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The classification of an area is defined in the National Electrical Code (among other places) as a way of determining how to make electrical equipment safe in a hazardous environment.
The classification system is slightly different in the USA from the way it is defined in Europe, but in general the ideas are similar. This note describes the American system.
The area classification is separated into three parts, the Class, the Division, and the Group.
The Class determines the type of hazard – in our case, it is explosive gases almost always. Other hazards might be explosive dusts, for example. Explosive gases are in Class 1.
The Division determines the degree of hazard. Division 1 means that the hazard is likely to occur; Division 2 means that it is possible for it to occur.
If it is very unlikely for a hazard to occur , we say the area is “General purpose” meaning that no particular safety standards apply other than the normal electrical hazard standards.
The Group refers to the type of gas we are dealing with. Group A is acetylene, group B is hydrogen, and groups C and D are pretty much everything else organic, such as methane or ethanol. A and B are a lot easier to ignite than the C or D materials.
Equipment used in a hazardous area is supposed to be approved by a National Recognized Testing Laboratory to meet the appropriate classification. There are many such laboratories – UL, FM and CSA are famous examples. When it has been approved, the agency will issue a certificate saying that the piece of equipment is approved for use in Class 1 Division 1 Group BCD areas, for example.
It takes a professional engineer to determine the hazard level in any particular area, but as a rough guide, if you are close to something which is pretty likely to leak flammable gases (such as a valve, compressor, storage tank etc.) you are likely to be in a division 1 area, whereas if you are far enough away that things would have to go wrong in a big way for a leak to be a problem, you’d be in a division 2 area. Over the fence in the next block you’d be in a general purpose area. Since flammable gas analyzers are of necessity measuring the flammable gas, in general they should be regarded as division 1 areas by themselves. Portable analyzers that only occasionally see the gas might be regarded as division 2 devices. Analyzers that are in well ventilated areas with no likelihood of a high pressure leak might well be regarded as division 2. These statements are just to give you an idea, they should not be regarded as definitive.
In this area it is quite likely that a leak can occur, and so the analyzer or other equipment used must be designed so it won’t ignite any leak (or more exactly, that it is no more likely to ignite it than a meteorite will hit it. You can’t ever achieve certainty, only a very small chance that the worst could happen). Such an analyzer can use explosion proof enclosures (designed so that if an explosion happens in one, the flames won’t leave the enclosure to ignite anything outside it); it can use a purge or be filled with oil or liquid so that no explosion can occur inside it, or it can be “intrinsically safe” which means that no matter what happens, there isn’t enough electrical energy to ignite the gas.
AMI uses a combination of explosion-proof enclosures to protect the high power sections, such as the relay contacts (which of necessity may arc and spark), and intrinsic safety on the analytical parts. This allows the use of relays and heaters, while still allowing for easy sensor replacement and simple operator controls.
It is essential that the installation of the analyzer meet the NEC requirements, with sealed-off rigid conduits and all the other requirements. If those requirements aren’t met all the safety precautions are voided and you can have very serious results including injury and death.
In this area it is possible that a leak can occur leading to an explosive atmosphere, but it isn’t likely. In this case the protection required of electronic equipment is reduced accordingly so that it is merely unlikely that the equipment can ignite the gases, not virtually impossible. The idea is that if the equipment in normal operation doesn’t do anything that can ignite such a mixture, it is OK. This means in practice that it doesn’t get hot, and it doesn’t contain any arcing sparking contacts, or switches that switch currents or voltages that in normal operation have enough energy to ignite the gas mixtures. For example, an electric motor that uses brushes would probably not be suitable, but a DC brushless motor probably would be OK.
You can always use equipment approved for Class 1 Division 1 in a Class 1 Division 2 area (with the same or higher group classification).
This would be an area that is very unlikely to see a hazardous gas mixture, such as a laboratory or a control room. Equipment used in this kind of area needs to meet no particular safety requirements other than the normal standards for electrical equipment such as are covered by the UL approvals you see on pretty much everything electrical you buy.