Class I
A location where there is a danger of explosion due to the presence of a flammable gas or vapour.

Class II
A location where there is a danger of explosion due to the presence of flammable dust.

Class III
A location where there is a danger of explosion due to the presence of flammable fibres or flyings.

Division 1
A location where an explosive mixture of gas, vapour, dust, fibres or flyings and air may exist under normal operating conditions.

Division 2
A location where an explosive mixture of gas, vapour, dust, fibres or flyings and air may exist under abnormal operating conditions, such as accidental rupture of a vessel or a pump seal leak.

Substances are categorized into natural groups based primarily on the similarity of their MESG and MIC values.

MESG (Maximum Experimental Safe Gap)
The distance through which an explosion can propogate if the gap between two machined surfaces exceeds a certain value.

MIC (Minimum Igniting Current)
The minimum level of current at which each gas or vapour will ignite when tested in a standard intrinsic safety apparatus.

Other factors include explosion pressure and time to peak pressure.

GROUP A GASES/VAPOURS:
Acetylene

GROUP B GASES/VAPOURS:
Butadiene, Ethylene Oxide, Acrolein (inhibited), Hydrogen, Propylene Oxide, manufactured gases containing more than 30% Hydrogen (by volume)

GROUP C GASES/VAPOURS:
Acetaldehyde, Allyl Alcohol, N-Butyraldehyde, Carbon Monoxide, Crontonaldehyde, Cyclopropane, Diethyl Ether, Djiethylamine, Epichlorohydrin, Ethylene, Ethylenimine, Hydrogen Sulphide, Morpholine, 2-Nitropropare Tetrahydrofuran, Isoprene, Unsymmetrical Dimethl Hydrazinc (UDMH)

GROUP D GASES/VAPOURS:
Acetic Acid (Glacial), Acetone, Acrylonitrite, Ammonia, Bensene, Butane, 1-Butanol (Butyl Alcohol), 2-Butanol (Secondary Butyl Alcohol), N-Butyl Acetate, Isobutyl Acetate, Di-Isobutylene, Ethone, Ethonol (Ethyl Alcohol), Ethyl Acetate, Ethyl Acrylate (inhibited), Ethylene Diamine (Anhydrous), Ethylene, Dichloride, Gasoline, Heptanes Hexanes, Isoprene, Isopropyl, Ether, Mesityl Oxide, Methane (Natural Gas), Methanol (Methyl Alcohol), 3-Methyl-1-Butanol (ISO-Amyl Alcohol), Methyl Ethyl Keytone, Methyl, Isobutyl Keytone, 2-Methyl-1-Propanol (Isobutyl Alcohol), 2-Methyl-2-Propanol (Tertiary Butyl Alcohol), Petroleum Naphtha, Pyridene, Ocianes, Pentanes, 1-Pentanol (Amyl Alcohol), Propane, 2-Propanol (Isopropyl Alcohol), Propylene, Styrene, Tolvene, Vinyl Acetate, Vinyl Chloride, Xylenes

GROUP E DUSTS:
Powdered metals such as Aluminum, Magnesium and their commercial alloys

GROUP F DUSTS:
Dusts such as Carbon Black, Coal or Coke dust

GROUP G DUSTS:
Dusts such as Flour, Starch or Grain dust

Developed by the International Electrotechnical Commission

GASES & VAPOURS

Zone 0: an area in which an explosive gas-air mixture is continuously present or present for long periods of time.

Zone 1: an area in which an explosive gas-air mixture is likely to occur in normal operation.

Zone 2: an area in whcih an explosive gas-air mixture is not likely to occur and, if it occurs, will exist only for a short time.

DUSTS

Zone 10: an area in which an explosive dust-air mixture is continuously present or present for long periods of time.

Zone 11: an area in which an explosive dust-air mixture is likely to occur in normal operation.

Zone 12: an area in whcih an explosive dust-air mixture is not likely to occur and, if it occurs, will exist only for a short time.

GROUP I
For mines susceptible to fireamp

GROUP II
For explosive gases and vapours in locations other than mines

GROUP IIA
For atmospheres containing propane and gases of equivalent hazard

GROUP IIB
For atmospheres containing ethylene and gases of equivalent hazard

GROUP IIC
For atmospheres containing hydrogen and gases of equivalent hazard

AIT (Auto-Ignition Temperature) is the temperature at which a gas, vapour or dust will ignite spontaneously without any other sources of ignition. Years ago, the IEC recognized that there was no relation between the gas, vapour and dust groupings, and the AITs of the various compounds. As such, temperature codes were established for equipment used in hazardous locations, and adopted by CEC and NEC.

IEC and North American Temperature Codes
IEC Temperature Codes (European Standard) Temperature, °C North American Temperature Codes
T1	450	T1
T2 - - - -	300 280 260 230 215	T2 T2A T2B T2C T2D
T3 - - -	200 180 165 160	T3 T3A T3B T3C
T4 -	135 120	T4 T4A
T5	100	T5
T6	85	T6
Notes: (1) As an alternative to the above temperature codes, the actual maximum temperature that a device can attain, under normal or overload conditions, may be shown. (2) In North America, if no temperature or temperature code is shown, the maximum external temperature is assumed to be 280°C for Groups A and B, 160°C for Group C and 215°C for Group D.

