Instrument system grounding is generally divided into four types: protective grounding, working grounding, intrinsically safe grounding, and antistatic grounding. The grounding requirements for each type of grounding type vary. Protective grounding requirements 1. The metal casing of the electric instrument and the metal parts of the self-control equipment that are normally uncharged may carry dangerous voltage due to various reasons (such as insulation damage). The following electrical meters and automatic control equipment should be used for protective grounding: 1 instrument panel, instrument console, instrument cabinet, instrument rack and instrument box; 2 instrument control system cabinet and operation station; 3 computer system cabinets and consoles; 4 power supply tray, power supply box, power meter housing, cable tray, protection tube, junction box and armored cable armor. Standard provisions This article proposes some occasions where no protective grounding can be provided. It complies with the power industry standard "Code for Grounding Design of AC Electrical Installations" 4.2. It stipulates the electrical measuring instruments, relays and other low-voltage electrical appliances installed on the power distribution panel, control panel and power distribution device, and the insulator metal base that does not cause dangerous voltage on the support when insulation damage occurs. The metal part may not be grounded. 2. The metal casing of small low-voltage electrical appliances such as buttons, signal lamps, relays, etc. installed on the metal dial of non-explosion-proof occasions can not only be used for protective grounding when it is in good electrical contact with the metal dial frame that has been protected and grounded. Standard provisions This article proposes some occasions where there is no protective grounding, which complies with the power industry standard "grounding design specifications for AC electrical installations") 4.2. It stipulates the electrical measuring instruments, relays and other low-voltage electrical appliances installed on the power distribution panel, control panel and power distribution device, and the insulator metal base that does not cause dangerous voltage on the support when insulation damage occurs. The metal part may not be grounded. 3. Field instruments, transmitters, local switches, etc. that are lower than 36V power supply, no special need or protection grounding. Standard provisions For the regulation of safe voltage values, countries are not exactly the same. I am accustomed to adopting 36V and 12V, and some foreign countries have regulations of 50V and 25V; while some Japanese companies stipulate that power meters below 60V can be used without protective grounding. This specification specifies field instruments, transmitters, local switches, etc. that are powered by less than 36V. If there is no special requirement, it may not be protected. Due to the complexity of the installation in the field, the metal casing of the field instrument below 36V power supply may also be exposed to other power supplies higher than 36V. In this case, the outer casing of these instruments should also be protected and grounded. 4. Self-control equipment that has been protected and grounded can be considered as electrostatic grounding. When using an anti-static raised floor in the control room, ground the static electricity. The electrostatic ground should be used in conjunction with the protective grounding system. Standard provisions According to the chemical industry standard "Chemical Enterprise Electrostatic Collision Design Regulations" HG20675-1990 Section 2.6.3 "Using Electrical Protective Earthing Trunks as Static Grounding Trunks" and Section 2.10.1 "Used in Computer (Electronic Instrument) Room When anti-static raised floor, it should be statically grounded to provide a good electrical path for electrostatic charge. The details about the anti-static of the instrument system are outside the scope of this specification, please refer to the relevant information. Working grounding requirements 1. The working grounding should include signal loop grounding and shield grounding. 2. The signal loop grounding should meet the following requirements: 1 In electronic equipment such as automation systems and computers, non-isolated signals need to establish a uniform signal reference point, and the signal loop should be grounded (usually the negative pole of DC power supply). 2 The isolated signal can be ungrounded. The isolation should be insulated for each input (output) signal and other input (output) signals. The ground is insulated and the power supplies are isolated and isolated. Standard provisions The spare core of the ordinary multi-core signal cable laid overhead is used for shielding grounding to prevent lightning from inducing high voltage on the signal line. 1 Components used in the instrument system to reduce electromagnetic jamming, such as the shield of the cable, the drain wire, and the shield grounding terminal on the instrument shall be shielded and grounded. 2 The spare core of the ordinary multi-core cable without overhead provided by the outdoor overhead should be grounded. 3 The shield of the shielded cable is grounded and the spare core can be ungrounded. 4 Multi-core cable spare core with protective tube can not be grounded. Intrinsically safe grounding requirements 1. Intrinsically safe instruments that require grounding in safety functions should be grounded intrinsically safe according to the requirements of the instrument manufacturer. 2. The bus bar of the Zener barrier should be connected to the common terminal of the DC power supply. The bus bar (or rail) of the Zener barrier should be grounded intrinsically. 3. The isolated safety barrier can be grounded. Standard provisions A. In the intrinsically safe instrumentation system, intrinsically safe related equipment that requires intrinsically safe grounding, such as the negative pole of the shunt diode and the grounding terminal of the safety barrier. B. In order to ensure that the Zener barrier is protected against dangerous places in the event of DC power failure, the safety barrier bus bar must be connected to the common end of the DC power supply. Anti-static grounding requirements 1. Install the control room, cabinet room and process control computer room of the automatic control system and other equipment, and do anti-static grounding. These indoor static conductive ground, anti-static raised floor, workbench, etc. should be grounded anti-static. 2. Instruments and equipment for protective grounding and working grounding have been made, and anti-static grounding is not required. 3. Anti-static grounding for large control room can be set separately with anti-static grounding plate. 4. The metal cable tray should be grounded with anti-static. 5. The non-metallic cable tray should be grounded. KNB6-40 Miniature Circuit Breaker
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3. Shield grounding should meet the following requirements: