series instruments are typical common-ground instruments. Although the non-isolated signal grounding of instruments is ultimately connected to the electrical grounding, it should not be directly mixed with the electrical grounding. The connection for the instrument's working grounding should use multi-strand copper core insulated wire. Before connecting to the grounding junction box, all grounding wires and grounding busbars, except for normal connection points, should be insulated. The final connection to the grounding electrode or grounding grid is wired separately from the grounding junction box.
The grounding of conventional instruments such as DDZ-Ⅲ type instruments, EK series instruments, I series instruments and YS80 series instruments is ultimately connected to the electrical grounding.
Most instrument and control system signals are low-frequency signals. The principle for grounding low-frequency signals is single-point grounding, and there are no special requirements for grounding resistance. Grounding loops should be avoided in signal circuits. If both the signal source and the receiving instrument on a line are unavoidably grounded, an isolator should be used to isolate the two grounding points.
2.3
The national standard GB50174-93, "Code for Design of Computer Rooms," stipulates in Article 6.4.3 that four types of grounding—AC working grounding, safety protection grounding, DC working grounding, and lightning protection grounding—should share a common grounding device, with the grounding resistance determined by the minimum value among them. Although the scope of application of GB50174-93 does not include industrial control rooms and microcomputer rooms, some of its provisions can be used as a reference.
The IEC standard "Grounding and Equipotential Bonding of Information Technology Installations" (IEC 364-5-548-1996) clearly stipulates that the functional grounding and protective grounding of information technology installations are connected through equipotential bonding and share a common grounding. The scope of application includes: information technology installations, devices requiring interconnection for data exchange, data communication equipment, data processing equipment, signaling devices with grounding return paths within buildings, communication networks of DC-powered information technology installations within buildings, local area networks, fire alarm systems and intrusion alarm systems, such as building service equipment for direct digital control systems, computer-aided manufacturing (CAM) and other computer-aided service systems. The standard also specifies that the following busbars are permitted to be connected to the grounding common conductor: shielding of remote communication cables or equipment, grounding busbars for overvoltage protection devices, grounding busbars for radio communication antenna systems, grounding busbars for DC power supply systems of information technology installations, and functional grounding busbars, etc.
IEEE Std 11000-1992 states that it is not recommended to use any so-called separate, independent, insulated, dedicated, clean, static, signal, computer, electronic, or other incorrect earth grounding body as a connection point for the equipment grounding conductor.
2.4
Maxwell advocated using the Faraday cage principle for lightning protection, which not only eliminates the need for grounding but is also safer and more economical than current methods. This is the principle of equipotential bonding. High-voltage live-line work utilizes the principle of equipotential bonding, rather than insulation protection.
The IEC standard "Grounding and Equipotential Bonding of Information Technology Equipment" stipulates that equipment grounding and protective grounding should share a common grounding system through equipotential bonding, without specifying requirements for the grounding resistance. Equipotential bonding is one of the effective measures to prevent interference signals from affecting the equipment. In this case, the grounding resistance has no impact on the information equipment; the core technology is equipotential bonding. In military and communications applications, mobile devices are only connected to the fuselage and not grounded, which cannot meet the various requirements for grounding resistance, yet they can still operate safely, normally, and reliably, precisely because of the application of the equipotential bonding principle.
"The shift from individual grounding to equipotential bonding has long been a consensus in the international electrical academic community and has been incorporated into IEC standards, ISA standards, and standards of some developed countries. Many manufacturers' product information has made the correct provisions or has been modified. However, some manufacturers still continue to use individual grounding and grounding requirements with stringent resistance values, which may be due to a lack of understanding of the current provisions of the standards or for some commercial purpose." [2]
Article 6.4.5 of the national standard GB50174-93 "Code for Design of Computer Room" stipulates that the grounding of the computer system should adopt single-point grounding and preferably adopt equipotential measures.
For automation professionals, protective grounding, instrument working grounding, and intrinsically safe system grounding are ultimately connected together on the grounding busbar, sharing a grounding electrode to achieve equipotential bonding.
03 Intrinsically Safe System Grounding
Safety barriers are divided into two types: isolated and Zener. Isolated safety barriers use isolation protection technology and do not require dedicated grounding, while Zener safety barriers require a good grounding system according to their protective operating principle. The discussion of intrinsically safe system grounding usually refers to the grounding issue of Zener safety barriers.
There are two types of power supply faults in non-intrinsically safe areas: one is a DC short circuit, which is typically supplied by a 24-30V DC power supply for two-wire or three-wire transmitters, therefore the safety barrier ground must be connected to the common terminal of the DC power supply; the other is an AC short circuit, which, to achieve the protection function, requires the safety barrier ground to be connected to the neutral line of the AC power supply. This determines that the safety barrier ground should ultimately be the electrical system ground.
