Is protective grounding required for the metal enclosures of field instruments?

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This post was last edited by cww9521 on 2023-5-30 at 23:49. I. Relevant regulatory provisions 1. SH/T3521-2013 “Technical Regulations for the Construction of Petrochemical Instrumentation Engineering”. Paragraph 9.5.1: The normally non-electrified metal parts of electrical instruments, such as enclosures, panels, cabinets, boxes, cable trays, protective conduits, supports, and bases, shall all be provided with protective grounding. When the supply voltage is below 36V and there are no special requirements in the design documents, protective grounding may be omitted. Paragraph 9.5.2: The metal enclosures of small low-voltage electrical devices such as buttons, signal lights, and relays installed on metal panels in areas that are not explosive hazard areas may not require protective grounding, provided that they are in good contact with grounded metal panels. 2. Article 3.2 of SH/T3081-2019 “Code for Design of Grounding Systems for Petrochemical Instruments”: Protective grounding is a type of grounding implemented to ensure personal safety and the safety of electrical equipment; it is also referred to as safety grounding. Clause 4.1.3: The exposed conductive parts of instruments and control systems that are in good electrical contact with or conductively connected to already grounded metal plates, platforms, boxes, cabinets, racks, etc., need not be separately protected by grounding. Paragraph 4.1.4: In areas that are not explosive hazard areas, the metal enclosures of field instruments, metal protection boxes, and metal wiring boxes whose supply voltage is below 36V do not require protective grounding; however, those that may come into contact with equipment operating at voltages higher than 36V do need to have protective grounding. Paragraph 4.1.5: In explosive hazardous environments, the metal enclosures, metal protective boxes, and metal wiring boxes of field instruments in non-intrinsically safe systems shall be provided with protective grounding, whereas such enclosures, protective boxes, and wiring boxes for field instruments in intrinsically safe systems need not have protective grounding. Main text of Clause 4.1.6: Protective grounding shall be implemented for the metal enclosures, metal protection boxes, and metal junction boxes of field instruments used for lightning protection. Text of Clause 4.1.7: The metal enclosures of field instruments, metal protection boxes, and metal junction boxes that require protective grounding should be connected to the grounding network in close proximity, or to metal components such as grounded metal cable trays, metal protective conduits, cable armor layers, metal supports, frames, platforms, fences, and other equipment. 3. Article 3.1.1 of HG/T20513-2014 \"Code for Grounding Design of Instrument Systems\": The metal enclosures of electrical instruments and the metal parts of automatic control equipment that are normally not charged may acquire dangerous voltages due to various reasons (such as insulation failure, etc.). The following electrical instruments and automatic control equipment shall be provided with protective grounding: (1) instrument panels, instrument consoles, instrument cabinets, instrument racks, and instrument boxes; (2) cabinets and operating stations of instrument control systems; (3) cabinets and consoles of computer systems; (4) power distribution panels, power supply boxes, external parts of electrical instruments, cable trays, protective conduits, junction boxes, and the armor layer of armored cables. Paragraph 3.1.2: The metal enclosures of small low-voltage electrical components such as buttons, signal lights, and relays installed on metal dials in non-explosion-proof environments need not be individually grounded, provided that they maintain good electrical contact with the metal dial frame, which is already protected by grounding. Paragraph 3.1.3: Field instruments, transmitters, local switches, etc., powered by less than 36V do not require protective grounding unless there is a specific need for it. 4. Article 10.2.1 of GB50093-2013 \"Code for Construction and Quality Acceptance of Automated Instrumentation Projects\": The non-electrified metal parts of field instruments with a supply voltage higher than 36V, such as their enclosures, dashboards, cabinets, boxes, brackets, and bases, shall all be provided with protective grounding. Paragraph 10.2.2: For field instruments and switches with a supply voltage of 36V or less, protection grounding is not required unless specified otherwise in the design documents. Paragraph 10.2.3: The metal enclosures of small low-voltage electrical devices such as buttons, signal lights, and relays installed on metal panels in non-explosion-proof hazardous areas may not require protective grounding, provided that they are in good contact with grounded metal panels. 5. Article 5.5.3 of GB50058-2014 “Code for Design of Electrical Installations in Explosive Atmospheres”: The protective grounding of equipment in explosive atmospheres shall comply with the following provisions: (1) In accordance with the relevant provisions of the current standard “Code for Design of Grounding of AC Electrical Installations” GB/T50065, the following parts that generally do not require grounding must still be grounded in explosive atmospheres: a. The normally non-electrified metal enclosures of equipment with an AC rated voltage of 1,000 V or less and a DC rated voltage of 1,500 V or less, when located on poorly conductive surfaces ； b. In a dry environment, the normally de-energized metal enclosure of equipment with an AC rated voltage of 127 V or less and a DC voltage of 110 V or less ； c. Equipment installed on a grounded metal structure. 6. Article 7.1.1 of GB50257-2014 \"Code for Construction and Acceptance of Electrical Installations in Explosive and Flammable Environments\": The non-live exposed metal parts of electrical equipment in explosive environments, such as its metal enclosures, metal frames, equipment installed on grounded metal structures, metal wiring conduits and their fittings, cable protection pipes, and the metal sheaths of cables, shall all be grounded. The explanatory text states that it has been revised in accordance with the relevant provisions of the current **standard, the ‘Code for Design of Electrical Installations in Explosive Atmospheres’ GB50058; different requirements are set for the installation of grounding wires depending on the different hazardous areas and electrical equipment. It should be noted in particular that the metal enclosures of all electrical equipment in explosive atmospheres must be grounded, whether they are installed on a grounded metal structure or not. To ensure the safety of people's lives and property, clause 7.1.1 is designated as a mandatory provision. 