【Daily Question 20090302】Which devices should be provided with protective grounding?

Almost all electrical devices need to be grounded

Those with metal casings and a relatively high power rating are generally grounded

DCS control cabinet and equipment power transmission control cabinet!

Our company has installed protection for all large electrical equipment. Moreover, grounding protection is also implemented for all three power supplies.

　　Protective grounding is suitable for ungrounded power grids. In such power grids, all metal parts that may develop dangerous voltages due to insulation failure or other reasons must be grounded, unless otherwise specified! Connecting the enclosures, frames, and supports of electrical equipment, which are not charged under normal conditions but may become charged in the event of a fault, to the ground through grounding is known as protective grounding. The function of protective grounding is to establish a good metallic connection between the non-live metal parts of electrical equipment and the grounding electrode, thereby reducing the voltage at those connections with respect to the ground and preventing the risk of electric shock to humans.

Protective grounding in the control room includes the operation console, control cabinets, etc.!

Electrical equipment and main control machinery, operating system

Control cabinets, distribution boxes, etc. with metal enclosures: lol

Everything in the factory is grounded; if you don’t believe it, ask a welder and you’ll find out. As required, all equipment must be grounded to prevent the generation of static voltage.

All electrical equipment must be reliably grounded

1. Technical requirements for grounding systems 1.1 Grounding systems in substations (distribution stations) ① The grounding electrodes of the grounding systems in substations (distribution stations) should be laid horizontally. Its grounding electrode is made of round steel with a length of 2.5 m and a diameter of not less than 12 mm, or angle steel with a thickness of not less than 4 mm, or steel pipe with a thickness of not less than 4 mm; these elements are connected to form a closed loop using flat steel with a cross-section of not less than 25 mm × 4 mm, and the outer corners of this loop are designed to be curved. ②The grounding electrode should be buried outside the wall of the substation (distribution station), at a distance of not less than 3 meters. The burial depth of the grounding grid should exceed the thickness of the local permafrost layer, with a minimum burial depth of not less than 0.6 meters. ③The main transformer in the substation must have its working ground and protective ground connected to the artificial grounding grid separately. ④Lightning rods (wires) should be equipped with a separate grounding system. 1.2 Protective grounding of electrical equipment in flammable and explosive areas ① Electrical equipment, mechanical equipment, metal pipes, and the metal structures of buildings in flammable and explosive areas shall all be grounded, with jumper wires installed at the pipe joints. ②In neutral-point grounded circuits below 1 kV, when the overcurrent protection device is a fuse, the operating safety factor must be no less than 4; when it is a circuit breaker, the operating safety factor must be no less than 2. ③The connection points between the grounding main and the grounding electrodes must be no fewer than 2, and they shall be connected to the grounding electrodes at both ends of the building. ④To prevent accidents caused by sparks generated when measuring the ground resistance, measurements should be carried out in areas free from explosion hazards, or the terminals used for measurement should be led to locations outside flammable and explosive areas. 1.3 Grounding of DC equipment: Due to the effect of direct current, metal corrosion is severe, which increases contact resistance. Therefore, when installing grounding devices on DC circuits, the following measures must be carefully considered. ①For the grounding of DC equipment, natural grounding electrodes cannot be used as the PE wire or as grounding electrodes and wires for repeated grounding, nor can they be connected to natural grounding electrodes. ②The artificial grounding electrodes in DC systems should have a thickness of not less than 5 mm, and their degree of erosion must be checked regularly. 1.4 Grounding of portable and mobile electrical equipment: The grounding wires for portable and mobile electrical equipment should be made of soft copper wire with a cross-sectional area of not less than 1.5 mm2 to ensure sufficient mechanical strength. The connection between the grounding wire and electrical equipment or the grounding electrode should be made using bolts or specialized clamps, in order to ensure good contact and to meet the requirements for mechanical and thermal stability under short-circuit current conditions. During the operation of the grounding system, the grounding wire and electrode may be damaged or broken due to external forces or corrosion, and the grounding resistance can also change as soil conditions vary. Therefore, it is necessary to regularly inspect and test the grounding system

1. Equipment that should be grounded or connected to the neutral wire: 1) The metal casings, bases, and transmission mechanisms connected to them of motors, transformers, electrical appliances, lighting fixtures, as well as portable and mobile electrical devices. 2) The metal frames or reinforced concrete frames of indoor and outdoor power distribution installations, as well as the metal barriers, fences, or metal doors located near live parts. 3) The metal frames or enclosures of distribution panels, consoles, and control boxes. 4) The secondary winding of the transformer. 5) The metal enclosures of AC/DC power cable junction boxes, the metal sheaths of cables, and the steel pipes used for wiring, etc. 6) Metal poles and reinforced concrete towers of power lines equipped with lightning conductors. 7) Electrical equipment such as switches and capacitors installed on power distribution line poles.

