Electrical grounding specifications

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To protect the safety of people and equipment, reduce electrical accidents in the company, and prevent damage to its personnel and property, all electrical equipment must be reliably grounded in accordance with regulations. Grounding Specifications 1. Scope of Application These specifications set out the requirements for grounding electrical circuits in the power systems of production and business entities, as well as in new constructions, expansions, maintenance, renovations, office areas, employee dormitories, etc. 2. Normative reference documents: GB14052—93 \"Forms of system grounding and safety requirements\", GB50054—95 \"Code for design of low-voltage power distribution\", GB 50169—2006 \"Code for construction and acceptance of grounding systems in electrical installation projects\". 3. Terms and definitions: The protection methods available for electrical systems include protective grounding, protective neutral grounding, repeated grounding, and working grounding, among others. A good electrical connection between a certain part of an electrical device and the ground is called grounding. A metal conductor or group of metal conductors in direct contact with the soil of the ground is called a grounding electrode; the metal conductor that connects the parts of electrical equipment that need to be grounded to the grounding electrode is called a grounding wire ； The grounding electrode and the grounding wire are collectively referred to as a grounding system. 4. Concepts and types of grounding: (1) Lightning protection grounding: Grounding intended to quickly direct lightning into the ground in order to prevent damage caused by lightning. When the lightning protection system shares a common grounding grid with the working ground of telegraph equipment, the grounding resistance must meet its minimum requirements. (2) AC working ground: A point in the power system is connected to the earth directly or through special equipment in a metallic manner. Working ground mainly refers to the grounding of the transformer’s neutral point or neutral wire (N wire). The N wire must be an insulated copper core wire. In power distribution, there are auxiliary equipotential terminals, and these equipotential terminals are generally located inside cabinets. It must be noted that this terminal cannot be exposed ； It cannot be connected together with other grounding systems, such as DC grounding, shielding grounding, and anti-static grounding ； It also cannot be connected to a PE wire. (3) Safety protection grounding: Safety protection grounding refers to establishing a good metallic connection between the non-live metal parts of electrical equipment and the grounding electrode. The electrical equipment inside the building, as well as some metal components located near such equipment, should be connected using a PE wire; however, it is strictly prohibited to connect the PE wire to the N wire. (4) DC grounding: To ensure high accuracy and stability in various electronic devices, in addition to a stable power supply, a stable reference potential is also necessary. Insulated copper wires with a larger cross-sectional area can be used as leads, with one end connected directly to the reference potential and the other end serving for the DC grounding of electronic equipment. (5) Anti-static grounding: The grounding performed to prevent interference with electronic equipment caused by static electricity generated in the dry environment of computer rooms in intelligent buildings is known as anti-static grounding. (6) Shield grounding: To prevent interference from external electromagnetic fields, grounding the enclosure of electronic devices as well as the shield wires inside and outside the devices or the metal tubes they are enclosed in is referred to as shield grounding. (7) Power grounding system: In electronic devices, AC/DC filters are installed to prevent interference voltages of various frequencies from entering through the AC and DC power lines and affecting the operation of low-level signals; the grounding of these filters is referred to as power grounding. (8) The standard requirements for grounding resistance are shown in the table below:
Name | Specific Requirements (Ohms)
--- | ---
Lightning protection grounding | The resistance of the independent lightning protection grounding system should be less than or equal to 10
Safety protection grounding | The resistance of the independent safety protection grounding system should be less than or equal to 4
AC operating grounding | The resistance of the independent AC operating grounding system should be less than or equal to 4
DC operating grounding | The resistance of the independent DC operating grounding system should be less than or equal to 4
Anti-static grounding | The resistance for anti-static grounding is generally required to be less than or equal to 100
Shared grounding electrode (combined grounding) | The grounding resistance should be less than 5

There are three types of grounding functions: protection grounding, operating grounding, and anti-static grounding.
