Scope of protective grounding for electrical installations

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Abstract: In a building, electrical installations are allowed to have only one common grounding device, and equipotential bonding is employed to eliminate or reduce potential differences. Information technology equipment shall be connected to the common grounding device only through PE wires, and equipotential bonding shall be implemented, using the potential of the equipotential bonding system as the reference potential for the information technology equipment. Keywords: electrical equipment, information technology equipment, protective grounding, scope. 1.1 Scope requiring protective grounding: The exposed conductive parts of the following electrical equipment shall be protected by grounding, unless otherwise specified: – the bases and enclosures of motors, transformers, electrical appliances, as well as portable and mobile electrical devices ； －Electrical equipment transmission mechanism ； －Secondary winding of the transformer ； – Metal frames for distribution panels (boxes), control panels (boxes), various types of cabinet control consoles, etc ； – Metal frames and reinforced concrete frameworks of indoor and outdoor power distribution installations, as well as metal fences and metal doors located near live parts ； – Metal enclosures of enclosed switchgear and box-type substations ； – Metal sheaths of power cables and control cables, metal tubes for passing wires through ； – Various metal frames and supports for electrical use ； －Cable trays, cable channels, and metal supports ； －Surge protector ； － Metal enclosures of the generator neutral point, generator outlet cabinets, and enclosed busbars (dense-type or air-insulated type) ； －Power line poles equipped with lightning conductors ； －In residential areas with non-asphalt surfaces, metal and reinforced concrete pole towers for overhead power lines with low grounding current and no lightning protection wires ； – Electrical equipment such as switchgear and capacitors installed on power distribution line poles. 1.2 Areas where protective grounding is not required: The exposed conductive parts of the following electrical apparatus may not require protective grounding, unless otherwise specified: – Electrical apparatus installed in non-conductive areas, where the insulation resistance of the floor and walls relative to ground is such that, for a rated voltage of 500V, the insulation resistance is not less than 50 kΩ ； When the rated voltage exceeds 500V, the insulation resistance must be at least 100kΩ, and Class 0 equipment can be used. Within that area, within a range of 2 meters from the human body’s arms, it is not possible to touch two exposed conductive parts simultaneously, or one exposed conductive part together with any external conductive part ； Outside the reach of the arm, this distance can be reduced to 1.25 m. Measures must be taken to prevent a potential from arising outside the area through external conductive parts. －Equipment for ultra-low voltage (SELV) applications ； – The enclosures of electrical measuring instruments, relays, and other low-voltage electrical devices installed on distribution panels, control panels, and power distribution units; as well as the metal bases of insulators that do not generate voltages dangerous to human safety when insulation is damaged, and which are mounted on supports ； – Equipment installed on grounded metal frames, such as bushings, etc. (good electrical contact must be ensured) ； – Supports in battery rooms with a rated voltage of 220V or less ； – Motor and electrical apparatus enclosures that have a reliable electrical connection to the grounded machine frame ； – Electrical equipment with double insulation ； – For electrical equipment powered by electrical isolation protection, each winding of the isolation transformer supplies power to only one piece of equipment ； When each winding supplies multiple devices, equipotential bonding without grounding should be established between these devices. Electrical products are classified into four categories based on their measures to prevent electric shock. The measures for preventing indirect contact electric shock are shown in the table below: Table 1-1 Protection Measures Against Electric Shock for Electrical Equipment and Apparatus. Equipment Category | Protection Measures | Equipment Part | Apparatus Part | Basic Protection | Additional Protection 0 | Basic insulation – In non-conductive environments, electrical isolation for each device; Type I | Basic insulation with protective connections and automatic power disconnection; Type II | Basic insulation plus additional insulation – Use of reinforced insulation or equivalent structures; Type III | Voltage limitation – SELV and PELV. 1.3 Grounding of Information Technology Devices. According to IEC standards, there shall be only one common grounding system within a building, and equipotential bonding should be used to eliminate or reduce potential differences. Information technology equipment shall be connected to the common grounding device only through PE wires, and equipotential bonding shall be implemented, using the potential of the equipotential bonding system as the reference potential for the information technology equipment. According to IEC standards, a 50 mm2 copper conductor is considered the best balance between material cost and impedance for use as a grounding main, while a 10 mm2 copper conductor represents the minimum cross-sectional area required for functional grounding. The grounding methods and equipotential bonding methods for information technology equipment