1. General Description
ATEX is the acronym of Atmosphere Explosive, explosive atmosphere, that is a mixture of flammable substances in form of gas, vapors, mist or dust, where combustion, after ignition, propagates through all the flammable mixture. A potentially explosive atmosphere is an atmosphere that is not explosive under normal operating condition, but can become such in case of unpredictable events, like gas leaks, equipment failure, temperature or pressure. To obtain the formation of an explosive atmosphere it is necessary that flammable substances are present in a determined concentration, otherwise a reaction of combustion will take place only or no reaction at all. This concentration range is defined for each flammable substance by the lower explosion limit (LEL) and the upper explosion limit (UEL), that is the minimum and maximum concentration, between which a given substance may be ignited, when given an ignition source.
2. Regulation force
Explosive atmospheres can arise in several industrial fields (oil& gas, chemistry, pharmaceutical, feed industry, metallurgic) during transport, manufacturing or storage processes and if ignited they can lead to harmful or catastrophic events for people and the environment. ATEX directive, in the European Union countries, aims to uniform the different legislations of its members with respect to the risk given by the presence of potentially explosive atmosphere: it is divided in 2014/34/UE and ATEX 99/92/CE. ATEX directive 2014/34/UE defines the essential safety requirements and protection systems intended to be used in potentially explosive atmosphere. It applies for manufacturers and require the certification of their products. ATEX directive 99/92/CE defines the minimum requirements for safety and health in working environments in potentially explosive atmospheres, where plants and certificated equipment are employed. It applies for final users.
3. Groups and Categories
According to ATEX directive, products are divided into categories based on the level of protection required and depending on the degree of danger of the environment I which they will be installed.
Group I: equipment intended for use in mines above and below ground. It is divided into two categories depending on the required protection level.
M1: equipment in this category is required to remain functional with an explosive atmosphere present. Level of protection “very high”.
M2: this equipment is intended to be de-energized in the event of an explosive atmosphere forming. Level of protection “high”.
Group II: : equipment intended for use in location, other than mines (surface industries). It is divided into three categories depending on the required protection level. For gaseous atmosphere categories are identified by the letter G, while for dusty atmospheres by the letter D.
1G or 1D: an explosive atmosphere is present continuously or for long periods. Level of protection “very high”.
2G or 2D: an explosive atmosphere is occasionally present. Level of protection “high”.
3G or 3D: an explosive atmosphere is not likely to occur of if it does, it will be only for short periods. Level of protection “normal”.
All electric equipment falling in categories 1 and 2 must be certified by an ATEX notified body and manufacturing companies have to undertake a quality surveillance system. For equipment falling in category 3, an auto-certification is sufficient. All product, whichever their group or category, must be CE certified and must be provided with the instruction manual.
4. Zone Clarification
According to ATEX 99/92/CE, explosive atmosphere are divided in zones by the employer by applying EN 60079-10.
Zone 0: an explosive atmosphere consisting of a mixture of air and flammable substances in form of gas, vapours or mists is present continuously or for long periods.
Zone 20: an explosive atmosphere consisting of a mixture of air and flammable substances in form of dusts is present continuously or for long periods.
Zone 1: an explosive atmosphere consisting of a mixture of air and flammable substances in form of gas, vapours or mists is occasionally present under normal operating conditions.
Zone 21: an explosive atmosphere consisting of a mixture of air and flammable substances in form of dusts is occasionally present under normal operating conditions.
Zone 2: an explosive atmosphere consisting of a mixture of air and flammable substances in form of gas, vapours or mists is never present, or if it occurs, it will be only for short periods.
Zone 22: an explosive atmosphere consisting of a mixture of air and flammable substances in form of dusts is never present, or if it occurs, it will be only for short periods.
The correspondence between EPL and ATEX categories in their respective installation zones is shown in the table below. Next figure shows instead an example of zones subdivision.
ATEX Category | EPL Protection Degree | Zone Installation | Atmosphere |
1G | Ga | 0 | