Hazardous Area Classification

History Of Hazardous Area 

In the 18th century, frequent accidents happened due to fire and explosions in coal mines, using ordinary lamps in the explosive atmosphere in the presence of methane and coal dust and other flammable gases led to fire and incidents. In 19th century, scientist started to invent safety lamps, to avoid frequent incidents. By 1813 Dr. William Clanny invented the first safety lamp, after that in 1815 Sir Humphry Davy, Dr. J. Mueseler, and others invented safety lamps, that comply with the explosive atmosphere in coal mines at this age.

That safety lamps were the beginning, after that by the inventing of petrochemical industries, the ordinary electrical apparatus also became ignition source in hydrocarbon environment, in the 19th century the United States of America introduced the National Fire Protection Association (NFPA).

Hazardous Area Classification
Hazardous Area Classification

Through the period from 1813 to 1911 many trials of inventing lamps, and methods to avoid frequent incident due to usage of ordinary electrical equipment.
We can say that, the systematic and sophisticated development started in 1911, When NFPA -National Fire Protection association _ developed the National Electrical Code, by 1943 Europe introduce the first rules for construction of electrical equipment in EX areas, the following lines describe a part of this development.

1947: The NEC introduces the class /division classification
1971: introduction of operating temperatures
1975: start to harmonize national guidelines in Europe
1994: unification of European legislation (ATEX).
1996: NEC introduces the Class/Zone Classification

2007: worldwide unification of guidelines (IECEx).

Hazard Area Classification Importance

 Hazard Area Classification Importance
 Hazard Area Classification Importance

There are many tragic accidents happened around the world. Texas City refinery explosion one of them, that explosion occurred on March 23,2005, when a hydrocarbon vapor cloud was ignited and violently exploded at Isomerization process unit at BP’s Texas City refinery. this explosion causes a fatal accident, many people died and injured, huge damage occurred in the plant. The blast was believed to have been caused due to sparks in the presence of hydrocarbons.
Lack of Knowledge in hazardous area classification can be very serious and fatal, causing irrevocable damage to life and property.
The hazardous area could be in Paint shops, mines, refineries, chemical plants, petrochemical, sugar plants...etc.

So, what is a hazardous area?

An area in which an explosive gas atmosphere is, or may be expected to be, present in a quantity such as to require special precautions for the construction, installation, and use of electrical apparatus. IEC60079-0

 There are two systems of Hazardous Area classification

The first system is known as Class / Division system this system mainly used in North America in the USA and Canada, National Electrical Code NEC 500, American petroleum institute API 500, describes the main features of that system.   

The Second system is known as a zone system. Inside North America via NEC505, API505 that system is identified, also in Europe and around the world there is harmonization on this system through IECEx Standards, IEC60079 describe the main topics of that system, IEC60079-10-1 for gases, while IEC60079-10-2 for flammable dusts.   

Fire/ Explosion Triangle
Fire/ Explosion Triangle

Fire/ Explosion Triangle

three factors have to be present at the same time to create a fire or explosion known as the fire or explosion triangle:
1.      flammable material, which could be gas, liquid, dust or fibers will exist with sufficient quantities, there are two limits
·        LEL: Low Explosive Limit or Low Flammability Limit (LFL)
·        The lowest concentration by the percentage of gas or vapor in the air that is capable of producing a flash of fire in the presence of an ignition source, concentration below the LEL, will cause no fire because of the flammable substance is not sufficient.
·        UEL: Upper Explosive Limit or Upper Flammability Limit (UFL)
·        The Upper concentration by a percentage of gas or vapor in the air that is capable of producing a flash of fire in the presence of an ignition source.
·        Concentration above the UEL also will not cause fire or explosion because of not enough or sufficient oxygen.
These limit refer to the concentration limits of explosive atmosphere
2.     oxidizer or oxygen
                oxygen is found in sufficient quantity in the air and open spaces
3.     An ignition source could be arc, spark, heat, or flame.
Explosion due to Flammable dust need two factors in addition to the mentioned three factors, that dust should be dispersed and in pressurized atmosphere.
Temperature effect

Ignition temperature: is the minimum the temperature required, at normal atmospheric pressure in the absence of a spark or flame, to set afire or cause self-sustained combustion independently of the heating or heated element
Auto-ignition temperature: the lowest the temperature at which a combustible material will spontaneously ignite.
Flash Point: the lowest temperature at which a liquid can vaporize to form an explosive mixture.
Or Lowest liquid temperature at which, under certain standardized condition, a liquid gives off vapors in a quantity such as to be capable of forming an ignitable vapor/air mixture.
By heating a liquid, it will vaporize at certain temperature, that temperature is the flashpoint, by increasing the temperature till the auto-ignition temperature, the fire will happen in normal atmosphere.at this point heat is the ignition source.

Zone or Division System

The NEC 500, API 500, and CEC (Canadian Electrical Code) work under the umbrella of Class/Division.
ATEX, API505, NEC 505, and IECEx have another umbrella called zone system, most of the world now harmonize with the Zone system IECEx.

Zones definition According to IEC Standard
Zone 0
An Area in Which an explosive gas atmosphere is present continuously or for long periods or frequently.

Zone 1
An Area in Which an explosive gas atmosphere is likely to occur periodically or occasionally in normal operation.

Zone 2
An Area in Which an explosive gas atmosphere is not likely to occur in normal operation but, if it does occur, it will exist for a short period only.

Flammable materials Classification

In addition to the classification of the material according to the state (liquid- gas /vapor- dust/solid), materials also classified into groups according to the severity of each material this classification based on the minimum ignition current ratio(MIC), and the maximum Experimental Safety Gap (MESG).

Temperature Class/ Temperature Rating

The Maximum temperature generated from the electrical apparatus in the ambient temperature 40 °C, this temperature may be the ignition source, if it is located in the wrong area.

The Main purpose of that classification is to prevent the electrical equipment from being the ignition source, the electrical apparatus shall comply with the area of service. We can use electrical apparatus designed for Zone 0 in Zone 1 and 2, but we cannot use Zone 2 apparatus or zone 1 apparatus in Zone 0.
The following tables describe the main features of the Hazardous area classification according to the Zone system and division system.

Division Vs Zone

Zone 0
Division 1
Zone 1
Zone 2
Division 2

We can use zone 0 electrical apparatus in Division 1 but we cannot use Division 1 apparatus in Zone 0, while intrinsic safety apparatus EX it can be used only in Zone 0.
Division 2 and Zone 2 electrical apparatus have the same features.

Temperature class/Rating

Method of Protection

Classified apparatus shall be marked, to clear the area of usage, the following examples show that marking.


1.     Environment
I mines/ underground
II (surface)

2.     Category risk level /zone
1 zone 0
2 Zone 1
3 Zone 2

3.     Combustible material
 G Gas
D Dust
4.     standard
EEX international
AEX American
EX European

5.     Protection method
Ex d, flameproof method

6.     groups
IIC groups
7.     Temperature
T4 Temperature rating /class, that equipment can produce heat up to 135 °C at 40 °C Ambient Temperature

Example 2

Class I, Zone 1 AEx d IIC T6
Class I for gases
Zone 1
American standard
Explosion Proof
Group C
Temperature class T6

Example 3

Apparatus group 2 (surface)
Zone 1
International standard
Group IIC
Temperature class T6

Example 4
Class I Division 1 Group B T6
Class I for gases
Division I
Group B
Temperature class T6

Hazardous area classification, IEC60079, NEC 500, NFPA70, API500, API505, NEC 505, 

written by 

Ahmed Khairi

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