Grounding of instrumentation and control systems is not a new topic. Many problems have early conclusions and correct design methods. However, some engineers and technicians still have some vague concepts and doubts. The role of grounding and the classification of grounding have been discussed in many literatures. Different methods can have different classifications and they all make sense. This paper will not discuss them. This article mainly discusses how to do grounding design, and why. The purpose of instrumentation and control system grounding is mainly two: First, for personal safety and the operation of electrical equipment, including protective grounding, intrinsically safe grounding, anti-static grounding and lightning protection grounding; the second is for signal transmission and anti-jamming work Grounding. But the two are related and cannot be completely separated. Regarding the instrument system grounding, China has not yet formulated a corresponding national standard. However, the relevant regulations in the national standards for protection grounding and lightning protection grounding in the electrical industry can be implemented by reference. The relevant standards of international organizations such as IEC and ISA provide a good reference. In particular, the functional grounding and protective earthing of information technology devices through equipotential bonding and combined grounding provide designers with an authoritative and clear basis for engineering design. 01 Protective grounding Protective earthing is a grounding (also called safety grounding) set up for personal safety and electrical equipment safety. The instrument's professional protective earthing is the same as the electrical professional protective earthing and belongs to the low voltage distribution system grounding. Therefore, it should be related to the electrical professional. Standards, specifications and methods are carried out. For example: GBJ65-83 "Grounding Design Specification for Industrial and Civil Power Devices" and so on. For the low-voltage distribution system grounding, the electrical professional has a series of well-developed standard specifications for design, calculation, test, construction and acceptance. It has complete theoretical, experimental and methodological aspects of the grounding system, and is definitely not a grounding resistance. The value can be summarized. The professional power consumption of the instrument is generally from uninterruptible power supply UPS or electrical building distribution, which can be roughly divided into power consumption in the control room and power consumption in the field. TN-S system is generally used for control room electricity (protection line and center line are separate in the whole system) [1]. Field instrument electricity generally uses TT system (distributed grounding). According to the principle of equipotential bonding, the protective grounding of the instrument's electricity should be an electrical grounding system. Not only the equipotential bonding is implemented in buildings, but petrochemical plants are generally also equipped with all-equipment equipotential bonding. Grounding works should be designed in accordance with the standards and methods of electrical specialties. Some designs separate the protective ground of the UPS-powered instrument system from the grounding system alone, which is inappropriate. One of the two power supplies of most UPSs is directly switched over without transformer isolation, which makes it impossible to have a separate grounding system. In addition, other distribution systems within the building (such as lighting distribution, maintenance distribution, etc.) are low-voltage power distribution systems that are specialized in electrical engineering, and are not power supplies for meters that come out of UPS. In this way, there are two grounding systems in the same building, and it is impossible to avoid the event of simultaneous contact, which violates the “distributive conductive part that can reach at the same time and should be connected to the same grounding system†in the electrical professional specification. Grounding regulations. Both the complete isolation of the two grounding systems cannot be achieved, and the equipotential connections within the building cannot be achieved at the same time, creating an unsafe factor. 02 Instrument working ground The purpose of instrumentation and control system work grounding is to resist interference. Many articles in this issue have been discussed very clearly. They are correct and feasible in theory, practice and method. This article is not repeated. Instrumentation and control system work ground can be divided into shield grounding, instrument signal grounding, etc. from the project. 