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Selection and Protection Design of Low-Voltage Assembled Switchgear in Humid Environments

source：Zhejiang CHBEST Power Technology Co., Ltd. Addtime：2026-02-25 Views：

Low-voltage assembled switchgear is a core component of power distribution systems, widely used in various industrial and civil fields to distribute electrical energy, control equipment operation, and protect electrical circuits. However, in humid environments—such as coastal areas, underground engineering, water treatment plants, chemical plants with high humidity, and rainy regions—moisture can easily penetrate the switchgear cabinet, leading to serious problems such as insulation degradation, component corrosion, short circuits, and even equipment failure or safety accidents. Unlike normal dry environments, humid environments impose higher requirements on the selection, structural design, and protective measures of low-voltage assembled switchgear. This article systematically elaborates on the key points of selection and protection design for low-voltage assembled switchgear in humid environments, based on relevant international standards and engineering practice, to ensure the safe, stable, and long-term operation of the switchgear under high humidity conditions.

Humid environments are defined as areas where the relative humidity (RH) is consistently above 75%, or where there is frequent condensation, water vapor accumulation, or direct contact with moisture (such as splashing water, dripping water, or immersion). In such environments, moisture can enter the switchgear through gaps, cable entries, and ventilation openings, causing two major types of damage: first, chemical corrosion of metal components (such as busbars, circuit breakers, and contactors) due to moisture and harmful gases (such as salt spray in coastal areas), leading to increased contact resistance, poor conductivity, and even component failure; second, degradation of insulation performance of insulating materials (such as cables, terminal blocks, and insulating panels), resulting in insulation breakdown, short circuits, and leakage currents, which pose serious threats to equipment and personnel safety. Therefore, the selection and protection design of low-voltage assembled switchgear in humid environments must focus on "moisture prevention, corrosion resistance, and insulation protection" to form a comprehensive protective system.

I. Key Selection Principles of Low-Voltage Assembled Switchgear in Humid Environments

The selection of low-voltage assembled switchgear in humid environments is not only based on the conventional parameters such as rated voltage, rated current, and short-circuit withstand current, but also needs to prioritize the adaptability to humid conditions, focusing on the selection of cabinet materials, protection levels, internal components, and insulation performance. The core selection principles are "corrosion resistance, moisture resistance, and high insulation", and the following key points should be strictly followed:

1. Selection of Cabinet Material and Surface Treatment

The cabinet is the first barrier to prevent moisture intrusion, so the selection of material and surface treatment directly determines the moisture resistance and corrosion resistance of the switchgear. In humid environments, traditional ordinary steel cabinets are prone to rust and corrosion, so it is necessary to select materials with strong corrosion resistance and perform professional anti-corrosion surface treatment.

Material Selection: Priority should be given to corrosion-resistant materials such as 304 or 316 stainless steel, aluminum alloy, or galvanized steel. 316 stainless steel has excellent corrosion resistance, especially suitable for coastal areas with salt spray and chemical environments with corrosive gases, as it can effectively resist the erosion of moisture and harmful substances. Aluminum alloy cabinets are lightweight, have good corrosion resistance, and are suitable for general humid environments such as underground garages and water treatment plants. Galvanized steel cabinets (hot-dip galvanized or electro-galvanized) can also be used in moderate humid environments, but their corrosion resistance is lower than that of stainless steel and requires regular maintenance.
Surface Treatment: On the basis of selecting corrosion-resistant materials, professional surface treatment should be carried out to further enhance moisture resistance and corrosion resistance. For stainless steel cabinets, passivation treatment can be performed to form a dense oxide film on the surface, preventing moisture and harmful gases from penetrating. For aluminum alloy cabinets, anodizing treatment is adopted to improve the surface hardness and corrosion resistance. For galvanized steel cabinets, after galvanizing, a layer of anti-corrosion paint (such as epoxy resin paint or polyurethane paint) should be applied to enhance the protective effect. It should be noted that the surface treatment layer must be uniform, free of cracks, bubbles, and peeling, to ensure the integrity of the protective barrier.

2. Selection of Protection Level (IP Rating)

The protection level (IP rating) of the switchgear cabinet is an important indicator to measure its ability to prevent the intrusion of solid foreign objects and moisture. In humid environments, it is necessary to select a switchgear with an appropriate IP rating according to the specific humidity level and moisture intrusion form, to ensure that moisture cannot enter the cabinet or can only enter in a small amount that does not affect the normal operation of the equipment.