7. Article 9.2.1 of SH/T3164-2012 \"Design Code for Lightning Protection of Instrumentation Systems in Petrochemical Industries\": The metal enclosures of field instruments, instrument protection boxes, wiring boxes, and the metal enclosures of cabinets should be grounded nearby or connected to grounded metal structures. Paragraph 9.2.2: The metal enclosures of field instruments can be grounded through metal mounting brackets or the metal equipment itself. Paragraph 9.2.3: The field instruments on metal equipment, vessels, towers, and operating platforms shall be equipotentially connected to the equipment and operating platforms. Paragraph 9.2.4: Instruments and metal supports located in explosive hazard areas in Zone 1 should be designed to prevent any gaps in connections, thereby avoiding sparks caused by lightning currents. 8.AQ3009-2007 \"Safety Codes for Electrical Explosion Protection in Hazardous Areas\", Clause 6.1.1.4.1: The non-live exposed metal parts of electrical equipment, such as its metal enclosures, metal frames, metal wiring ducts and their accessories, cable protection tubes, and the metal sheaths of cables, shall all be grounded. II. Summary of the regulatory provisions: The three requirements for protective grounding specified in the aforementioned standards SH/T3521, SH/T3081, HG/T20513, and GB50093 are as follows: (1) Protective grounding must be provided for those exposed metal parts of field instruments whose supply voltage is higher than 36 V or that may be subject to hazardous voltages, provided that these parts are not normally charged. (2) For the exposed metal parts of field instruments with a supply voltage not exceeding 36V, protection grounding is not required unless there are special specifications. (3) Metal panels and boards in non-explosion-proof hazardous areas, on which small low-voltage electrical devices such as buttons, signal lights, and relays are installed, are properly grounded; therefore, protective grounding is not required. The requirements specified in the aforementioned standards GB50058, GB50257, and AQ3009 state that the exposed metal parts of electrical equipment, cable protection conduits, junction boxes, etc., in explosive hazardous environments must be grounded. The requirements specified in the aforementioned standard SH/T3164 state that the metal enclosures of field instruments, instrument protection boxes, wiring boxes, and the metal enclosures of cabinets must be grounded. III. Individual analysis based on standard interpretations: (1) The terms “electrical installations” in GB50058, “electrical apparatus” in GB50257, and “electrical equipment” in AQ3009 refer not to electronic devices related to high-voltage electricity in a narrow sense, but rather to electronic devices that are related to electrical energy in a broader sense. Refer to the definitions of the term “electrical equipment” in GB3836.1-2010 and GB12476.1-2013: Equipment that utilizes electrical energy, in whole or in part. Note: These devices include power generation, power transmission, power distribution, energy storage, electrical measurement, regulation, power conversion, electrical consumption equipment, and communication equipment, etc. (2) In the cases specified in SH/T3521, HG/T20513, and GB50093 where the voltage is 36V or less, no protective grounding is required unless otherwise specified. These regulations are in place to ensure personal safety and the safety of electrical equipment. 36V is merely a general threshold for safe voltage; in actuality, the safe voltage can be 48V, 24V, or some other value in certain situations. (3) SH/T3081, GB50058, GB50257, and AQ3009 specify the grounding requirements in explosive hazardous environments. These requirements are unrelated to the protective grounding necessary for personal safety and the safety of electrical equipment as mentioned in 3(2); they constitute safety measures specific to explosive hazardous environments. Under abnormal conditions, when a metal conductor becomes charged and there is a certain gap between it and adjacent metal objects, electric sparks can be generated. If explosive substances and oxygen are present in the environment at that time, an explosion may occur. (4) The SH/T3164 standard specifies grounding measures to prevent direct lightning strikes, backstroke lightning, and induced lightning, thereby avoiding or reducing the impacts that lightning strikes may cause; theoretically, lightning strikes can also generate electric sparks that may ignite explosive materials in the vicinity. IV. Personal opinions are for reference only. (1) Whether in an explosive hazardous environment or not, electrical equipment with a supply voltage of over 36 V, or those that may have hazardous voltages and whose exposed metal parts are normally uncharged, must be provided with protective grounding. (2) In non-explosive hazardous environments, where the voltage is not higher than 36V and there is no possibility of hazardous voltages, protective grounding may not be required. (3) For the purpose of protecting against lightning strikes, the metal parts of electrical equipment should be connected to ground at equal potential. In addition, lightning protection measures include lightning rods, lightning conductors, and the installation of surge protectors. (4) In explosive hazardous environments, regardless of the supply voltage of electrical measuring equipment, all exposed metal parts such as electrical equipment, cable protection conduits, and junction boxes must be grounded. (5) In explosive hazardous environments, electrical measuring instruments whose bodies or enclosures have obvious grounding marks shall be grounded. Theoretically, the metal enclosures of intrinsically safe devices in intrinsically safe circuits do not need to be grounded. According to Clause 6.3 of GB/T3836.15-2017, \"The metal enclosures of intrinsically safe devices or devices with energy limitation do not need to be connected to an equipotential bonding system, unless specified in the device documentation or required to prevent the accumulation of static electricity.\" Clause 4.1.4 of SH/T3081-2019 states that \"In non-explosive environments, where the supply voltage is below 36 V, the metal enclosures of field instruments, metal protection boxes, and metal wiring boxes do not require protective grounding; however, such enclosures that may come into contact with devices operating at voltages higher than 36 V do need to be grounded.\" In explosive hazardous environments, grounding should also be implemented for lightning and static electricity protection, as well as to ensure consistency with the overall engineering design. Automatic control instruments