2.1 Shield ground Instrument shield grounding is divided into two types. One is the grounding of cable protection tubes, cable trays, etc. This type of grounding should be connected to the device's electrical grounding network, which is an equipotential connection. The other is grounding of the signal shielded cable. Different connections should be made according to the different conditions of the signal source and the receiving instrument. For example, the common transmitter internal circuits are mostly ungrounded, so the signal shielded cable is generally grounded on the side of the control room. Signal shielding cable grounding should be single-point grounding. From the role of shielding can be divided into: electric field shielding, magnetic shielding, electromagnetic field shielding, etc., in order to solve the interference problem. Electrostatic shielding is electrostatic shielding to solve the problem of interference generated by distributed capacitance. The use of high-conductivity materials should be grounded. The magnetic shield uses a material with a high magnetic permeability and requires the magnetic circuit to be closed. When the frequency is low, it may not be grounded. Electromagnetic field shielding against all kinds of electromagnetic radiation interference, using low-resistance materials, the shield can be grounded or not grounded. 2.2 Instrument signal grounding Instrumentation signals are divided into isolated signals and non-isolated signals. Isolated signals can generally not be grounded. The isolation here should be that the circuit of each input signal (or output signal) is insulated from the circuitry of other input signals (or output signals), is isolated from ground, and the power supply is isolated from each other. Non-isolated signals are usually referenced to the negative side of the 24VDC power supply and grounded. Signal allocation is used as a reference point. The common mode rejection voltage of this circuit is usually very small, and grounding is the main measure to eliminate such interference. When designing grounding engineering, care should be taken to avoid voltage drops on the ground when the equipment is in operation and to interfere with signals. Different series of conventional meters have different ground connection regulations. This is because the signal transmission between the secondary instrument of the conventional instrument is more complicated. For example: Regarding I-Series instrument signal grounding, it is described in detail in "I-Series Electronic Meter System Design Guide." Instrumentation signal common grounding, distributed control system (DCS), and non-isolated input grounding of the programmable logic controller (PLC) should all be connected to the ground plane summary board from the connection terminal strip or bus bar. This is essentially an equipotential connection. EK series meters are typical common ground meters. Non-isolated signal grounding of the meter, although ultimately connected to the electrical ground, should not be directly mixed with the electrical ground. The wires of the instrument's working ground should use stranded copper core insulated wires. Before connecting to the grounding summary board, the grounding wires and grounding bus bars should be insulated except the normal connection points. The final connection to the grounding body or grounding grid is separately wired from the grounding strap. The grounding of conventional instruments such as the DDZ-III type instrument, the EK series instrument, the I series instrument and the YS80 series instrument is finally connected with the electrical ground. The vast majority of instrumentation and control system signals are low-frequency signals. The principle of low-frequency signal grounding is single-point grounding. There is no special requirement for grounding resistance. A ground loop should be avoided in the signal loop. If both the signal source and the receiving instrument on a line are unavoidably grounded, an isolator should be used to isolate the two points from the ground. 2.3 Electronic information equipment grounding and protective earthing ground electrode The national standard GB50174-93 "Code for design of computer rooms", Clause 6.4.3 stipulates that four types of grounding devices such as AC work grounding, safety protection grounding, DC working grounding, and lightning protection grounding shall share a group of grounding devices, and the grounding resistance shall be The minimum is determined. Although the scope of application of the GB50174-93 standard does not include the industrial control room and the computer room, some of them can be referred to. The IEC standard "grounding and equipotential bonding of information technology equipment" (IEC364-5-548-1996) clearly states that the functional grounding and protective earthing of information technology equipment are connected via equipotential bonding and grounding. The scope of application includes: information technology devices, devices requiring data exchange, devices for data exchange, data processing devices, signal devices with ground return paths in buildings, communication networks for information technology devices with DC power supply in buildings, and local areas. Communication networks, fire alarm systems, and intrusion alarm systems, such as building services equipment, computer-aided manufacturing (CAM), and other computer-aided service systems such as direct digital control systems. The standard also stipulates that the bus bars that are allowed to be connected to the grounding summary conductor are: shields of telecommunication cables or devices, ground busbars of overvoltage protection devices, ground busbars of radio communication antenna systems, and DC power supply systems of information technology devices. Ground busbars, functional ground busbars, etc. IEEE Std 11000-1992 stipulates that it is not recommended to use any kind of so-called separate, independent, insulated, dedicated, clean, stationary, signal, computer, electronic, or other such incorrect earth grounding body as A connection point for the equipment ground conductor. 