The simplest and most reliable way to connect the safety barrier grounding busbar to the starting point of the AC power supply neutral line is by using a wire.
The IEC standard IEC60079-14 "Installation of electrical equipment in hazardous locations" specifies the following regarding the grounding of intrinsically safe circuits: "The grounding terminal of a safety barrier without electrical isolation (e.g., a Zener barrier) shall: 1) be connected to an equipotential bonding system via the shortest feasible path; or 2) for a TN-S system, be connected to a highly intact grounding point, the connection of which shall ensure that the impedance between this point and the main power supply system grounding point is less than 1Ω."
However, some companies use the earth as a conductor, leading some designers to mistakenly believe this is a standard and reasonable method, resulting in inappropriate engineering designs. This causes waste and construction difficulties, and also creates potential safety barrier hazards. The method of using the earth as a conductor was first seen in the two-wire-one-ground system of power transmission, but the grounding specifications for power equipment emphasize that the earth is strictly prohibited from being used as a phase or neutral wire in low-voltage power grids. Currently, using the earth as a conductor is only seen in TT systems. However, many engineering designs use a separate grounding electrode for Zener barriers, and it is precisely this method that creates the conditions for ground potential breakdown of the safety barrier. This is one of the root causes of Zener barrier damage accidents due to grounding problems. Therefore, the author believes that the inclusion of this method of setting a separate grounding electrode for Zener barriers in some standards and specifications (such as the "Design Specification for Grounding of Petrochemical Instruments" SH3081-1997) warrants further discussion.
ISA-RP12.6-1995, "Grounding Implementation for Instruments in Hazardous Locations," Part 1, "Intrinsic Safety," specifies that the connection resistance between the grounding busbar of the safety barrier and the neutral point of the AC power supply should be less than 1Ω, and clearly provides a diagram of a direct connection. MTL's grounding guidelines also specify this, but add the statement: "If it can reach 0.1Ω, it is even more suitable," and mention that connecting with a wire is the easiest method. As for the resistance to earth, none of the above documents specify it. Some safety barrier companies only vaguely state that 1Ω is generally recommended, but they do not provide the basis for this recommendation. It should be noted that in foreign documents, earth connection is called "Earthing" or "Grounding," while grounding connection is called "Bonding," and these have different meanings. Documents discussing the grounding resistance of intrinsically safe instruments generally specify the grounding connection (Bonding) resistance.
Foreign data only focus on the bonding resistance. Both ISA-RP12.6-1995 and MTL's grounding data suggest using two grounding wires connected repeatedly to measure the bonding resistance, rather than measuring the resistance to earth.
Intrinsically safe instruments in the field generally do not have their signal terminals grounded; the purpose of grounding the instrument casing is not for intrinsic safety. Furthermore, the ground potential only acts on the insulation of the transmitter with a grounded casing and will not reach the level that breaks down the insulation of the field instrument. Some literature treats transmitter casing grounding as signal terminal grounding and incorrectly discusses the breakdown of safety barriers based on the potential difference between the casing grounding point and the Zener barrier grounding point in non-hazardous locations. If the field terminal of the instrument signal is inherently grounded, the loop forms a two-point grounding, and the ground potential difference may act on the safety barrier. In this case, using a Zener barrier is incorrect; an isolated safety barrier should be used to avoid multiple grounding points. This meets both signal transmission requirements and intrinsic safety requirements.
04 Lightning protection grounding
When discussing the lightning protection grounding issue for instrumentation and control systems, the design of lightning protection engineering for instrumentation and control systems should be discussed first, as lightning protection grounding for instrumentation and control systems is only one component of lightning protection engineering for instrumentation and control systems. Reference [3] has already discussed this issue in detail. This article is not specifically for discussing instrumentation lightning protection engineering, but only adds a few points regarding instrumentation lightning protection grounding.
4.1
The national standard GB50057-94, "Code for Design of Lightning Protection of Buildings," provides a good basis and reference. Although the standard does not directly specify lightning protection design for electronic equipment, it does offer some explanations. GB50057-94 specifies protection against direct lightning strikes, lightning induction, and lightning surge intrusion. IEC1024-1-1993 divides lightning protection into external and internal lightning protection. External lightning protection refers to protection against direct lightning strikes, while internal lightning protection includes protection against lightning induction, backflashover, lightning surge intrusion, and protection against life-threatening situations, with equipotential bonding being the core method.
The explanation of Clauses 5 and 6 of Section 3.2.4 of GB50057-94 standard points out the equipotential bonding for direct lightning strike protection and induced lightning protection: "From a lightning protection point of view, it is better to install a common grounding device, which is suitable for all grounding purposes (e.g., lightning protection, low-voltage power systems, telecommunications systems)." "The arrangement and size of the grounding device are more important than the specific value of the grounding resistance."
