Is the metal shell of the field instrument such as the instrument shell and the junction box protected and grounded?
In response to this question, the following is analyzed according to the normative provisions.
01. Relevant normative provisions
01. SH/T3521-2013 "Technical Regulations for the Construction of Petrochemical Instrument Engineering"
Clause 9.5.1 Text: The shell, instrument panel, cabinet, box, box and cable tray, protective tube, bracket, base and other normal non-charged metal parts of the electrical instrument shall be protected and grounded. When the power supply voltage is lower than 36V, when there is no special requirement in the design file, the protective grounding can be omitted.
Clause 9.5.2 Text: The metal shell of small low-voltage electrical appliances such as buttons, signal lights, relays and other small low-voltage electrical appliances installed on metal disks and plates in non-explosive hazard areas may not be protected and grounded when they are in good contact with the grounded metal disks and plates.
02. SH/T3081-2003
Clause 2.1.1 Body: Protective grounding (also known as safety grounding) is a grounding set up for personal safety and the safety of electrical equipment. The exposed conductive part of the instrument and control system is not charged under normal conditions, and may have dangerous voltage when it is faulty, damaged or abnormal, and such equipment should be protected and grounded.
Clause 2.1.2 Text: Field instruments with power supply below 36V may not be protected and grounded, except for those that may be in contact with equipment with voltage higher than 36V.
Clause 2.1.3 Text: When the instrument installed on the metal dashboard, box, cabinet and frame has good electrical contact with the metal dashboard, box, cabinet and frame that has been grounded, it may not be protected and grounded.
03. HG/T20513-2014 "Code for Grounding Design of Instrument System"
Clause 3.1.1 Text: The metal shell of the electrical instrument and the metal part of the automatic control equipment that is normally not charged may have dangerous voltage due to various reasons (such as: insulation damage, etc.). The following electrical instruments and automatic control equipment should be protected and grounded:
(1) Instrument panel, instrument console, instrument cabinet, instrument rack and instrument box;
(2) Instrument control system cabinet and operation station;
(3) Computer system cabinet and console;
(4) The armor sheath of the power supply panel, power supply box, electrical instrument shell, cable tray, protective pipe, junction box and armored cable.
Clause 3.1.2 Text: The metal shell of small low-voltage electrical appliances such as buttons, signal lights, relays, etc., installed on the metal dial of non-explosion-proof occasions, can be used for protective grounding only when it is in good electrical contact with the metal dial frame for which it has been protected and grounded.
Clause 3.1.3 Text: Field instruments, transmitters, local switches, etc. with power supply below 36V do not need to be protected and grounded without special needs.
04. GB50093-2013 "Code for Construction and Quality Acceptance of Automated Instrument Engineering"
Clause 10.2.1 Text: The shell, dashboard, cabinet, box, bracket, base and other normally uncharged metal parts of the field instrument with a power supply voltage higher than 36V shall be protected and grounded.
Clause 10.2.2 Text: Field instrument switches with a power supply voltage of no higher than 36V, etc., when there are no special requirements in the design documents, protective grounding may not be done.
Clause 10.2.3 Text: The metal shell of small low-voltage electrical appliances such as buttons, signal lights, relays and other small low-voltage electrical appliances installed on metal disks and plates in non-explosion-proof hazardous areas may not be protected and grounded when they are in good contact with the grounded metal disks and plates.
05. GB50058-2014 "Code for the Design of Electrical Installations in Explosion Hazardous Environments"
Clause 5.5.3 Text: The protective grounding of equipment in explosive atmospheres shall comply with the following provisions: (1) In accordance with the relevant provisions of the current national standard "Code for Grounding Design of AC Electrical Installations" GB/T50065, the following parts that do not need to be grounded shall still be grounded in explosive atmospheres:
a. At the poor conductive ground, the equipment with an AC rated voltage of less than 1000V and a DC rated voltage of 1500V and below is a normal and uncharged metal shell;
b. In a dry environment, the normal and uncharged metal shell of the equipment with AC rated voltage of 127V and below and DC voltage of 110V and below;
c. Equipment installed on grounded metal structures.
06. GB50257-2014 "Code for Construction and Acceptance of Electrical Installations in Explosion and Fire Hazard Environments of Electrical Installation Engineering"
Clause 7.1.1 Text: Non-electrified bare metal parts of electrical equipment in explosion hazard atmospheres, metal frames, equipment installed on grounded metal structures, metal wiring pipes and their fittings, cable protection pipes, metal sheaths of cables, etc., shall be grounded.
Its provisions are explained: according to the relevant provisions of the current national standard "Code for the Design of Electric Installations in Explosion Hazardous Environments", the relevant provisions of the GB50058 are revised, and the setting of the grounding wire is treated differently according to different dangerous areas and different electrical equipment. Special care should be taken that the metal enclosure of all electrical equipment in an explosion hazard atmosphere, whether or not mounted on a grounded metal structure, should be grounded. In order to ensure the safety of people's lives and property, Article 7.1.1 is listed as a mandatory provision.
07. SH/T3164-2012 "Code for Design of Lightning Protection Engineering of Petrochemical Instrumentation System"
Clause 9.2.1 Body: The metal shell of the field instrument, the instrument protection box, the junction box and the metal shell of the cabinet shall be grounded nearby or connected with the grounded metal body.
Clause 9.2.2 Body: The metal enclosure of the field instrument can be naturally grounded by means of a metal mounting bracket or a metal device.
Clause 9.2.3 Body: Field instruments on metal equipment, vessels, towers and operating platforms shall be equipotential connected to the equipment and operating platforms.
Clause 9.2.4 Text: Instruments and metal brackets located in explosion hazard locations in Zone 1 shall be protected from connection gaps and sparks caused by lightning currents.
08. AQ3009-2007 "Electrical Explosion-proof Safety Code for Hazardous Places"
Clause 6.1.1.4.1 Text: The metal shell, metal frame, metal wiring pipe and its accessories, cable protection pipe, metal sheath of the cable and other non-live bare metal parts of electrical equipment shall be grounded.
02. Summary of relevant normative provisions
The 3 protective grounding requirements given in the above specifications SH/T3521, SH/T3081, HG/T20513 and GB50093:
(1) If the power supply voltage is higher than 36V or the exposed metal part of the field instrument with a dangerous voltage is normal and not electrified, it shall be protected and grounded.
(2) If the power supply voltage is not higher than 36V, the exposed metal part of the field instrument can not be protected and grounded when there are no special requirements.
(3) The metal disks and plates of small low-voltage electrical appliances such as metal disks in non-explosion-proof hazardous areas, buttons, signal lights, relays installed on the boards have been well grounded, and protective grounding is not required.
The requirements given in the above specifications GB50058, GB50257 and AQ3009: Exposed metal parts such as electrical equipment, cable protection pipes, junction boxes, etc., in explosive hazardous atmospheres should be grounded.
The requirements given in the above specification SH/T3164: the metal shell of the field instrument, the instrument protection box, the junction box and the metal shell of the cabinet should be grounded.
03, Analysis
(1) The "electrical installation" in the GB50058, the "electrical installation" in the GB50257 and the "electrical equipment" in AQ3009 are not "strong current" electronic equipment in the narrow sense, but electronic equipment related to electrical energy in the broad sense. See GB3836.1-2010 and GB12476.1-2013 for the definition of the term "electrical equipment": equipment that utilizes electrical energy in whole or in part. Note: These equipment include power generation, transmission, distribution, power storage, electrical measurement, regulation, conversion, electrical equipment and communication equipment.
(2) SH/T3521, SH/T3081, HG/T20513 and GB50093 not higher than 36V, when there are no special requirements, no protective grounding can be done. This is for the sake of personal safety and the safety of electrical equipment, 36V is just a general sense of the safety voltage, in fact the safety voltage may be 48V in some cases, or it may be 24V or others.
(3) GB50058, GB50257 and AQ3009 are grounding requirements in explosive hazardous environments, and this grounding requirement has nothing to do with the protective grounding required by 3 (2) for personal safety and electrical equipment safety, and belongs to the safety precautions for explosive hazardous environments. Under abnormal circumstances, the metal conductor is electrically charged, and if there is a certain degree of gap with the adjacent metal body, an electric spark can be generated, and if there are explosive substances and oxygen in the environment at this time, an explosion may occur.
(4) SH/T3164 specification is for the protection of direct lightning, anti-counterattack lightning, anti-induction lightning, to avoid or reduce the possible impact of lightning strikes and the required grounding, theoretically lightning strikes may also produce electric sparks and ignite the surrounding explosive substances.
04. In addition
(1) Regardless of whether it is in an explosive hazardous environment or not, if the exposed metal part of the electrical equipment with a power supply voltage higher than 36V or a dangerous voltage is normally not electrified, it shall be protected and grounded.
(2) In a non-explosive hazardous environment, if it is not higher than 36V and cannot have a dangerous voltage, it can not be protected and grounded.
(3) For the purpose of lightning protection, equipotential grounding should be considered for the metal part of electrical equipment. In addition, lightning protection measures include lightning rods, lightning belts, and the installation of surge protectors.
(4) In an explosive hazardous environment, regardless of the power supply voltage of electrical equipment, exposed metal parts such as electrical equipment, cable protection pipes, and junction boxes should be grounded.
Theoretically, the metal shell of the intrinsically safe equipment of the intrinsically safe circuit can not be grounded, see Article 6.3 of GB/T3836.15-2017, which requires that "the metal shell of the intrinsically safe equipment or energy-limiting equipment does not need to be connected to the equipotential connection system, except when the equipment documents stipulate or there are requirements for anti-static charge accumulation". It can be seen that for the sake of lightning protection and anti-static considerations, and the consistency of the project, grounding should also be done.