The metal enclosures of electrical meters and automatic control equipment, as well as their normally non-electrified metal parts, should all be provided with protective grounding when they may develop dangerous voltages due to insulation failure. 　　 They include: dashboards, instrument cabinets, instrument boxes, PLC and DCS cabinets, control stations and auxiliary equipment, power distribution panels, power supply boxes, junction boxes, cable trays, cable channels, conduit pipes, and the shielding layers of armored cables. 2.0.2 Field instruments, transmitters, local switches, etc., powered by 24V or below 24V, do not require protective grounding unless there are specific requirements. 2.0.3 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 they are in good electrical contact with the grounded frame of the metal panel. 3 Working Grounding 3.0.1 Instruments, PLCs, DCS systems, computer systems, etc., should be provided with a working ground. Working ground includes: signal circuit grounding, shielding grounding, and intrinsically safe instrument system grounding. 3.0.2 When electronic devices such as instruments, PLCs, DCS systems, and computer systems require a unified reference potential, the signal circuit grounding should be carried out. 3.0.3 When PLCs, DCSs, computer systems are used in conjunction with analog instruments, a common signal loop ground point should be provided for both the analog and digital systems. 3.0.4 Components in the instrumentation system used to reduce electromagnetic interference (such as cable shielding layers, shielded twisted pairs, and shielding grounding terminals on instruments) should be grounded for shielding. Except for those signal sources that are grounded themselves, shielding grounding should be carried out on the control room side. 3.0.5 The intrinsically safe associated equipment that must be grounded in an intrinsically safe instrument system shall be reliably grounded in accordance with the requirements of the instrument manufacturer. 3.0.6 The signal circuit ground and shielding ground of the intrinsically safe instrument system can be connected to the intrinsically safe ground through a grounding busbar. 4 Lightning Protection and Grounding for Instrumentation Systems 4.0.1 In petrochemical plants located in areas with frequent or severe lightning strikes, if surge protectors are already installed at the points where the PLC, DCS, and computer system cables enter the control room, as well as at the field instruments, then those surge protectors shall be used for the lightning protection and grounding of the instrumentation systems. 4.0.2 For multi-core cables installed outdoors in areas prone to severe lightning strikes, and not housed in metal cable trays or conduits, their spare cores should be used for lightning protection grounding. 5 Grounding Connection Methods and Ground Resistance Requirements 5.0.1 The protective grounding of electronic devices such as instruments, PLCs, DCS systems, and computer systems shall be connected to the plant’s electrical system grounding grid, with a ground resistance of less than 4Ω. 5.0.2 The working grounding of electronic devices such as instruments, PLCs, DCS systems, and computer systems (signal circuit grounding and shielding grounding) can be carried out in one of the following two ways: 5.0.2.1 When the grounding resistance value of the plant’s electrical system grounding grid is less than 4Ω, and it meets the requirements of the instrument system, with no special requirements from the instrument manufacturer, these devices can be directly connected to the plant’s electrical system grounding grid ； 5.0.2.2 When the grounding resistance of the grounding grid in the plant’s electrical system is high, or when the instrument manufacturer has specific requirements, an independent instrument grounding system should be established, with a grounding resistance of less than 4Ω (or as specified by the instrument manufacturer). 5.0.3 Under normal circumstances, instrument circuits and systems should have only one grounding point for the signal circuit. When using a transformer-coupled isolator or an optocoupled isolator, signal circuit ground points can also be provided on each side of the isolator. 5.0.4 The shielding layer of the wires used for transmitting signals shall be grounded at the grounding terminals or grounding busbars of the instrument panel (cabinet); it shall not remain ungrounded or be grounded multiple times. 5.0.5 The grounding system for the Zener-type safety barriers in intrinsically safe instrumented systems should be installed separately, with a grounding resistance of less than 1Ω. The grounding electrode of the intrinsically safe instrument system should be kept separate, and the distance between it and the grounding grid of the plant’s electrical system or that of other instrument systems should not be less than 5.0 m. 5.0.6 Lightning protection (surge protectors) for the instrument system on the control room side shall be grounded; if the grounding resistance value of the existing instrument system is 1Ω or less, it is possible to share the grounding electrode with the protective grounding and working grounding of the instruments ； Otherwise, a separate grounding system should be installed to ensure that the grounding resistance value for lightning protection (surge protectors) of the instrumentation system is no greater than 1Ω. 5.0.7 For the grounding of lightning protection (surge protection) devices for field transmitters, this can be achieved by connecting the instrument itself to a grounded metal cable conduit or similar method. 6 Installation of grounding electrodes 6.0.1 When the electrical system’s grounding grid meets the requirements of these specifications, the instrumentation system does not require its own separate grounding electrodes. 6.0.2 In the following situations, a separate grounding electrode for the instrument system should be provided: 6.0.2.1 When a separate instrument system that is intrinsically safe is required ； 6.0.2.2 DCS or computer systems that require separate setup ； 6.0.2.3 When the grounding resistance of the electrical system’s grounding grid does not meet the requirements for the grounding of the instrumentation system ； 6.0.2.4 When the instrumentation system is sensitive to noise and requires high resistance to interference ； 6.0.2.5 When it is more economical and reasonable to install a grounding electrode separately. 7 Grounding Wires and Connection Requirements 7.0.1 The grounding wires for the instrumentation system shall be made of multi-strand copper-core insulated wires or cables. 7.0.2 The grounding connections of the instrumentation system shall be connected to the following facilities according to different requirements: 7.0.2.1 A separately installed grounding electrode for the instrumentation system ； 7.0.2.2 Plant electrical system grounding grid ； 7.0.2.3 Grounding distributors installed in different devices or different zones within the electrical system. 7.0.3 The protective grounding connections for individual field instruments, cable junction boxes, etc., may be connected to nearby grounded metal components or metal pipes, but shall not be connected to metal pipes that carry flammable substances. When using the above facilities for grounding connections, it is necessary to ensure the continuity and reliability of the grounding, as well as to meet the requirements regarding the grounding resistance of the instrumentation system. 7.0.4 The cross-sectional area of the grounding conductor can be selected according to the requirements for grounding resistance value, as well as the number of instruments connected and the length of the grounding conductor, as specified in Table 7.0.4.