4.2
The national standard GB50057-94, "Code for Design of Lightning Protection of Buildings," specifies that lightning protection grounding resistance is impulse grounding resistance and provides the relationship between impulse grounding resistance and power frequency grounding resistance. The ratio of impulse grounding resistance to power frequency grounding resistance is called the impulse coefficient. When lightning strikes the ground, there is a breakdown phenomenon and a sparking effect; the resistance exhibited by the grounding electrode is the impulse grounding resistance. The current dissipation resistance when lightning strikes the ground is a non-linear resistance, related to the peak value and waveform of the lightning current; therefore, Ohm's law cannot be simply applied.
Arbitrarily lowering the grounding resistance value is irresponsible. Grounding resistance regulations should be based on sound principles, as they directly impact grounding engineering. For example, before the Ministry of Water Resources and Electric Power issued the "Regulations for Grounding Devices of Electrical Equipment" in 1959, the grounding resistance of electrical equipment in the power system was 0.5Ω. As a result, many power plants consumed large amounts of steel, reportedly ranging from 10 to 40 tons, with grounding device areas of 100×100m². Each year, several tons of steel needed to be buried to maintain and improve the grounding resistance; after several years, tens of tons had been buried, rendering further burial unnecessary. Later, the balanced grounding design method was adopted, resolving the conflict between grounding resistance and potential harm to personnel. The current national standard in my country stipulates a grounding resistance of 4Ω. Lightning protection grounding resistance should comply with the national standard GB50057-94 "Code for Design of Lightning Protection of Buildings" and should not be arbitrarily lowered.
4.3
The grounding resistance of a grounding system is usually not constant, and grounding devices are not a one-time solution. Grounding systems should be inspected and maintained regularly to promptly detect and repair faults such as corrosion, broken wires, and damage, in order to maintain the integrity of the entire system, especially the grounding connections. Numerous examples demonstrate the crucial importance of inspecting and maintaining lightning protection projects.
4.4
Modern lightning protection technology is a comprehensive prevention and control technology. Lightning protection for instrument systems cannot be achieved solely through grounding. It cannot be simply assumed that grounding will solve the problem. Buried cables are also susceptible to lightning strikes, only the probability of them being struck is lower than that of overhead cables.
Modern lightning protection technology is a comprehensive approach, which can be summarized as: conduction, balanced connection, grounding, current shunting, and shielding. For more information, please refer to relevant literature.
4.5
Lightning has extremely high power; the typical peak value of a current pulse is 10⁴ A, the potential difference of a lightning channel is 10⁷–10⁹ V, and the median power is 10¹² W (1 billion kilowatts). However, the energy of a single lightning strike is not large, lasting approximately 10–40 μs, with a charge of about 20 C (coulombs) and electrical energy of about 10⁹ J (joules), equivalent to a 100W light bulb providing illumination for just over 100 days [4]. Understanding the characteristics of lightning is beneficial for understanding lightning protection technology.
5
The characteristics of electrostatic discharge (ESD) in anti-static grounding are high voltage, low current, short duration, and high power. For instrument systems, the discharge of static electricity from the human body onto the metal casing of electronic devices is the most common ESD phenomenon. Various measures should be taken to suppress or eliminate ESD; besides minimizing the generation of static electricity, timely discharge is one of the effective means. Anti-static grounding for instruments and control systems is relatively simple. The discharge resistance of a static conductor to ground is typically on the order of 10⁴ to 10⁶ Ω; therefore, many relevant documents specify a resistance of 100 Ω for ESD grounding. Furthermore, anti-static grounding can be shared with other grounding systems.
6. DCS and PLC grounding
6.1
The reason why the grounding of distributed control systems (DCS) and programmable logic controllers (PLC) is not a separate grounding category is that, due to various reasons, people have intentionally or unintentionally separated them. There are no separate grounding standards and specifications for DCS and PLC, either domestically or internationally (in fact, there is no need for them at all). Sometimes, due to some misunderstandings or incorrect regulations, it can cause considerable trouble.
6.2
Grounding for DCS (or PLC) equipment can be divided into signal processing and data processing sections. The signal processing section comprises the input and output (I/O) parts of the controller and detectors. This is the same as conventional instruments and falls under instrument working grounding. Therefore, the relevant descriptions (2.0.1, 3.0.1) in the "Petrochemical Instrument Grounding Design Specification" SH3081-1997 are correct. The data processing section includes controllers, operating consoles, engineering workstations, and other processors or network station equipment. These devices are essentially single-board computers, microcomputers, workstations, minicomputers, etc. The grounding of these devices is protective grounding; the grounding of their switching power supplies, motherboards, and other components or boards is either floating or connected to the chassis. Therefore, the grounding method for DCS (or PLC) is also clear.
