Due to the instability of plastic casings and materials, as well as the danger of electrostatic charge accumulation, most explosion-proof products generally adopt metal casings or metal components to reduce the incidence of explosion risks. However, the application of metal shells and metal components also needs to consider the characteristics of the metal. For example, high activity materials such as aluminum and magnesium are not suitable for direct use on the casing. The more common materials are cast aluminum, cast iron, cast steel, stainless steel, etc. In Chapter 8 of GB/T 3836.1-2021 Explosive Atmospheres Part 1: General Requirements for Equipment, there are also specific basic requirements for metal casings and metal components of casings.

8 Metal casings and metal components of casings

8.1 Material composition

The materials of the casing or casing components should be specified in the documents required in Chapter 24.

Note 1: The document does not require experimental verification of the chemical composition of the material.

Note 2: The paint or coating applied to the metal casing is a non-metallic component of the casing, and the requirements of GB/T 3836.7.4 "Electrostatic charges on external non-metallic materials" apply.

Note 3: Zinc and zinc alloys containing 80% or higher zinc are prone to rapid deterioration (especially in tensile strength), especially in warm and humid air. Zinc is also considered more active than other metals. Some explosion-proof types do not allow the use of zinc in the casing.

8.2 Class I equipment

The total content (mass fraction) of Class I EPL Ma or Mb equipment casing materials for manufacturing should not exceed:

15% aluminum, magnesium, titanium, and zirconium, with magnesium, titanium, and zirconium not exceeding 7.5%.

The above requirements do not apply to Class I portable measuring equipment, but the equipment should be marked with the symbol "X" according to the requirements of 29.3e), and special precautions during storage, transportation, and use should be specified in special usage conditions.

8.3 Class II equipment

The total content (mass fraction) of Class II equipment casing materials manufactured for different levels of equipment protection should not exceed:

For EPL Ga: 10% aluminum, magnesium, titanium, and zirconium, with magnesium, titanium, and zirconium not exceeding 7.5%;

For EPL Gb: 7.5% magnesium, titanium, and zirconium;

For EPL Gc: There are no other requirements except that the fan, fan hood, and ventilation baffle should meet the requirements of EPL Gb.

If the material composition limit of the EPL Ga or Gb level equipment mentioned above is exceeded, the equipment should be marked with an "X" according to 29.3e), and special usage conditions should contain sufficient information to ensure that users can determine whether the equipment is suitable for use under specific conditions, such as preventing ignition hazards caused by impact or friction.

8.4 Class III equipment

The total content (mass fraction) of Class III equipment casing materials manufactured for different levels of equipment protection should not exceed:

For EPL Da: 7.5% magnesium, titanium, and zirconium;

For EPL Db: 7.5% magnesium, titanium, and zirconium;

For EPL Dc: There are no other requirements except that the fan, fan hood, and ventilation baffle should meet the requirements of EPL Db.

If the material composition limit of the EPL Da or Db level equipment mentioned above is exceeded, the equipment should be marked with an "X" according to 29.3e), and special usage conditions should contain sufficient information to ensure that users can determine whether the equipment is suitable for use under specific conditions, such as preventing ignition hazards caused by impact or friction.

8.5 Copper alloy

If intended for use in environments containing ethyl acetate, the equipment casing and the Ex component casing for external installation, if made of copper or copper alloy, shall meet the following requirements:

Apply zinc, nickel or other coatings; or

The maximum copper content in the alloy shall not exceed 65%.

The cable entry devices, sealing components, threaded pipe joints, and insulation sleeves specified in Appendix A are not considered as shell surfaces that require coating or copper content limitations.

Note: The limitation of using copper in a fast environment is due to the possibility of forming acetylene compounds on the outer surface that can be ignited by friction or impact.

From the standards, we can summarize:

1. For Class I EPL Ma and EPL Mb equipment, there are clear requirements for the aluminum content in the shell metal material. And equipment with aluminum metal casings should be marked with the symbol "X" according to the requirements of 29.3e), and special precautions during storage, transportation, and use should be specified in special usage conditions.

2. For Class II EPL Ga equipment, there are clear requirements for the aluminum content in the shell metal material. We generally do not recommend using aluminum alloy shells. If the customer's product uses an aluminum alloy shell, a material report for the metal shell 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" in accordance with the requirements of 29.3e, and special usage conditions should contain sufficient information to ensure that users can determine whether the equipment is suitable for use under specific conditions, such as preventing ignition hazards caused by impact or friction.

(2) Reduce the level of equipment protection, such as lowering the explosion-proof symbol from Ex ia IIC T4 Ga to Ex ia IIC T4 Gb.

4. Requirements for using copper or copper alloys in environments containing acetylene. Specific requirements have been made for the metal casing and metal components of equipment used in acetylene environments in GB/T 3836.1-2021 and GB/T 3836.2-2021, respectively. But according to the principle that specialized standards take priority over general standards. The copper content in the alloy is required to not exceed 60%.

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