Basic Protection Level Requirements: For general humid environments (relative humidity 75%-85%, no direct water contact), the switchgear should have a protection level of at least IP54. IP54 means that it can prevent the intrusion of dust that may affect the normal operation of the equipment, and can prevent splashing water from all directions from entering the cabinet. For harsh humid environments (relative humidity above 85%, frequent condensation, or occasional dripping water), the protection level should be upgraded to IP55 or IP65. IP55 can prevent water jets from all directions from entering the cabinet, while IP65 can completely prevent dust intrusion and strong water jets from entering, which is suitable for outdoor humid environments or areas with frequent water splashing (such as washing areas in food processing plants).
Special Considerations: When the switchgear is installed in areas with condensation (such as underground engineering with large temperature differences), in addition to selecting a high IP rating, it is also necessary to consider the ventilation and dehumidification function of the cabinet to avoid moisture accumulation inside the cabinet due to condensation. For areas with direct water immersion (such as underwater engineering or areas prone to flooding), special waterproof switchgear with IP67 or higher protection level should be selected, which can be temporarily immersed in water without being damaged.

3. Selection of Internal Components

The internal components of low-voltage assembled switchgear (such as circuit breakers, contactors, relays, terminal blocks, and cables) are the core of the system’s normal operation. In humid environments, these components are easily affected by moisture, leading to performance degradation or failure. Therefore, the selection of internal components must focus on moisture resistance and corrosion resistance.

Circuit Breakers and Contactors: Priority should be given to components with moisture-proof and corrosion-resistant designs, such as sealed circuit breakers and contactors with stainless steel or corrosion-resistant plastic shells. The contacts of the components should be made of silver-nickel alloy or other corrosion-resistant materials to avoid contact oxidation and poor contact caused by moisture. In addition, the operating mechanism of the components should be sealed to prevent moisture from entering and causing jamming.
Terminal Blocks and Insulating Components: Terminal blocks should be made of high-quality flame-retardant and moisture-proof engineering plastics (such as PA66 or PC), and the metal parts should be galvanized or nickel-plated to enhance corrosion resistance. Insulating panels, busbar supports, and other insulating components should be made of insulating materials with high moisture resistance and insulation performance, such as epoxy resin boards or phenolic resin boards, to avoid insulation degradation caused by moisture absorption.
Cables and Wires: The cables and wires used in the switchgear should be moisture-proof and corrosion-resistant, such as cross-linked polyethylene (XLPE) insulated cables or polyvinyl chloride (PVC) insulated cables with moisture-proof sheaths. The cable joints should be sealed with waterproof connectors to prevent moisture from entering the cable and causing insulation breakdown. In addition, the cross-sectional area of the cables should be appropriately increased to avoid overheating caused by increased resistance due to moisture corrosion.

4. Selection of Insulation Performance

In humid environments, the insulation performance of the switchgear is easily degraded by moisture, which may lead to insulation breakdown and short circuits. Therefore, when selecting the switchgear, it is necessary to pay special attention to its insulation performance indicators, and select products with higher insulation levels and better moisture resistance.

Insulation Level: The switchgear should meet the insulation level requirements specified in IEC 61439 or GB 7251 standards. For humid environments, the insulation level should be appropriately improved. For example, the rated insulation voltage of the switchgear should not be less than 690V, and the insulation resistance should be not less than 100MΩ at room temperature. In addition, the switchgear should pass the dielectric strength test under humid conditions to ensure that it can maintain good insulation performance in high humidity environments.
Moisture-Proof Insulating Materials: The internal insulating materials of the switchgear should be moisture-proof and non-hygroscopic. Avoid using insulating materials that are easily hygroscopic (such as ordinary paper insulation materials), to prevent insulation performance degradation caused by moisture absorption. At the same time, the insulation surface should be smooth and free of cracks, to avoid moisture accumulation and leakage current.

II. Protection Design of Low-Voltage Assembled Switchgear in Humid Environments

In addition to selecting appropriate switchgear products, reasonable protection design is also essential to ensure the safe operation of low-voltage assembled switchgear in humid environments. The protection design should focus on "preventing moisture intrusion, reducing moisture accumulation, and enhancing corrosion resistance", and comprehensively adopt structural design, ventilation and dehumidification, sealing treatment, and anti-corrosion measures to form a multi-level protection system.

1. Structural Design Optimization

The structural design of the switchgear cabinet directly affects its moisture-proof effect. By optimizing the cabinet structure, the intrusion of moisture can be effectively prevented, and the accumulation of moisture inside the cabinet can be reduced.

