IEC is the zone system. The class/division system is not used outside North America. They look similar but definitely not interchangeable. The difference is that the boundaries are slightly different so the assumed service conditions are also different. You can mix IEC and zones because the zone definitions come from IEC.
A NEMA 4X box is not explosion proof rated service. It is dust and water tight. The design for Exe is not to propagate a flame. This means doors able to withstand internal pressures, flame arresting vents, and conduit seals. NEMA 4X is none of that so a conduit seal would be not necessary either. I deal with a lot of wood plants, coal plants, and feed mills which are all Class 3. NEMA 4X is used a lot because it is dust proof. As long as the panel stays clean they don’t have to do anything more. You cannot take the same design and use it in gas service because it is not vapor proof. Nothing is except hermetically sealed equipment which is extremely limited.
This is at least one system that you mentioned. You could do it with purged and pressurized where the box construction doesn’t matter because you are creating a non hazardous atmosphere within a hazardous atmosphere. This sounds ridiculous but I was at a natural gas storage facility a couple years ago (liquify natural gas and store in million gallon tanks over the summer months, vaporize in winter) and all their panels were purged and pressurized with pressure alarms. Very nice setup for 4160 V 5000 HP compressors where an Exe panel is completely impractical.
Suggest you read up on the “Ex” classification system. That is what IEC uses. It has a lot more detail and explanation behind why hazardous location equipment is rated the way it is where in North America we just look for say a “Class 1 Division 2” label and buy it without understanding how it works. If you understand the Ex_ rating system hazardous location design is intuitive.
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