2.4 Equipotential principle Maxwell advocates the principle of lightning protection of the Faraday cage, not only can not be grounded, but also safer and more economical than the current method, which is the principle of equal potential. High-voltage electrification uses the principle of equipotentiality instead of insulation protection. The IEC standard "grounding and equipotential bonding of information technology equipment" stipulates that equipment grounding and protective grounding are used in conjunction with equipotential bonding, and there is no requirement for the size of the grounding resistance. The equipotential connection is one of the effective measures to prevent the influence of the interference signal. At this time, the magnitude of the grounding resistance has no effect on the information equipment, and the core technology is the equipotential connection. In military and communications, mobile devices only connect to the fuselage without connecting to the ground, and cannot meet the various requirements for grounding resistance, but they can work safely, normally, and reliably. It is the result of using the principle of equipotential bonding. “From the stand-alone grounding to the equipotential bonding, we have already achieved consensus in the international electrical academic community and wrote IEC standards, ISA standards, and standards of some developed countries. Many manufacturers’ product data have been properly stipulated or have been made. Modifications, but some manufacturers continue to use grounding requirements for separate grounding and harsh resistance, which may be the current regulations that do not understand the standard, or for some commercial purpose."[2] The national standard GB50174-93 "Code for the design of computer rooms", Clause 6.4.5 states that: The grounding of the computer system should adopt a single point of grounding and should take equipotential measures. For the automatic control professional, the protective grounding, instrumentation grounding, and intrinsically safe system grounding are finally connected together on the grounding busbar, and the grounding electrodes are used together to achieve equipotential bonding. 03 Intrinsically safe system grounding The safety barrier is divided into two types: isolated and Zener. Isolated barriers use isolation protection technology and do not require special grounding, while Zener barriers require a good grounding system based on their protective working principle. Intrinsically safe system grounding is usually discussed in the Zener safety barrier grounding problem. There are two kinds of power failures in the non-intrinsically safe area. One is a DC short circuit. Normally, two-wire or three-wire transmitters are powered by 24 to 30 V DC power supplies. Therefore, the safety grid grounding must be common to the DC power supply. Connected; the other is the AC short circuit, in order to achieve the protection function, safety barrier grounding must be connected to the AC power supply neutral. This determines that the grounding of the safety barrier should ultimately be the electrical system grounding. The simplest and most reliable method of connecting the safety bar ground bar to the starting point of an AC-powered neutral line is to use a wire connection. IEC standard IEC 60079-14 "Electrical Equipment Installation in Hazardous Locations" states that intrinsically safe circuits are grounded: For grounding terminals that do not have an electrically isolated barrier (such as a Zener barrier), they should: 1) be the shortest possible The path is connected to the equipotential bonding system; or 2) For the TN-S system, a highly complete grounding point is connected in such a way that the impedance between this point and the ground point of the main power system is less than 1Ω.†However, some companies use the earth as a conductor, which makes some designers mistakenly believe that this is a formal and reasonable method, which leads to improper engineering design. Therefore, the waste in the project and the construction troubles are caused, and the hidden danger of the safety barrier is also formed. The method of using the earth as a conductor was first seen in the two-wire system of power transmission, and the grounding specification of power equipment emphasizes that it is forbidden to utilize the earth as the phase or neutral wire in the low-voltage power network. At present, the use of ground as a conductor is only seen in the grounding of the TT system. However, there are a lot of engineering designs that use separate ground poles for the Zener type safety barrier grounds, and it is precisely this way that ground potential breaks through the safety barriers. This is one of the root causes of the damage caused by the Zener-type safety barrier due to grounding problems. Therefore, the author believes that this way of setting the grounding electrode separately for the Zener-type safety grid ground is also written in some standard specifications (such as the "Petrochemical Instrumentation Grounding Design Specification" SH3081-1997), which is worthy of further discussion. ISA-RP12.6-1995 "Earthing Implementation of Instruments in Hazardous Locations" Part I: "Intrinsic Safety" stipulates that the connection resistance between the barrier grounding bus bar and the neutral point of the AC power supply is less than 1Ω, and is clearly given Direct connection icon. MTL's grounding guideline also stipulates this way, just one more sentence: "If it can reach 0.1Ω more appropriate." And mention: Connect with wires is the easiest way. As for the resistance to earth, there are no provisions in the above information. Some barrier companies have only vaguely stated that they are generally recommended to be 1Ω, but if they are consulted, they do not. It should be noted that in foreign sources, Earthing or Grounding is called Earth Bonding and Bonding is grounded. The meaning is not the same. Basically, the grounding resistance (Bongding resistance) is basically stated in the information concerning the grounding resistance of the intrinsically safe instrument. Foreign data only attaches importance to the grounding resistance. ISA-RP12.6-1995 and MTL's grounding data all propose a method of repetitive connection using two grounding conductors in order to measure the bonding resistance instead of measuring the grounding resistance. Earthing resistance. The signal terminal of the on-site intrinsically safe instrument is generally not grounded, and the purpose of the instrument housing grounding is not for intrinsic safety. In addition, the ground potential acts only on the insulation of the grounded transmitter of the housing and does not achieve the degree of insulation of the field instrument. Some literature ground the transmitter housing ground as a signal terminal. It is not correct to discuss the breakdown of the barrier by the potential difference between the grounding point of the enclosure and the Zener barrier grounding point of the non-hazardous location. If the field end of the instrument signal is inherently grounded, the loop forms two points of ground, and the ground potential difference may act on the safety barrier. In this case, the use of a Zener barrier is not correct, and an isolated barrier should be used to prevent multipoint grounding. This will not only meet the requirements of signal transmission, but also meet the requirements of intrinsic safety. 04 Lightning grounding When discussing the lightning protection and grounding of instrumentation and control systems, it is necessary to discuss the lightning protection engineering design of the instrument and the control system. Because the lightning protection of the instrument and control system is only an integral part of the lightning protection engineering of the instrument and control system. Literature [3] has discussed this issue in more detail. This article is not designed to discuss the lightning protection of the instrument. It only supplements the lightning protection and grounding of the instrument. 4.1 National standard The national standard GB50057-94 "Code for the design of lightning protection in buildings" provides a good basis and reference. Although the standard does not directly stipulate lightning protection design for electronic equipment, it also makes some explanations. The GB50057-94 standard provides protection against direct lightning strikes, lightning strikes, and lightning surges. IEC1024-1-1993 classifies lightning protection into external lightning protection and internal lightning protection. External lightning protection is to prevent direct lightning, internal lightning protection including lightning induction, counter-attack, lightning wave intrusion and prevent life-threatening, and the core method is the equipotential connection. In the description of the clauses of No. 5 and No. 6 of Article 3.2.4 of GB50057-94 standard, the equipotential connection between direct lightning strike and inductive lightning protection is pointed out: “From the viewpoint of lightning protection, common grounding is preferred. The device, which is suitable for all grounding purposes (eg lightning protection, low voltage power systems, telecommunication systems), "The arrangement and size of the grounding device is more important than the specific value of the grounding resistance." 4.2 About grounding resistance The national standard GB50057-94 "Lightning Protection Design Code for Buildings" stipulates that the lightning protection grounding resistance is the impact grounding resistance, and the relationship between the impact grounding resistance and the power frequency grounding resistance is given. The ratio of the impact grounding resistance to the power frequency grounding resistance is called the impact factor. There is a breakdown phenomenon when the lightning strikes the ground, there is a spark effect, and the resistance exhibited by the grounding electrode is the impact grounding resistance; the scattering resistance when the lightning strikes the ground is a nonlinear resistance, which is related to the peak value and the waveform of the lightning current, so it cannot be applied in a narrow sense. Ohm's law. It is irresponsible to reduce the value of ground resistance at will. The provisions of grounding resistance should be based on the fact that this directly affects grounding engineering. For example, before the publication of the "Regulations for Electrical Equipment Grounding Devices" of the Ministry of Water and Electricity in 1959, the electrical equipment grounding resistance was 0.5Ω. For this reason, many power plants consume a large amount of steel, and it is said that some of them reach 10 to 40 tons, and the grounding device area is 100×100 m2. Every year, in order to maintain and improve grounding resistance, it is necessary to continue burying several tons of steel. After several years, it has buried dozens of tons and it is no longer useful to bury it again. Later, a balanced grounding design method was adopted to solve the contradiction of human injury. The grounding resistance of China's current national standard is specified as 4Ω. Lightning grounding resistance should be in accordance with the national standard GB50057-94 "Lightning Protection Design Code for Buildings," and should not be arbitrarily reduced. 4.3 Grounding system maintenance The grounding resistance is usually not constant and the grounding device cannot be used once and for all. The grounding system should be regularly inspected and maintained. Faults such as corrosion, breakage, and damage should be promptly discovered and repaired in a timely manner to maintain the integrity of the entire system, especially the integrity of the grounding connection. Numerous examples have proved that it is very important for the inspection and maintenance of mine protection projects. 4.4 Modern Lightning Protection Technology Is Comprehensive Prevention Technology The lightning protection of the instrument system must not rely solely on grounding. It cannot be simply considered that the grounding will solve the problem, and the buried cable will also be struck by lightning, but only that it is less likely to suffer a lightning strike than an overhead cable. Modern lightning protection technology is a comprehensive prevention and control technology, summed up are: conduction, balanced connection, grounding, shunting, shielding and so on. This can be found in relevant literature. 4.5 Lightning energy The power of thunder and lightning is extremely high, the typical value of the current pulse peak is 104A, the potential difference of the lightning channel is 107 to 109V, and the median power is 1012W (1 billion kilowatts), but the energy of a lightning is not large, and the time is about 10 to 40μs. The charge is about 20C (coulomb) and the power is about 109J (joules), which is equivalent to 100W bulb lighting for more than 100 days [4]. Understanding the characteristics of lightning helps to understand lightning protection technology. 5 Anti-static grounding Electrostatic discharge is characterized by high voltage, low current, short time, and high power. For meter systems, the discharge of static electricity from the human body on the metal housing of electronic devices is the most common phenomenon of electrostatic discharge. To suppress or eliminate electrostatic discharge, various measures should be taken. In addition to avoiding static electricity as much as possible, discharging static electricity in time is one of the effective means. The anti-static grounding of the instrument and control system is relatively simple, and the discharge resistance of the electrostatic conductor to the ground is usually of the order of 104 to 106 Ω. Therefore, many corresponding data regulations stipulate that the resistance used for electrostatic grounding is 100 Ω. Also, anti-static grounding can be shared with other grounding systems. 6 DCS and PLC ground 6.1 cause The grounding of decentralized control system (DCS) and programmable logic controller (PLC) is not originally a separate grounding classification, but for various reasons, it has been intentionally or unintentionally separated from it. Domestically and internationally, there is no separate DCS. The grounding standard specification compiled by PLC and PLC (which is actually not necessary at all) is sometimes caused by some misunderstandings or incorrect regulations. 6.2 Grounding basis DCS (or PLC) equipment can be divided into signal processing and data processing. The signal processing part is the input and output (I/O) part of the controller and detector. This is the same as the conventional meter and belongs to the instrument working ground. Therefore, the relevant narrative (2.0.1, 3.0.1) of the “Specifications for the Design of Petrochemical Instrument Grounding†SH3081-1997 is correct. The data processing part includes controllers, consoles, engineer stations and other processors or network site equipment. The essence of these devices is a single board computer, a microcomputer, a workstation, a minicomputer, and the like. The grounding of these devices is a protective grounding, and the switching power supply, main board or other device or board card is grounded, or is floating or connected with the chassis. Therefore, the DCS (or PLC) grounding can also be defined. 6.3 Relevant provisions In DCS manufacturers' grounding engineering manuals, most of them have provisions that should comply with local government, national, or international standards for electrical grounding and specifications, and then recommend a local government, national, or international standard based on DCS (or PLC) manufacturers. Grounding method. Some regulations in our country are often written as "according to the requirements of the instrument manufacturer". This is not appropriate. Foreign standards and regulations rarely stipulate such regulations. International standards do not have such a regulation at all, and all manufacturers must comply with local government, national or international standards and norms. This is not only a scientific and technical issue, but in some areas or in some aspects even political and economic issues. Such examples are not uncommon and should be taken seriously. Some manufacturers conduct experiments, research, design, production, and manufacture in strict accordance with the standards of ISO, IEC, and ISA, and their results can be used directly. 6.4 A wrong instance The DCS (or PLC) grounding method is the same as a conventional meter, which is already clear. The example given here is not a demonstration, but an incorrect way. Please pay attention to the reader. In the engineering design data of a DCS, the AC ground and DCS's “primary reference ground†are respectively provided with a grounding electrode, and there is a description of the grounding: “The safety grounding system is required to provide a grounding resistance of 0.1 to 5Ω, and the required values ​​are as follows: Way to decide: "1. When the equipment has neither a safety barrier nor a lightning protection ground, the AC safety grounding only needs to meet the minimum grounding resistance specified in the local electric code, usually 5Ω. "2. When the device uses a Zener safety barrier, the AC grounding should be less than 0.1 Ω and the primary reference ground is less than 0.9 Ω. The reason for this is shown in lightning protection grounding. "3. When the equipment is considered for lightning protection, each lightning protection grounding rod should have a maximum ground resistance of 0.1Ω. “In the discussion of lightning protection in this manual, the lightning protection principle is that the grounding resistance of each lightning protection grounding rod is less than 0.1 Ω. If a lightning strike of 100 kA is applied and a resistance of 0.1 Ω is applied, a potential of 10 kV will be generated. Experience shows that 10 kV does not Causes spark discharge in the wire groove or creepage of the terminal. Without spark or electromagnetic induction, the system can work perfectly when lightning strikes." In this document, the grounding resistance of lightning protection grounding is less than 0.1Ω. It is not difficult to find the mistakes in comparison with these regulations, and the grounding of lightning protection grounding resistance is absurd. These unreasonable regulations have brought difficulties to the design work, resulting in engineering waste and difficulties in construction, and even become a pretext for the failure of the system. 07 relevant factor A type of grounding is usually one of the methods used to achieve a certain purpose, such as intrinsically safe grounding in an intrinsically safe system, lightning protection grounding in a lightning protection technology, and the like. The lack of other methods and links will affect the effect of the grounding system, or even the intended purpose of the grounding system. Therefore, in the engineering design, not only should pay attention to the design of the grounding system, but also pay attention to other related designs. Many engineering matters are random events, and some are accidental events. The occurrence of an accident is often the result of a combination of factors. Faults caused by grounding are typical multi-factor random events. An erroneous engineering design does not necessarily lead to an operational accident. The correct engineering design, but sometimes the accident caused by other reasons cannot be ascertained and is mistaken for the design. This is the complexity and randomness of accidents in the production process. It is sometimes very difficult to test, simulate, reproduce, and find out the cause of accidents. This is also the reason why engineering designs such as grounding engineering that prevent certain random events from happening are more confusing. This is beyond the scope of the issues discussed in this paper, but it is indeed a factor that affects ground engineering design. 08 in conclusion 8.1 Basic principles of grounding engineering The basic principle for the various purposes of grounding and various methods of implementing grounding engineering is equipotential bonding, and absolute equipotential bonding is not possible. In order to achieve an approximate equipotential connection, multiple methods need to be designed and paid Project cost. Grounding engineering is system engineering. It consists of conduction, overlap, equipotential plates, grounding lines, and grounding poles. The failure of each link will affect the effectiveness of the grounding system. The effectiveness of grounding projects is also an integrated result and cannot be simply characterized by the value of the grounding resistance. The principle of low frequency signal grounding is single point grounding. The grounding of the instrumentation and control system should eventually be connected to the grounding device of the electrical system. 8.2 Earthwork economic constraints The grounding project must take into account economic factors, and it must not raise one aspect of the index or overemphasize one grounding mode regardless of the cost and difficulty of its implementation. The correct engineering design method can achieve the purpose of grounding engineering in a simple and easy manner and at a relatively low cost. 8.3 Intrinsically safe system grounding Intrinsically safe instrumentation system grounding should not be a separate grounding system and should be combined with the electrical system grounding. 8.4 Lightning grounding resistance Lightning grounding resistance is impact resistance. This 500w FM Transmitter has an audio output port that can be connected with your computer to the car's cigarette lighter power. 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