IMPORTANT NOTE
When considering a particular gas, vapour or dust, we must not only consider the Class and Group of the compound, but also the AIT. For example, Ethyl Ether is rated as Class I, Group C, but the AIT is 160°C (320°F), which is lower than the Ruffneck™ FX5 Explosion-Proof Electric Air Unit Heater's T3B rating. Another example would see Hydrogen's AIT at 550°C (1022°F), but it is rated as a Class I, Group B gas. The FX5 is rated for Group C, D, E, F & G only.

An explosion-proof enclosure is an enclosure that will withstand an internal explosion of gas or vapour without rupture and without causing the ignition of an external gas or vapour. It is important to remember that EXPLOSION-PROOF DOES NOT MEAN WATERPROOF.

To prevent the ignition of an external explosive atmosphere, the enclosure must not only be strong enough to withstand the internal explosion pressure, but all of the openings (e.g. cover joints, conduit or cable entries, operating shafts, etc.) must be tight enough to cool the hot burning gases before they can come into contact with the external atmosphere.

Three basic conditions must be satisfied for a fire or explosion to occur. First, a flammable liquid, vapor or combustible dust must be present in sufficient quantity. Second, the flammable liquid, vapor or combustible dust must be mixed with air or oxygen in the proportions required to produce an explosive mixture. Finally, a source of energy must be applied to the explosive mixture.

In applying these principles, the quantity of the flammable liquid or vapor that may be liberated and its physical characteristics must be recognized. Vapors from flammable liquids also have a natural tendency to disperse into the atmosphere, and rapidly become diluted to concentrations below the lower explosion limit, particularly when there is natural or mechanical ventilation.

In order to have an explosive gas atmosphere, the concentration of the gas or vapor must be above the Lower Explosive Limit (LEL) but below the Upper Explosive Limit (UEL). The possibility that the gas concentration may be above the upper explosion limit does not afford any degree of safety, as the concentration must first pass through the explosive range to reach the upper explosion limit.

Electrical equipment approved or listed for use in North America will bear the following information:

1. The class or group of the hazardous location.


2. The temperature code (T1-T6) or the maximum surface temperature of the equipment.

Electrical equipment approved to the IEC standards will bear the following markings:

1. The letter "E", indicating that it is suitable for use in an explosive atmosphere


2. The letter indicating the type of protection used (e.g., "d" for explosion-proof)


3. 3) The group for which the equipment is approved


4. The temperature code (T1-T6) or the maximum surface temperature of the equipment

Oil & Gas
Heating the various buildings associated with oil and gas transmission and processing facilities. Typical buildings that require explosion-proof heating are: compressor buildings, pumping buildings, water separator buildings, fresh water knock out buildings, produced water disposal buildings, analyzer buildings, pressure reducing stations, oil tank farms, gas & oil production satellites, well head enclosures, and drilling rigs (dog house, manifold buildings and other enclosed areas).

Printing & Packaging
Heating storage facilities for the various inks and volatile solvents associated with the printing and packaging industries. Manufacturers of cans, boxes, cartons and other containers that require printing services may use hazardous materials in the printing and labeling processes.

Ammunition Storage/Manufacturing
Heating areas where ammunition manufacturing or storage takes place.

Wastewater Treatment Plants
Heating process areas at sewage treatment plants where the potential for build-up of explosive concentration of methane or sour gas exists.

Distilleries & Breweries
Heating areas exposed to, or with the potential for exposure to, alcohol vapor in explosive concentrations.

Transport Equipment - Wash Bays
Heating wash bays used for the cleaning of tanker trucks or railcars that transport hazardous chemicals, gasoline, etc. Tanks are cleaned or purged after transport to avoid contamination of products that are not compatible with previously transported materials. Large volumes of vapor may be produced when tanks are purged. Heating equipment may need to be of explosion-proof design.

Hospitals
Heating medical gas storage buildings, where oxygen and flammable gases are present.

Chemical Hazardous Wastes
Heating storage lockers and containment buildings used to store chemicals, paints and other hazardous wastes.

Fuel Storage & Refuelling Facilities
Heating areas that use extremely volatile fuels, such as airplane hangers, jet fuel storage / metering facilities, natural gas refueling stations (e.g., city buses), etc.

Fiberglass Industry
Heating areas where fiberglass products are produced or cured after production. Factories that produce truck toppers, canoes, body panels for cars and snowmobiles or other fiberglass products may require explosion-proof heating. The fiberglass industry uses volatile solvents in its manufacturing process.

Painting
Heating areas that may contain explosive concentrations of vapors associated with various painting processes.

Agricultural
Heating areas where explosive concentrations of dusts may be present. These include grain handling facilities, feed mills, etc.

Coal
Heating conveyer areas at coal handling facilities, as found in coal mines, coal fired power generation plants, or any other areas where large quantities of coal are handled or consumed.

Chiller Rooms
Heating chiller rooms where large quantities of fluorocarbon refrigerants are present. Freon gas decompresses to form phosgene gas when it comes in contact with surfaces with temperatures exceeding 1094ºF (590ºC). Phosgene gas is a toxic substance. Standard type heaters have elements with surface temperatures exceeding 1094ºF (590ºC). The XL4 series heaters have no surfaces exceeding 1094ºF (590ºC). The City of Chicago recently purchased heaters for this application.

A Hazardous Location is defined as a location where fire or explosion hazards may exist due to the presence of:

•  Flammable gases or vapours
•  Flammable liquids
•  Combustible dusts
•  Ignitable fibres or flyings