In industrial power distribution systems, the grounding of switchgear-especially enclosed electrical switchgear, high voltage electrical switchgear, and medium voltage switchgear-is the "last line of defense" for ensuring the safe operation of equipment and the safety of personnel. Its core function is to quickly conduct the leakage current into the ground, preventing the equipment casing from becoming electrified or causing electrical fires. However, during actual installation, many construction workers, due to cognitive biases or seeking convenience, have carried out numerous violations, ultimately laying the groundwork for leakage accidents. As a professional manufacturer specializing in electrical equipment, Zhejiang Lvma Electric Co., Ltd. (LVMA) has accumulated rich experience in the R&D, production, and technical support of enclosed electrical switchgear, high voltage electrical switchgear, and medium voltage switchgear.
Misconception 1: Pressing multiple grounding wires onto a single grounding terminal, thinking it's a shortcut and thus causing hidden dangers
Misconception manifestation:
Some construction workers, in an attempt to reduce the workload of wiring, press 3 or more grounding wires onto the same grounding terminal of enclosed electrical switchgear or medium voltage switchgear, believing that "as long as it conducts electricity, there's no problem" and even considering this operation as an industry norm. Even worse, they connect the grounding wires of multiple high voltage electrical switchgear or medium voltage switchgear in series to the same terminal, omitting the independent grounding step. This irregular operation is also frequently found in the on-site inspections conducted by Zhejiang Lvma Electric Co., Ltd. (LVMA) for customers.
Core risks:
Excessive contact resistance: The pressing area and tightening torque of the bolt-type terminal are fixed. When multiple wires are stacked, some wires may be partially disconnected, resulting in a significant increase in contact resistance, and when fault current passes through enclosed electrical switchgear or high voltage electrical switchgear, it is prone to generating high temperatures, burning the terminal and the insulation layer of the wire;
Grounding continuity failure: A single terminal loosening will cause all connected enclosed electrical switchgear or medium voltage switchgear to lose grounding protection simultaneously, resulting in a "chain failure";
Maintenance risks: During maintenance, removing one wire may cause other wires to fall off, and it is impossible to quickly distinguish which wire corresponds to which high voltage electrical switchgear or medium voltage switchgear, prolonging the time for fault handling.
Correct and standardized practice:
According to the mandatory requirements of Article 4.3 of GB 50169-2016, each electrical device's grounding-whether for enclosed electrical switchgear, high voltage electrical switchgear, or medium voltage switchgear-should be connected to the grounding busbar with a separate grounding wire. It is strictly prohibited to connect multiple devices that need grounding in a single grounding wire. Zhejiang Lvma Electric Co., Ltd. (LVMA) also clearly specifies this requirement in the installation manuals of its switchgear products to guide construction teams in standardized operations. At the same time, the following should be observed:
Each side of a single grounding terminal should not exceed 2 wires; a flat washer should be added between the bolt connection;
The grounding of multiple enclosed electrical switchgear or high voltage electrical switchgear should be connected separately to the grounding busbar, and it is strictly prohibited to connect them in series;
When pressing the grounding wire onto the terminal of medium voltage switchgear, ensure the contact surface is clean, use anti-loosening bolts for tightening, and the torque should comply with the specification requirements.
Misconception 2: Grounding resistance exceeds the limit, ignoring soil conditions and anti-resistance measures
Misconception manifestation:
During construction, the appropriate grounding electrode was not selected based on the soil resistivity for high voltage electrical switchgear or medium voltage switchgear, or anti-resistance treatment was omitted, and only a single metal rod was simply buried as the grounding electrode, resulting in a grounding resistance far higher than the standard limit (usually ≤ 4Ω). Some projects involving enclosed electrical switchgear even did not conduct grounding resistance tests and directly accepted and delivered. In the after-sales service of Zhejiang Lvma Electric Co., Ltd. (LVMA), many equipment failure complaints are related to this misconception.
Typical accident case:
A 220kV substation's 10kV feeder line outlet connected to high voltage electrical switchgear experienced a single-phase grounding fault. Due to the grounding resistance of the high voltage electrical switchgear exceeding the limit, the fault current could not be quickly discharged, causing the B and C phase voltages to rise, triggering arc overvoltage, and the insulation between the busbars broke down, developing into a three-phase short circuit. The high-temperature arc caused severe damage to the cabinet of the enclosed electrical switchgear and internal components, resulting in the 10kV busbar tripping and power outage, affecting regional power supply. After receiving the customer's request for help, Zhejiang Lvma Electric Co., Ltd. (LVMA) dispatched technical personnel to the site for investigation. The fault analysis revealed that the grounding electrode of this medium voltage switchgear did not adopt anti-resistance measures based on the soil conditions, and the measured grounding resistance reached 12Ω, far exceeding the standard requirement. The LVMA technical team provided a targeted transformation plan and successfully restored the safe operation of the equipment.