Cabinet Structure Design: The cabinet should adopt a sealed structure with no obvious gaps. The top of the cabinet should be designed with a slope (angle not less than 15°) to prevent water accumulation and dripping. The bottom of the cabinet should be raised by 100-150mm to avoid direct contact with the humid ground and prevent moisture from penetrating from the bottom. In addition, the cabinet door should be designed with a double-layer structure or a labyrinth seal to enhance the sealing effect and prevent moisture from entering through the door gap.
Cable Entry Design: The cable entry of the switchgear is a key part of moisture intrusion. The cable entry should be designed at the bottom or side of the cabinet, and waterproof cable glands should be used to seal the gap between the cable and the cabinet. For multiple cables, a collective cable entry box with a sealing gasket should be used to ensure that each cable is tightly sealed. In addition, the cable entry should be designed to be downward-facing to prevent water from flowing into the cabinet along the cable.
Internal Compartmentalization: The internal of the switchgear should be divided into multiple compartments (such as busbar compartment, circuit breaker compartment, and control compartment) using metal partitions. This not only prevents mutual interference between components but also reduces the spread of moisture between compartments. The control compartment, which contains sensitive electronic components, should be designed as a separate sealed compartment to provide better moisture protection.

2. Ventilation and Dehumidification Design

Even with a good sealed structure, a small amount of moisture may still enter the switchgear, and condensation may occur inside the cabinet due to temperature differences. Therefore, it is necessary to design a reasonable ventilation and dehumidification system to remove the moisture inside the cabinet and maintain a dry environment.

Ventilation Design: For switchgear installed in well-ventilated humid environments, natural ventilation can be adopted. Ventilation holes should be designed at the top and bottom of the cabinet, and the top ventilation holes should be equipped with rain covers to prevent rainwater from entering. The bottom ventilation holes should be equipped with dust-proof and moisture-proof filters to prevent dust and moisture from entering. For switchgear installed in poorly ventilated environments (such as underground engineering), forced ventilation should be adopted, and axial fans or centrifugal fans should be installed to enhance air circulation and remove moisture.
Dehumidification Design: In environments with high humidity and frequent condensation, it is necessary to install dehumidification devices inside the switchgear. Common dehumidification devices include semiconductor dehumidifiers, condensation dehumidifiers, and silica gel dehumidifiers. Semiconductor dehumidifiers are small in size, low in power consumption, and suitable for small and medium-sized switchgear; condensation dehumidifiers have high dehumidification efficiency and are suitable for large switchgear or environments with extremely high humidity; silica gel dehumidifiers are simple in structure and low in cost, but need to be replaced regularly. The dehumidification device should be set to automatically start and stop according to the relative humidity inside the cabinet (usually set to start when the relative humidity is above 70% and stop when it is below 60%), to maintain the internal humidity at a safe level.

3. Sealing Treatment

Sealing treatment is the key to preventing moisture intrusion. All gaps and joints of the switchgear should be sealed to form a complete sealed barrier. The following key parts should be focused on:

Cabinet Door Sealing: A high-quality EMC gasket or rubber gasket should be installed around the cabinet door. The gasket should be closely attached to the cabinet body to ensure no gaps. The gasket should be made of moisture-resistant and aging-resistant materials (such as nitrile rubber or silicone rubber) to ensure long-term sealing performance.
Panel and Cover Sealing: All removable panels, covers, and observation windows of the switchgear should be sealed with gaskets or sealant. The observation window should be made of tempered glass with a sealed edge to prevent moisture from entering through the gap between the glass and the frame.
Component Installation Sealing: The installation holes of internal components (such as circuit breakers, contactors, and terminal blocks) should be sealed with sealant or rubber plugs to prevent moisture from entering through the installation holes. The joints of busbars and cables should be wrapped with waterproof tape or heat-shrinkable sleeves to enhance the sealing effect.

4. Anti-Corrosion Design

In humid environments, especially in coastal areas or chemical environments, the switchgear is not only affected by moisture but also by corrosive gases (such as salt spray, sulfur dioxide) and liquids, which can cause corrosion of metal components. Therefore, anti-corrosion design is an important part of the protection design.

Internal Anti-Corrosion Treatment: All metal components inside the switchgear (such as busbars, mounting rails, and component brackets) should be galvanized, nickel-plated, or chrome-plated to enhance corrosion resistance. For key components, such as busbar connections, anti-corrosion grease or conductive paste should be applied to prevent contact corrosion. The internal surface of the cabinet should be painted with anti-corrosion paint (such as epoxy resin paint) to form a protective layer.
External Anti-Corrosion Treatment: The external surface of the cabinet should be treated with anti-corrosion and anti-rust treatment. For stainless steel cabinets, passivation treatment can be performed; for aluminum alloy cabinets, anodizing treatment can be performed; for galvanized steel cabinets, a layer of anti-corrosion paint can be applied. In addition, the external surface of the cabinet should be regularly cleaned and maintained to remove dust, salt spray, and other corrosive substances.
Corrosion-Resistant Fasteners: All fasteners (such as bolts, nuts, and screws) used in the switchgear should be made of corrosion-resistant materials (such as stainless steel or galvanized steel) to avoid corrosion and loosening caused by moisture.