are as follows: (1) Radial connection of protective conductors. As shown in Figure 1-1, this method uses a protective conductor together with the power conductors. The protective conductor of each device provides a path with a relatively high impedance for electromagnetic interference (except for transients caused by the power supply), thereby causing the signal cables between information technology devices to bear most of the incoming noise. Therefore, the device itself must have satisfactory high interference resistance. Since information technology equipment is equipped with dedicated power circuits and grounding systems, and is isolated from other power circuits, grounding systems, and external metal objects, this significantly reduces the interference introduced. In some cases, the function grounding of information technology equipment with a radial connection and the star-shaped grounding point of the protective conductors (such as the PE busbar in the relevant distribution panel) can be grounded through a separate dedicated insulated conductor connected to the main grounding terminal. Figure 1-1 shows the protective conductor (2) in a radial connection. The use of a local horizontal equipotential bonding system (network) – as shown in Figure 1-2 – to equipotentially connect the various components of the information technology system to a local network (bonding material) can complement the role of conventional protective conductors. This approach provides a low-impedance reference potential plane near the equipotential mesh, between the various components interconnected by signals, with an impedance that depends on the frequency and the spacing of the mesh nodes. As with Method 1, the interference resistance is improved because the power supply circuits and grounding systems of the entire information technology system, including the equipotential bonding network, are isolated from other power supply circuits and grounding systems as well as external conductive parts (such as metal components of buildings). Figures 1-2 show a horizontal and vertical equipotential bonding network system using a local horizontal equipotential bonding system (3). As shown in Figure 1-3, an equipotential bonding network is installed on each floor of the building, which enhances the function of the conventional protective conductors. These equipotential networks are repeatedly connected to the metal components of buildings, the exposed conductive parts of electrical installations, and other metal objects for various purposes, thereby achieving vertical equipotential connection between floors. This grounding method can also use a ring-shaped grounding bus to extend the building’s main grounding terminal. This method can provide an impedance low enough to address most noise issues in devices with only moderate interference resistance; the effectiveness of this solution depends on the operating and interference spectra as well as the mesh spacing. However, if the entire network cannot be kept closed, problems will arise, as all possible noise sources will be connected to the system. Therefore, special attention should be paid to the spacing of the mesh to eliminate interference from such noise sources. Figures 1–3 Horizontal and vertical equipotential bonding systems (4) Further requirements for devices with a leakage current exceeding 10 mA When a device’s leakage current exceeds 10 mA, it must be connected in accordance with one of the three alternative requirements listed below: 1) A highly reliable protection (grounding) circuit. The protective conductor shall have a cross-sectional area that meets the requirements for thermal stability, or a cross-sectional area specified below, whichever is larger. a) When an independent protective conductor is used, it shall be a conductor with a cross-sectional area of not less than 10 mm2, or two conductors with separate ends, each having a cross-sectional area of not less than 4 mm2. b) When the protective conductor is combined with the supply conductor in a multi-core cable, the total cross-sectional area of all conductors in the cable shall be not less than 10 mm2. c) When the protective conductor is placed inside a rigid or flexible metal conduit and connected in parallel with the conduit, a conductor with a cross-sectional area of not less than 2.5 mm2 shall be used. d) Rigid or flexible metal conduits, metal busbars and trunking that meet the requirements, as well as metal shielding and armor. 2) Monitoring of ground continuity: One or more electrical devices should be installed to cut off the power supply to the equipment as required in the event of an interruption in the protective conductor. 3) Use a double-winding transformer: When the equipment is powered by a double-winding transformer or by units whose input and output circuits are separated from each other (such as motor-generators), it is recommended to use a TN system for its secondary circuit. The purpose is to localize the path of the leakage current and reduce the likelihood of that path being interrupted. For ease of representation, only single-phase systems are shown in Figures 1–4; the system can also be three-phase. The control and protection measures for the primary and secondary circuits are not indicated in the diagram. C is the filtering capacitor. L1 and L2 (or N) are the connection conductors connected to the power supply inlet. PE is the connecting conductor from the accessible parts of the equipment to the main grounding terminal of the electrical installation; it serves as both the protective conductor for Class I equipment and the functional grounding conductor for Class II equipment.