What are the requirements regarding the materials of metal enclosures and their metal components in product explosion-proof certification?

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Due to the instability of plastic casings and materials, as well as the risk of static charge accumulation, most explosion-proof products use metal casings or metal components in order to reduce the likelihood of explosions. However, when using metal casings and metal components, the properties of the metal also need to be taken into consideration. Metals with higher reactivity, such as aluminum and magnesium, are not suitable for use in casings directly. More common materials include cast aluminum, cast iron, cast steel, stainless steel, etc. Chapter 8 of GB/T 3836.1-2021 Explosive atmospheres – Part 1: Equipment – General requirements also specifies the basic requirements for metal enclosures and the metal components of enclosures. 8 Metal housing and metal components of the housing 8.1 Material composition The material of the housing or its components shall be specified in the documents required in Chapter 24. Note 1: The document does not require testing to verify the chemical composition of the materials. Note 2: The paint or coating applied to the metal enclosure is a non-metallic component of the enclosure, and the requirements of GB/T 3836.7.4 \"Static charge on external non-metallic materials\" apply. Note 3: Zinc and zinc alloys containing 80% or more zinc tend to deteriorate rapidly (especially in terms of tensile strength), particularly in warm and humid air. Zinc is also considered more reactive than other metals. Some explosion-proof types do not allow the use of zinc for the enclosure. 8.2 For the manufacture of Class I equipment with Class I EPL Ma or Mb rating, the total content (by mass fraction) of aluminum, magnesium, titanium, and zirconium in the casing material shall not exceed 15%; among these, the amount of magnesium, titanium, and zirconium shall not exceed 7.5%. The above requirements do not apply to Class I portable measuring equipment, but the equipment shall be marked with the symbol “X” as required by 29.3e), and special precautions for storage, transportation, and use shall be specified in the conditions for special use. 8.3 For the manufacture of Class II equipment, the total content (by mass fraction) of the shell materials used for different protection levels of Class II equipment shall not exceed: for EPL Ga, 10% for aluminum, magnesium, titanium, and zirconium, with magnesium, titanium, and zirconium together not exceeding 7.5% ； For EPL Gb: 7.5% magnesium, titanium, and zirconium ； For EPL Gc: Apart from the fan, fan cover, and ventilation opening baffles needing to meet the requirements of EPL Gb, there are no other requirements. If the material composition limits of the aforementioned EPL Ga or Gb class devices are exceeded, the device shall be marked with an “X” in accordance with 29.3e), and the special operating conditions shall contain sufficient information to enable the user to determine whether the device is suitable for use under specific conditions, such as to prevent the risk of ignition caused by impact or friction. 8.4 For the manufacture of Class III equipment – the total content (by mass fraction) of magnesium, titanium, and zirconium in the casing materials used for Class III equipment intended for different protection levels shall not exceed: 7.5% for EPL Da ； For EPL Db: 7.5% magnesium, titanium, and zirconium ； For EPL Dc: Apart from the fan, fan cover, and ventilation opening baffles needing to meet the requirements of EPL Db, there are no other requirements. If the material composition limits for the EPL Da or Db class devices mentioned above are exceeded, the device shall be marked with an “X” in accordance with requirement 29.3e), and the special operating conditions shall contain sufficient information to enable the user to determine whether the device is suitable for use under specific conditions, such as to prevent the risk of ignition caused by impact or friction. 8.5 If copper alloys are to be used in environments containing ethylene, the enclosures of equipment and those of Ex components intended for external installation, when made of copper or copper alloys, shall meet the following requirements: zinc plating, nickel plating, or other coatings ； or the maximum copper content in the alloy shall not exceed 65%. The cable entry devices, sealing elements, threaded pipe fittings, and insulating sleeves specified in Appendix A are not considered as the enclosure surfaces subject to coating or copper content restrictions. Note: The use of copper is restricted in acetylene environments due to the possibility of acetylide compounds forming on the outer surface, which can be ignited by friction or impact. From the standards, we can conclude the following: 1. For Class I EPL Ma and EPL Mb devices, there are specific requirements regarding the aluminum content in the metal material of the enclosure. Furthermore, devices with casings containing aluminum metal shall be marked with the symbol “X” as required by 29.3e), and special precautions during storage, transportation, and use shall be specified under special conditions of use. 2. For Class II EPL Ga devices, there are specific requirements regarding the aluminum content in the metal material of the enclosure. Generally, we do not recommend using an aluminum alloy casing. If the customer’s product features an aluminum alloy casing, a material report for the metal casing must be provided, specifying the aluminum content. If the aluminum content exceeds the specified limit, there are two common solutions. (1) The equipment must be marked with the symbol “X” as required by 29.3e, and the special conditions of use must contain sufficient information to enable the user to determine whether the equipment is suitable for use under specific conditions, such as to prevent the risk of ignition caused by impact or friction. https://pic1.zhimg.com/80/v2-0aa427d0c24780ea182aafb7e4125b62_1440w.webp?source=d16d100b（2）Reduce the equipment’s protection level, for example, lowering the explosion-proof rating from E*a IIC T4 Ga to E*a IIC T4 Gb. https://pic1.zhimg.com/80/v2-6defed0d3ad1eaec75fb40151f3bb6dc_1440w.webp?source=d16d100b2. For Class II EPL Gc and Class III EPL Dc devices, material requirements apply only to certain metal components of the enclosure; no requirements exist for other components. https://picx.zhimg.com/80/v2-0438ffbe93026533570bb695b0a65e31_1440w.webp?source=d16d100bhttps://pic1.zhimg.com/80/v2-251dc928521765559fe55d9f204fffed_1440w.webp?source=d16d100b3. For Class III devices, the material requirements for the metal enclosures of EPL Da and EPL Db devices are identical. 4. Requirements for the use of copper or copper alloys in environments containing acetylene. GB/T 3836.1-2021 and GB/T 3836.2-2021 specify specific requirements respectively for the metal enclosures and metal components of enclosures of acetylene equipment used in hazardous environments. But in accordance with the principle that dedicated standards take precedence over general standards. The copper content in the alloy is required to be no more than 60%. https://pic1.zhimg.com/80/v2-9c722aeceae14e81d7880218fac14017_1440w.webp?source=d16d100b Copying and reproduction on any platform are prohibited!