Core risks:
Overvoltage breakdown: When the grounding resistance exceeds the limit, the leakage current of enclosed electrical switchgear or medium voltage switchgear cannot be promptly discharged, causing the equipment casing to have an elevated potential, triggering insulation breakdown between phases;
Protection device failure to operate: The fault current does not reach the threshold for the protection switch of high voltage electrical switchgear to act, preventing the rapid cut-off of the power supply, expanding the impact of the accident;
Risk of personnel electric shock: When the equipment casing of enclosed electrical switchgear is electrified, the grounding system cannot effectively reduce the ground voltage, and the person in contact is prone to an electric shock accident.
Correct and standardized practice:
According to the "anti-resistance" requirements for grounding devices in Article 4.4 of GB 50169-2016:
Before construction, the soil resistivity should be tested, and the type of grounding electrode (vertical or horizontal) should be selected based on the test results for high voltage electrical switchgear and medium voltage switchgear; Zhejiang Lvma Electric Co., Ltd. (LVMA) recommends that customers cooperate with professional testing institutions to obtain accurate soil data before installation;
The grounding resistance should meet the following requirements: for general enclosed electrical switchgear, ≤ 4Ω; in explosive hazardous environments for high voltage electrical switchgear, ≤ 1Ω; in areas with high soil resistivity for medium voltage switchgear, anti-resistance measures (such as soil replacement, adding anti-resistance agents, increasing the number of grounding electrodes) should be taken;
The grounding electrode should be away from underground metal pipelines, cables, etc. The burial depth of vertical grounding electrodes for enclosed electrical switchgear should not be less than 0.6m, and the spacing of horizontal grounding electrodes for high voltage electrical switchgear should not be less than 5m;
During project acceptance, the grounding resistance of medium voltage switchgear and enclosed electrical switchgear must be measured, and if it is unqualified, it must be rectified to meet the standards before being put into use. LVMA's after-sales team can provide professional testing and rectification guidance.
Misconception 3: Incorrect selection of grounding wires, with insufficient cross-sectional area or incompatible materials
Misconception manifestation:
Using ordinary wires instead of dedicated grounding wires for enclosed electrical switchgear or high voltage electrical switchgear, or having a grounding wire with a cross-sectional area less than the required specification (such as a copper core wire with a cross-sectional area less than 25mm²), some projects involving medium voltage switchgear use aluminum conductors to save costs, and the connections are not treated for corrosion. Zhejiang Lvma Electric Co., Ltd. (LVMA) reminds customers that the quality of grounding wires directly affects the safety of the entire power distribution system, and it is strictly prohibited to cut corners.
Typical accident case:
When an electrician was inspecting a 35kV transformer connected to medium voltage switchgear, he used a copper core wire with a cross-sectional area of 10mm² as the grounding wire for the medium voltage switchgear, and the grounding end was not tightened, merely "placed on" the grounding electrode. During the inspection, the dispatch mistakenly sent power, and the fault current passed through the grounding wire of the high voltage electrical switchgear. Due to the small cross-sectional area and poor contact, a high-temperature arc was generated, burning the grounding wire and burning the electrician's arm. After the accident, the customer contacted Zhejiang Lvma Electric Co., Ltd. (LVMA) for technical support. It was detected that the load capacity of this grounding wire for enclosed electrical switchgear was far below the short-circuit current requirement, unable to withstand the fault impact. The LVMA team replaced the grounding wire with a compliant copper core wire of 35mm² and standardized the connection, eliminating potential safety hazards.
Core risks:
Wire burnout: When short-circuit current passes through a thin cross-sectional grounding wire of high voltage electrical switchgear or medium voltage switchgear, it will overheat and melt, causing the grounding protection to fail;
Poor contact: Aluminum conductors used for enclosed electrical switchgear are prone to oxidation, and the resistance at the connection point increases, which is prone to virtual connection over the long term;
Corrosion failure: Uncoated grounding wires of medium voltage switchgear corrode in humid environments, reducing the reliability of grounding.
Correct and standardized installation:
According to the "Selection of Grounding Devices" and industry practical standards:
The material of the grounding wire for enclosed electrical switchgear, high voltage electrical switchgear, and medium voltage switchgear should preferentially use copper conductors; in explosive hazardous environments for high voltage electrical switchgear, copper core wires must be used; aluminum conductors are prohibited. Zhejiang Lvma Electric Co., Ltd. (LVMA) provides matching dedicated grounding wires for its switchgear products to ensure compliance with specifications;
The cross-sectional area of the grounding wire should match the equipment voltage level and short-circuit current: for 10kV and below medium voltage switchgear, the copper core cross-sectional area of the grounding wire should not be less than 25mm², and for 35kV high voltage electrical switchgear, it should not be less than 35mm²;
The connection of the grounding wire for enclosed electrical switchgear should be tightened with bolts or welded; when connecting copper and aluminum for medium voltage switchgear, a transition joint should be installed, and all connection points should be treated for corrosion (galvanized or coated with anti-corrosion paint);
The grounding wire of high voltage electrical switchgear should avoid flammable and explosive environments and should not be laid parallel to cables and should have a distance of no less than 0.5m; when passing through walls, a protective pipe should be added.