(1) The metal casings of electrical equipment, as well as concrete structures and utility poles, may become charged due to insulation failures. To prevent such situations from posing a threat to human safety and avoiding electric shock accidents, the metal casings of electrical equipment are connected to a grounding system through protection grounding. When a person comes into contact with an electrical device whose casing is charged, since the contact resistance of the grounding electrode is much lower than that of the human body, most of the current flows into the ground through the grounding electrode, with only a small portion passing through the human body, thus preventing any harm to life. Image (2) The grounding carried out to ensure the reliable operation of electrical equipment under normal and fault conditions is known as service grounding; examples of this include direct and indirect grounding of the neutral point, as well as repeated grounding of the neutral wire and lightning protection grounding. To direct lightning underground, the grounding terminal of lightning protection equipment (such as lightning rods) is connected to the ground, in order to eliminate the damage caused by lightning overvoltage to electrical equipment as well as to people and property; this type of grounding is also known as overvoltage protection grounding. (3) The grounding of flammable oils, natural gas storage tanks and pipelines, electronic equipment, etc., to prevent the effects of static electricity hazards is referred to as anti-static grounding. 6. The resistance between electrical equipment and the ground through the grounding device is called grounding resistance, which consists of five components: (1) the contact resistance between the electrical equipment and the grounding wire. (2) The resistance of the grounding wire itself. (3) The resistance of the grounding electrode itself. (4) Contact resistance between the grounding electrode and the ground. (5) The resistance of the earth. 7. Different electrical equipment has varying requirements regarding ground resistance: (1) In systems with high ground short-circuit currents, R≤0.5 ohms ； (2) For transformers or generators with a capacity of over 100 kVA, R ≤ 4 ohms ； (3) Valve-type arrester R≤5 ohms ； (4) For independent lightning rods, systems with low grounding current, transformers or generators with a capacity of 100 kVA or less, and shared grounding for high-voltage and low-voltage equipment, R ≤ 10 ohms ； (5) The grounding resistance of metal poles, concrete poles, and chimneys in low-voltage lines shall be R≤30 ohms. 8. Requirements for installing grounding devices: (1) Grounding wires are generally made of galvanized flat steel with dimensions of 40mm×4mm. (2) The grounding electrode shall be made of galvanized steel pipe or angle steel. The steel pipe has a diameter of 50 mm, a wall thickness of not less than 3.5 mm, and a length of 2 to 3 m. An angle steel of 50mm×50mm×5mm is suitable. (3) The top of the grounding electrode should be 0.5–0.8 m above the ground level, so as to avoid the frost layer. The number of steel pipes or angle steels used depends on the soil resistivity surrounding the grounding electrode; generally, there should be no fewer than two electrodes, with a spacing of 3–5 m between each one. (4) The distance between the grounding electrode and any building should be at least 1.5 m, while the distance from independent lightning rod grounding electrodes must be greater than 3 m. (5) The connection between the grounding wire and the grounding electrode shall use lap welding. 9. Methods to reduce soil resistivity: (1) Before installing the grounding system, it is necessary to determine the resistivity of the soil surrounding the grounding electrodes; if it is too high, appropriate measures should be taken to ensure that the grounding resistance remains within acceptable levels. (2) Changing the soil structure around the grounding electrode: Within a range of 2–3 meters around the grounding electrode, water-impermeable materials with good water absorption properties such as charcoal, coke, cinder, or slag are mixed into the soil. This method can reduce the soil resistivity to 15–110 times its original value. (3) Reduce soil resistivity using table salt and charcoal by compacting them in layers. Evenly spread a layer of charcoal and fine mixture, about 10–15 cm thick, then add 2–3 cm of salt on top. Repeat this process to make a total of 5–8 layers. After laying, drive in the grounding electrode. This method can reduce the resistivity to 13–15 times its original value. However, table salt gradually loses itself through running water, and it generally needs to be replenished every two years or so. (4) Using long-acting chemical resistivity reducers: This method can reduce the soil resistivity to 40% of its original value. The grounding resistance of electrical equipment should be tested once each in spring and autumn, during periods when rainfall is low, to ensure that the grounding is proper. Generally, specialized instruments (such as the ZC-8 ground resistance tester) are used for testing, but the ammeter-voltmeter method can also be employed. 10. The items to check for grounding include (1) whether the connection bolts are loose or corroded. (2) The corrosion condition of the grounding wires and grounding electrodes below the ground surface, as well as whether there is any welding failure. (3) Check whether the grounding wires at the ground level are damaged, broken, corroded, etc. For the power cables of overhead feeders, including the neutral wire, the cross-sectional area should meet the specified requirements: it should be no less than 16 mm2 for aluminum wires and no less than 10 mm2 for copper wires. (4) To facilitate the identification of the different uses of various wires, the live wires, working neutral wires, and protective wires should be distinguished by different colors, in order to prevent the mixing of live wires and neutral wires or of working neutral wires and protective neutral wires. This helps to ensure proper wiring of various sockets, and a three-phase five-wire power distribution system is used for this purpose. (5) A single-phase leakage protector should be installed in the automatic air switch or fuse of the user-side power supply. User circuits that are in poor condition due to lack of maintenance, aged insulation, increased load, or insufficient cross-sectional area should be replaced as soon as possible, in order to eliminate the risk of electrical fires and ensure proper operation of the leakage protectors. (6) In power and electrical systems with a three-phase five-wire configuration, the protective ground wire and neutral wire must under no circumstances be less than half of the thickness of the phase wires. In lighting systems, whether they use a three-phase five-wire or single-phase three-wire configuration, the ground wire and neutral wire must have the same thickness as the phase wires. (7) The main conductors for working grounding and protective grounding may be shared, but their cross-sectional area must not be less than half of the cross-sectional area of the phase conductors. (8) The grounding of each electrical device shall be connected to the main grounding wire through a separate grounding wire; several electrical devices that need to be grounded shall not be connected in series within one grounding wire. (9) The cross-sectional area of the bare copper wires used for grounding in 380V distribution boxes, maintenance power boxes, and lighting power boxes should be >4 mm2; for bare aluminum wires, it should be >6 mm2. For insulated copper wires, the cross-sectional area should be >2.5 mm2, while for insulated aluminum wires, it should be >4 mm2. (10) The distance between the grounding wire and the ground should be 250–300 mm. (11) The working ground should be marked with yellow-green stripes on the surface, the protective ground should be marked in black, and the equipment’s neutral wire should be marked in light blue. (12) Snake pipes, the metal outer layer of pipe insulation, metal mesh, or the metal shielding of cables shall not be used as grounding wires. (13) When welding the ground wire, lap welding should be used; the lap length must be 2 times the width of the flat steel (with welding on at least 3 edges), or 6 times the diameter of the round steel (with welding on both sides). When connecting a round steel rod to a flat steel bar, the lap length for lap welding should be 6 times the diameter of the round steel rod (with welding on both sides). (14) The connection between copper and aluminum wires and the ground bar must be made using fixing screws; winding together is not allowed. When flat copper flexible wires are used as grounding wires, their length should be appropriate, and the wire terminals must be connected to the grounding screws. (15) During equipment operation, the operators shall check that the grounding wires of electrical equipment are properly connected to the ground grid, and that there are no breaks or other conditions that could reduce the cross-sectional area of the grounding wires; otherwise, it shall be treated as a defect. (16) When conducting acceptance inspections for equipment maintenance, it is necessary to ensure that the grounding wires of electrical equipment are in good condition. (17) The Equipment Department should regularly check the grounding status of electrical equipment, and notify relevant parties to make corrections promptly in case any issues are found. (18) The grounding resistance of electrical equipment should be monitored during periodic inspections or when the equipment is overhauled; any issues detected should be promptly analyzed for their causes and addressed. (19) The grounding resistance of high-voltage electrical equipment and its grounding grids is measured by the Equipment Department in accordance with the \"Regulations for Handover and Preventive Testing of Electrical Equipment\", while the grounding of low-voltage electrical equipment is carried out by the department responsible for that equipment. (20) The short-circuit current into the ground for the grounding device shall be taken as the maximum value of the symmetric component of the largest short-circuit current flowing into the ground through the grounding device in the case of internal and external short circuits within it. This current should be determined based on the maximum operating conditions of the system after 5–10 years of development, taking into account the distribution of short-circuit currents among the various grounded neutral points in the system, as well as the short-circuit current diverted by the lightning protection conductors. 11. The following equipment must be protected by grounding: (1) The secondary coil of the current transformer. (2) Enclosures for distribution panels and control panels. (3) The enclosure of the motor. (4) The enclosure of the cable junction box and the metal sheath of the cable. (5) The metal base or housing of the switch and its actuating mechanism. (6) Metal bases of high-voltage insulators and bushings. (7) Metal pipes for indoor and outdoor wiring. (8) Ground terminal of the meter. (9) Enclosures for electrical and lighting equipment. (10) The metal frames of indoor and outdoor power distribution installations, as well as the metal enclosures surrounding live parts. 12. Requirements regarding motor grounding: (1) The motor’s grounding wire should preferably be connected to the plant’s overall grounding network using flat iron. If the distance from the main grounding conductor is large, or if the use of flat iron grounding wires affects the aesthetic appearance of the area, it is advisable to use natural grounding elements instead, or flat copper wires as grounding wires. (2) For motors with grounding screws on the enclosure, the grounding wire must be connected to the grounding screw. (3) For motors without grounding screws on their casing, it is required to install grounding screws at appropriate positions on the motor casing to connect with the grounding wire. (4) The motor casing, which has a reliable electrical connection to the grounded frame, does not need to be grounded; the grounding wires should be arranged neatly and aesthetically. 13. Requirements regarding the grounding of distribution panels: (1) The grounding wire for distribution panels should preferably be made of flat iron and connected to the plant’s overall grounding network. If the distance from the main grounding conductor is large, or if using flat iron grounding wires would affect the aesthetic appearance, it is advisable to use natural grounding elements or soft copper wires as grounding wires. (2) When bare copper wires are used for the grounding wire of low-voltage distribution panels, their cross-sectional area shall be no less than 6 mm2; when copper wires with insulated covers are used, the cross-sectional area shall be no less than 4 mm2. (3) For distribution boards with grounding screws on the enclosure, the grounding wire must be connected to the grounding screw. (4) For distribution boards without grounding screws on their enclosures, it is required to install grounding screws at appropriate locations on the enclosure to connect with the grounding phase wire. (5) The enclosure of the distribution panel, which is in reliable electrical contact with the grounding electrode, may not be grounded. 14. Inspection and measurement methods for grounding wires: (1) Before testing, maintain a sufficient safety distance from the equipment being tested to avoid accidental contact with live or rotating parts, and the task should be carried out by two persons. (2) Before testing, use the resistance setting of a multimeter to short-circuit its two probes, thereby setting the indication on the meter’s resistance scale to 0. (3) Connect one end of the test lead to the ground wire, and the other end to the device’s dedicated ground terminal. (4) When the equipment under test does not have a dedicated grounding terminal, the other end of the test leads should be connected to the enclosure or metal components of the electrical equipment for measurement. (5) The grounding point must be selected at the main grounding grid or a location that is reliably connected to it, with the surface oxides removed to ensure good contact. (6) The value should be read after the meter indication stabilizes, and the ground resistance value must comply with the regulations.