CompEx - Basic Principles |

CompEx - Basic Principles


Since the early 1920`s many standards have evolved both in Britain and all over the world. Most of these standards are now obsolete so there is one international standard that is really important to remember - IEC 60079.

IEC 60079-10 - Classification of Hazardous Areas

IEC 60079-14 - Construction of electrical installations

IEC 60079-17 - Inspection and maintenance

ATEX Directives

ATEX is the European Directive 94/9/EC which applies to all equipment, both electrical and mechanical, used in hazardous atmospheres (both dust and gas). It came in to effect on 1th July 2003.

ATEX 95 - deals with all equipment and protective systems in use in potentially explosive environments.

ATEX 137 - workplace directive that sets the minimum requirements for the safety and health protection of workers in potentially explosive environments.


Spark - a form of electrostatic discharge; it can occur in various types (short circuit, lightning, static electricity, operation of switches and relays). It is important to remember that there can be two types of sparks - incentive spark and non-incentive spark.

An Incentive spark has sufficient energy to cause an explosion in a hazardous environment (i.e. Ignite a gas source).

A non-incentive spark does not have the sufficient energy to cause an explosion in a hazardous environment.

Flashpoint - the lowest temperature at which sufficient vapour is given off a liquid, to form a flammable mixture with air that can be ignited by an arc, spark or naked flame.

Ignition temperature - the minimum temperature at which a flammable material will spontaneously ignite.

Ambient temperature - from -20 celsius to 40 celsius.


Area classification is a method of analysing and classifying the environment where explosive gas atmospheres may occur. The main purpose is to give guidance in the selection and installation of apparatus to be used in that environment, taking into account the properties of the flammable materials that will be present.

Hazardous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere.

Zone 0 - an area in which an explosive atmosphere is present continuously, or for long periods, or frequently (more than 1000 hours per year).

Zone 1 - an area in which an explosive atmosphere is likely to occur in normal operation occasionally (more than 10, but less than 1000 hours per year).

Zone 2 - an area in which an explosive atmosphere is not likely to occur in normal operation, but if it does occur, will persist for a short period only (less than 10 hours per year).

Explosive atmosphere equipment groups

Group I - Mining activities - equipment designed for use below ground and associated areas where methane is the flammable gas (not covered in the CompEx course).

Group II - Surface industries - Gas - Sub-divided IIA, IIB, IIC (types of gases) - equipment that is used primarily used above ground where any other flammable gas may be present.

Group III - Surface industries - Dust - Sub-divided IIIA, IIIB, IIIC - equipment that is used where flammable dust may be present.

Equipment Protection Levels (EPL)

EPL`s are the IEC`s equivalent of the ATEX categories. It is a complicated and illogical concept, but the candidate must be able to remember the table below in order to successfully complete the CompEx course.

Zone ATEX Category IEC EPL
0 1G Ga
1 2G Gb
2 3G Gc


It is important to understand that equipment designed for a higher level of protection (i.e. Zone 0) can be used in areas of lower requirements (i.e. Zone 1 or Zone 2). However, equipment designed for lower levels of protection (i.e. Zone 1) must never be used in areas with higher levels of protection requirements (i.e. Zone 0).

Gas sub-division groups

Gas sub-division Gas Ignition Temp. °C
IIA Benzene
IIB Ethylene 425
IIC Hydrogen


Note: this table consists only of the most important gases and their ignition temperatures. During the course you will learn about more gases in each group. In order to successfully complete the CompEx course the candidate will have to memorise the contents of this table.

Temperature Classification

Temperature classification is based on the maximum temperature reached by any part of the equipment that could be in contact with a flammable gas.

T Class Max . surface temp.
T1 450 °C
T2 300 °C
T3 200 °C
T4 135 °C
T5 100 °C
T6 85 °C


In order to successfully complete the CompEx course the candidate will have to memorise the contents of this table.


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