5. Installation and Maintenance Design

Reasonable installation and regular maintenance are essential to ensure the long-term stable operation of the switchgear in humid environments. The installation and maintenance design should be user-friendly and facilitate daily inspection and maintenance.

Installation Design: The switchgear should be installed in a well-ventilated, dry, and non-corrosive area. It should be away from water sources (such as water pipes, drainage ditches) and areas with frequent condensation. The installation foundation should be level and dry, and the bottom of the cabinet should be isolated from the ground with insulating pads to prevent moisture from penetrating from the ground. In addition, the switchgear should be installed vertically to avoid water accumulation in the cabinet.
Maintenance Design: The switchgear should be designed with easy-to-open doors and removable panels to facilitate daily inspection and maintenance. The dehumidification device should be easy to replace or maintain, and the filter of the ventilation system should be easy to clean. In addition, the switchgear should be equipped with a humidity and temperature monitoring device, which can display the internal humidity and temperature in real time, facilitating timely adjustment of the dehumidification system and maintenance.

III. Engineering Application Cases and Attention Points

To further verify the effectiveness of the selection and protection design of low-voltage assembled switchgear in humid environments, this section combines practical engineering cases to summarize the key attention points in actual application.

1. Engineering Application Case

A water treatment plant in a coastal area has a relative humidity of 85%-90% all year round, and the air contains a small amount of salt spray, which is a typical harsh humid environment. The low-voltage assembled switchgear used in the plant initially adopted ordinary steel cabinets with IP54 protection level, but after half a year of operation, serious corrosion of internal busbars and contactors occurred, and insulation breakdown occurred twice, affecting the normal operation of the water treatment system. Later, the switchgear was replaced with 316 stainless steel cabinets with IP65 protection level, and the following protection measures were adopted: 1) Installed semiconductor dehumidifiers inside the cabinet to maintain the internal relative humidity below 60%; 2) Used waterproof cable glands and sealed all gaps with silicone sealant; 3) All internal metal components were nickel-plated, and anti-corrosion grease was applied to busbar connections; 4) The cabinet was installed 150mm above the ground, away from water sources. After the transformation, the switchgear has been operating stably for 3 years, with no corrosion, insulation degradation, or other problems, which verifies the effectiveness of the selection and protection design.

2. Key Attention Points

Adapt to the Specific Humid Environment: The selection and protection design should be based on the specific humidity level, moisture intrusion form, and corrosive substances of the environment. For example, coastal areas need to focus on salt spray corrosion resistance, while underground engineering needs to focus on condensation dehumidification.
Comprehensive Protection, Not Single Reliance on Sealing: Sealing is an important part of moisture protection, but it cannot be relied on alone. It is necessary to combine ventilation and dehumidification, anti-corrosion treatment, and other measures to form a comprehensive protection system.
Regular Inspection and Maintenance: Even with perfect selection and protection design, regular inspection and maintenance are still required. Regularly check the sealing performance, dehumidification device, corrosion of components, and insulation performance of the switchgear, and deal with potential problems in a timely manner.
Comply with Relevant Standards: The selection and protection design of the switchgear should comply with international standards such as IEC 61439 and IEC 60529, as well as national standards, to ensure the quality and safety of the equipment.

IV. Conclusion

In humid environments, the low-voltage assembled switchgear is faced with the risks of moisture intrusion, component corrosion, and insulation degradation, which seriously affect the safe and stable operation of the power distribution system. Therefore, the selection and protection design of the switchgear in such environments must be given sufficient attention.

The selection of switchgear should focus on the cabinet material, protection level, internal components, and insulation performance, and select products that are adaptable to humid environments. The protection design should adopt a multi-level protection system including structural optimization, ventilation and dehumidification, sealing treatment, and anti-corrosion measures to effectively prevent moisture intrusion, reduce moisture accumulation, and enhance corrosion resistance. At the same time, reasonable installation and regular maintenance should be carried out to ensure the long-term stable operation of the switchgear.

With the continuous development of industrial technology, the application scenarios of low-voltage assembled switchgear are becoming more and more complex, and the requirements for moisture resistance and corrosion resistance are also getting higher and higher. In the future, it is necessary to further optimize the selection and protection design, adopt new materials, new technologies, and new processes, to improve the adaptability of the switchgear to humid environments, and provide a more reliable guarantee for the safe operation of power distribution systems in various humid environments.

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