Hazardous locations are defined as premises, buildings or parts thereof where fire or explosion hazards may exist due to the presence of flammable gases or vapors, flammable liquids, combustible dusts, or easily ignitable fibers or flyings. Although, flammable gases, vapors and combustible dusts exist almost everywhere, fortunately, they are usually present only in minute quantities. Simply because flammable gasses or vapors, or combustible dusts are present, there is not necessarily a hazardous location. The quantities or concentrations must be sufficient to present a potential explosion hazard.

Note: The electrical codes that deal with these types of hazardous locations areas do not deal with materials such as high explosives, dynamite, munitions, or fireworks. Other rules and regulations deal with areas involving these materials.

Hazardous Locations as defined by the National Electrical Code (NEC)

In North America, the National Electrical Code (NEC) uses a “Class, Division” System as the basis for area classification of hazardous (classified) locations. Because the hazards and methods of protecting electrical equipment against these hazards differ for different materials, hazardous locations are divided into three Classes, and two Divisions. The Classes are based on the type of hazard and the explosive characteristics of the material with the Divisions being based on the occurrence or risk of fire or explosion that the material presents. While Canada and the United States have some differences in acceptable wiring methods and product standards, their systems are very similar. Class I, Division 1, Group A denotes the most severely and continually hazardous condition.

Class I Locations	Are those in which flammable bases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitable mixtures
Class II Locations	Are those which are hazardous because of the presence of combustible dust.
Class III Locations	Are those which are hazardous because of the presence of easily ignitable fibers or flyings, but in which such fibers or flyings are not likely to be in suspension in air quantities sufficient to produce ignitable mixtures.
Division 1	Locations in which hazardous concentrations in the air exist continuously, intermittently, or periodically under normal operating conditions.
Division 2	Locations in which hazardous concentrations are handled, processed, or used, but are normally confined within closed containers or closed systems from which they can escape only in case of accidental rupture or breakdown.
Group A	Atmospheres containing acetylene.
Group B	Atmospheres containing hydrogen, or gases or vapors of equivalent hazard, such as manufactured gas.
Group C	Atmospheres containing ethyl-ether vapors, ethylene or cyclopropane.
Group D	Atmospheres containing gasoline, hexane, naphtha, benzine, butane, propane, alcohol, acetone, benzol, lacquer solvent vapors or natural gas.
Group E	Atmospheres containing metal dust, including aluminum, magnesium, and their commercial alloys and other metals of similarly hazardous characteristics.
Group F	Atmospheres containing carbon black, coal or coke dust.
Group G	Atmospheres containing flour, starch, or grain dusts.

Hazardous Locations as defined by IEC, CENELEC and ATEX standards

In the rest of the world and mostly in Europe, areas containing potentially explosive atmospheres are dealt with using a “Zone System”. Zones are based predominantly on the International Electrotechnical Commission (IEC) and European Committee for Electrotechnical Standardization (CENELEC) standards.

Apparatus Group I. Mines and parts of its surface installations where a potential danger of firedamp or fuel powders exists.

Apparatus Group II. Places where exist a danger of explosive atmospheres' formation (not included in Group I). Additionally, these locations, depending on the type of dangerous substances contained, are classified in two different classes:

Class I Locations. Places where there is or might appear gasses, vapors or fogs in sufficient quantity to produce explosive or flammable atmospheres. Included in this class are places where there are or might appear liquids that can produce flammable vapors. Locations are classified, according to MI BT 026, ATEX directive and the probability of flammable environment, as:

• Zone 0. Locations that have a continuous explosive gas, vapor or fog atmosphere or it is foreseen to be present during long periods of time or continuous short periods of time.

• Zone 1. Locations where it is foreseen that explosive gases, vapor or fog atmospheres will be occasional during the normal functioning.

• Zone 2. Locations that it is foreseen that explosive gases, vapor or fog atmospheres will not be present during the normal functioning or it will be rare and with a short duration.

Class II Locations. Locations where the risk is due to the presence of fuel powder, excluding explosives per se. Class II Locations are divided in 3 zones:

• Zone 20. An area in which a combustible dust, as a cloud, is present continuously or frequently during normal operations in sufficient quantities to produce an explosive mixture.

• Zone 21. An area in which a combustible dust, as a cloud, is likely to occur during normal operations in sufficient quantities to produce an explosive mixture.

• Zone 22. An area in which combustible dust clouds may occur infrequently and persist for only short periods of time or in which accumulations or layers may be present under abnormal conditions.

Class III Locations. Equally treated under the MI BT 026 and ATEX, zones where the risk is due to the presence of fibers or easily volatile flammable materials, but in which these fibers or volatile matters may not likely be in a sufficient quantity to produce explosive atmospheres.