The motor, main control unit, operating system, distribution box, and so on must all be reliably grounded

Our company has installed protection for all large electrical equipment. Grounding protection is necessary when the motor’s power exceeds 7.5 KW

As the protective ground for instruments, it mainly includes control cabinets such as instrument cabinets, DCS cabinets, and PLC cabinets, as well as secondary instruments such as digital displays, regulators, and paperless recorders.

The metal enclosures of electrical meters and automatic control equipment, as well as their normally non-electrified metal parts, should all be provided with protective grounding when they may develop dangerous voltages due to insulation failure. 　It generally includes: dashboards, instrument cabinets, instrument boxes, PLC and DCS cabinets, operation stations and auxiliary equipment, power distribution panels, power supply boxes, junction boxes, cable trays, cable channels, conduit pipes, and the shielding layers of armored cables. Field instruments, transmitters, local switches, etc., powered at 24V or below, do not require protective grounding unless there are specific requirements. The metal enclosures of small low-voltage electrical devices such as buttons, signal lights, and relays mounted on metal dashboards in areas free from explosion hazards do not require protective grounding, provided they are in good electrical contact with the grounded metal dashboard frame.

The following metal parts of power grids and electrical equipment shall be grounded or connected to neutral, unless otherwise specified: ① the metal bases and enclosures of motors, transformers, electrical appliances, and lighting fixtures; ② the metal transmission components of electrical equipment; ③ the secondary windings of current transformers; ④ the frames of distribution panels and control consoles; ⑤ the metal frameworks and reinforced concrete structures of indoor and outdoor distribution installations, as well as the metal fences and doors located near live parts; ⑥ the enclosures of junction boxes and terminal boxes for AC and DC power cables, as well as the metal sheaths of cables and the steel conduits used for wiring; ⑦ the metal towers of power lines equipped with lightning protection conductors; ⑧ the metal and reinforced concrete towers of low-voltage overhead distribution lines in residential areas with non-asphalt surfaces that do not have lightning protection conductors; ⑨ electrical equipment such as switching devices and capacitors installed on utility poles along distribution lines; ⑩ the outer sheath of armored control cables, and 1–2 shielding wires of unarmored or non-metallic sheathed cables. The following metal parts of electrical equipment do not need to be grounded or connected to neutral, unless otherwise specified: ① The enclosures of electrical equipment with an AC rated voltage of 380 V or less and a DC rated voltage of 440 V or less, in dry rooms with poor-conducting floors such as wood or asphalt; except in cases where maintenance personnel may come into contact with both the equipment enclosure and other grounded objects, as well as in areas where there is a risk of explosion. ②In dry locations, the enclosures of electrical equipment with an AC rated voltage of 127 V or less and a DC rated voltage of 110 V or less, except in locations where there is a risk of explosion. ③The enclosures of electrical measuring instruments, relays, and other low-voltage electrical devices installed on distribution panels, consoles, and the walls of distribution equipment compartments, as well as the metal bases of insulators that do not generate hazardous voltages on any supports in the event of insulation failure. ④Equipment installed on grounded metal frameworks (with good electrical contact to be ensured), such as bushings, except in areas where there is a risk of explosion. ⑤Metal brackets in battery rooms with a rated voltage of 220V or less. ⑥ Enclosures of motors and electrical appliances that are in reliable electrical contact with the grounded machine base, except in areas where there is a risk of explosion. ⑦Rail tracks extending from power plants, substations, and industrial enterprise areas, except those used for transporting flammable and explosive materials. To ensure the reliability of grounding, the same part of electrical equipment sometimes requires more than one grounding point, which is known as repeated grounding. For example, in low-voltage overhead lines, the neutral wire (the grounded neutral conductor) requires repeated grounding at both ends and at branching points, as well as every 1 km. Low-voltage distribution systems with the neutral point directly grounded (see Low-voltage Distribution)