6.3
Most grounding engineering manuals from DCS manufacturers contain clauses stipulating that grounding should comply with local, national, or international electrical grounding standards and specifications, and then recommend a grounding method based on the DCS (or PLC) manufacturer's local, national, or international standards.
Some Chinese regulations often state that the requirement should be "according to the instrument manufacturer's requirements," which is inappropriate. Foreign standards and regulations rarely stipulate this, and international standards do not include such provisions. Instead, manufacturers must comply with the regulations of local governments, national standards, or international standards and regulations. This is not only a scientific and technological issue, but also, in some fields and aspects, a political and economic one. Such examples are not uncommon and should be taken seriously.
Some manufacturers conduct experiments, research, design, production, and manufacturing in strict accordance with standards such as ISO, IEC, and ISA, and the results can be directly utilized.
6.4
It is clear that an incorrect example of DCS (or PLC) grounding is the same as that of conventional instruments. The example given here is not an illustration, but rather an incorrect approach; please take note.
In the engineering design documents of a certain DCS, AC grounding and the DCS's "main reference ground" are respectively equipped with grounding electrodes, and there is a description of grounding: "The safety grounding system requires a grounding resistance of 0.1 to 5 Ω, and the required value is determined by the following method:
"1. When the equipment has neither a safety barrier nor lightning protection grounding, the AC safety grounding only needs to meet the minimum grounding resistance specified in the local electrical code, which is usually 5Ω."
"2. When the equipment uses a Zener barrier, the AC grounding should be less than 0.1Ω and the main reference ground should be less than 0.9Ω. For the reasons, please refer to the lightning protection grounding section."
"3. When lightning protection is considered for the equipment, each lightning protection grounding rod should have a grounding resistance of up to 0.1Ω."
"In the discussion of lightning protection in this manual, the principle of lightning protection is that the grounding resistance of each lightning protection grounding rod is less than 0.1Ω. Suppose a lightning strike of 100kA passes through a 0.1Ω resistor and generates a potential of 10kV. Experience shows that 10kV will not cause spark discharge or terminal creepage in the cable tray. Without sparks or electromagnetic induction, the system can function perfectly well in the event of a lightning strike."
The document repeatedly states that the grounding resistance of lightning protection grounding should be less than 0.1Ω.
A comparison with these regulations reveals their errors, and the justification given for lightning protection grounding resistance is absurd. These unreasonable regulations have created difficulties for the design work, caused waste in the project and difficulties in construction, and have even become an excuse when the system malfunctions.
07 Related Factors
A specific grounding method is typically used to achieve a particular purpose, such as intrinsically safe grounding in intrinsically safe systems or lightning protection grounding in lightning protection technology. The absence of other methods and components can affect the effectiveness of the grounding system, and may even prevent it from achieving its intended purpose. Therefore, in engineering design, attention must be paid not only to the design of the grounding system itself, but also to other related design aspects.
Many engineering events are random, some even accidental. Accidents often result from the simultaneous action of multiple factors. A grounding fault is a typical example of a multi-factor random event. A flawed engineering design does not necessarily lead to operational accidents, while a correct design may sometimes be mistakenly attributed to an accident caused by other factors that cannot be identified. This illustrates the complexity and randomness of accidents in production processes. Furthermore, the experimentation, simulation, and reproduction of accident phenomena, as well as the identification of their causes, are sometimes extremely difficult. This is why engineering designs aimed at preventing random events, such as grounding systems, are more likely to cause confusion.
This is beyond the scope of this article, but it is indeed a factor that affects the design of grounding projects.
08 Conclusion
8.1
The basic principle of grounding engineering is equipotential bonding, which is the fundamental principle for achieving various grounding purposes and implementing various grounding engineering methods. However, absolute equipotential bonding is impossible. To achieve approximate equipotential bonding, various methods need to be designed and engineering costs need to be incurred.
Grounding engineering is a systematic project, composed of multiple components such as conduction, bonding, equipotential bonding, grounding wires, and grounding electrodes. Defects or faults in any of these components will affect the effectiveness of the grounding system. The effectiveness of grounding engineering is a comprehensive result and cannot be simply characterized by the numerical value of grounding resistance.
The principle for grounding low-frequency signals is single-point grounding. The grounding of instruments and control systems should ultimately be connected to the grounding device of the electrical system.
8.2
Economic Constraints of Grounding Engineering: Grounding engineering must consider economic factors. It is crucial to avoid unilaterally increasing the requirements of a particular indicator or overemphasizing a specific grounding method while disregarding its implementation costs and difficulties. A correct engineering design approach can achieve the purpose of grounding engineering effectively in a simple, easy-to-implement manner and at a relatively low cost.
8.3
Intrinsically safe system grounding: The grounding of intrinsically safe instrument systems should not be a separate grounding system, but should be integrated with the grounding of the electrical system.
8.4
Lightning protection grounding resistance is an impulse resistance.