Temperature classes

Ignition temperature or auto-ignition temperature (AIT) is the minimum temperature of a surface at which an explosive atmosphere ignites. Flammable vapors and gases can be classified into temperature classes according to their ignition temperature. The maximum temperature of a piece of equipment must always be lower than the ignition temperature of the gas-air mixture or vapor-air mixture in which it is placed. Accordingly these classes of temperature are classified as:

North American (NEC500-503)	IEC/CENELEC/NEC505	Surface Maximum Temperature
Temperature code	Temperature classes	°C	°F
T1	T1	450ºC	842°F
T2	T2	300ºC	572°F
T2A		280ºC	536°F
T2B		260ºC	500°F
T2C		230ºC	446°F
T2D		215ºC	419°F
T3	T3	200ºC	392°F
T3A		180ºC	356°F
T3B		165ºC	329°F
T3C		160ºC	320°F
T4	T4	135ºC	275°F
T4A		120ºC	248°F
T5	T5	100ºC	212°F
T6	T6	85ºC	185°F

Types of protection

The use of electrical materials within potentially explosive areas creates a risk of ignition due to warm-ups by the Joule effect, hysteresis, and arcs and sparks caused in opening and closing processes of a circuit. There are different methods of protecting electrical equipment, which prevent an explosion when used in a flammable gas atmosphere, in the presence of combustible dust or easily ignited fibers. Those methods are based on one of the following 3 principles:

• Reducing the energy or avoiding its contribution in the formation of arcs, sparks or excessive warm-ups.

• Isolating explosive atmosphere from energy source.

• Restraining the eventual explosion by controlling its effects.

Below are the commonly-used approvals and their methods of protection:

Flameproof type of protection "d" or Explosion proof equipment
Although the North American term “explosion proof” and IEC term "flameproof" are identical concepts, the requirements in the product standards are different. This means that while a product may be certified to both standards, it must be specifically approved for the location. For example, flameproof equipment is not permitted in Class I, Division 1 locations. Since flammable gases and vapors are expected inside an enclosure, the equipment must be capable of withstanding an explosion caused by sparking contacts of devices, high temperatures, or an electrical fault. The enclosure is designed so that hot gases generated during an internal explosion are cooled below the ignition temperature of the surrounding flammable atmosphere as they escape through the joints of the unit. In addition, the external surfaces of the enclosure must not become hot enough to ignite the surrounding atmosphere due to heat energy within the unit. This heat energy may be the result of normal operation of heat-producing equipment, or the result of an electrical arc to the enclosure from an arcing ground fault. Safety factors are applied to all testing of this type of enclosure to ensure the unit will not rupture as a result of an internal explosion.

Encapsulation type of protection “m”
Encapsulation is a type of protection in which the parts that can ignite an explosive atmosphere are enclosed in a resin. The resin must be sufficiently resistant to environmental influences so that the explosive atmosphere cannot be ignited by either sparking or heating, which may occur within the device. This is typically used with electronic devices.

Increased safety “e”
Additional measures are taken to achieve a higher level of safety and avoid the risk of impermissible high temperatures and the occurrence of sparks and arcs internally or external parts of electrical equipment, which in normal use produce neither sparks, arcs nor dangerous temperatures.

Hermetically sealed
A common type of hermetically sealed equipment is a contact block or reed switch. In this method, the arcing components of the switch are encased in a glass tube. The connecting wires are fused to the glass sealing the unit to prevent any ingress of flammable gases. Hermetically sealed equipment is suitable for Class I, Division 2 or Zone 2 only.

Intrinsically safe equipment types of Protection “i”, “ia” and “ib”
North America now identifies three versions of this protection method. Types “i” and “ia” are identical since type “i” is based on the IEC 60 079-11 Standard. In Zone 0 the only acceptable type of equipment is types “i” and “ia”. Type “ib” is acceptable in Zone 1 and 2 locations.

Oil Immersion type of protection “o”
Oil immersion excludes the outer atmosphere from coming in contact with the arcing, sparking or heat producing parts of the apparatus. In the past, large transformers or circuit breakers used this technique. Its use today is limited.

Purged and pressurized type of protection “p”
This type of protection prevents the surrounding atmosphere from entering an enclosure by maintaining a positive pressure within the unit. Clean air or inert gas is used to maintain a higher pressure than the surrounding atmosphere. In purging, the electrical equipment is interlocked with a system which cycles clean air within the unit to remove explosive gases before start up.

Sand Filled Apparatus Type of Protection “q”
In this type of protection, the enclosure, or electrical apparatus, is filled with finely granulated quartz (or very small glass beads) to prevent any arc which may occur within the enclosure from igniting the surrounding atmosphere. The equipment can have no moving parts, which are in direct contact with the filling materials, and the enclosure must have a minimum protection of IP 54.

Methods of protection

METHOD	MODE
Restrain the explosion	d
Isolate the explosive atmosphere from the energy source	p, m, q, o
Reduce the energy or avoid sparks or arcs	e, ia, ib

The protection methods used in Zone 0 can also be used in Zone 1 and 2, and the protection methods used in Zone 1 can also be used in Zone 2.