Explosive atmospheres have different chemical properties that affect the likelihood and severity of an explosion. Such properties include flame temperature, minimum ignition energy, upper and lower explosive limits, and molecular weight. Empirical testing is done to determine parameters such as the maximum experimental safe gap (MESG), minimum igniting current (MIC) ratio, explosion pressure and time to peak pressure, spontaneous ignition temperature, and maximum rate of pressure rise. Every substance has a differing combination of properties but it is found that they can be ranked into similar ranges, simplifying the selection of equipment for hazardous areas.
Flammability of combustible liquids are defined by their flash-point. The flash-point is the temperature at which the material will generate sufficient quantity of vapor to form an ignitable mixture. The flash point determines if an area needs to be classified. A material may have a relatively low autoignition temperature yet if its flash-point is above the ambient temperature, then the area may not need to be classified. Conversely if the same material is heated and handled above its flash-point, the area must be classified for proper electrical system design, as it will then form an ignitable mixture.
Each chemical gas or vapour used in industry is classified into a gas group.
| NEC Division System Gas & Dust Groups | ||
| Area | Group | Representative Materials |
| Class I, Division 1 & 2 | A | Acetylene |
| B | Hydrogen | |
| C | Ethylene | |
| D | Propane | |
| Class II, Division 1 & 2 | E (Division 1 only) | Metal dusts, such as magnesium (Division 1 only) |
| F | Carbonaceous dusts, such as carbon & charcoal | |
| G | Non-conductive dusts, such as flour, grain, wood & plastic | |
| Class III, Division 1 & 2 | None | Ignitible fibers/flyings, such as cotton lint, flax & rayon |
| NEC & IEC Zone System Gas & Dust Groups | ||
| Area | Group | Representative Materials |
| Zone 0, 1 & 2 | IIC | Acetylene & Hydrogen(equivalent to NEC Class I, Groups A and B) |
| IIB+H2 | Hydrogen(equivalent to NEC Class I, Group B) | |
| IIB | Ethylene(equivalent to NEC Class I, Group C) | |
| IIA | Propane(equivalent to NEC Class I, Group D) | |
| Zone 20, 21 & 22 | IIIC | Conductive dusts, such as magnesium(equivalent to NEC Class II, Group E) |
| IIIB | Non-conductive dusts, such as flour, grain, wood & plastic(equivalent to NEC Class II, Groups F and G) | |
| IIIA | Ignitible fibers/flyings, such as cotton lint, flax & rayon(equivalent to NEC Class III | |
| Mines susceptible to firedamp | I (IEC only) | Methane |
Group IIC is the most severe Zone system gas group. Hazards in this group gas can be ignited very easily indeed. Equipment marked as suitable for Group IIC is also suitable for IIB and IIA. Equipment marked as suitable for IIB is also suitable for IIA but NOT for IIC. If equipment is marked, for example, Ex e II T4 then it is suitable for all subgroups IIA, IIB and IIC
A list must be drawn up of every explosive material that is on the refinery/chemical complex and included in the site plan of the classified areas. The above groups are formed in order of how explosive the material would be if it was ignited, with IIC being the most explosive Zone system gas group and IIA being the least. The groups also indicate how much energy is required to ignite the material by energy or thermal effects, with IIA requiring the most energy and IIC the least for Zone system gas groups.
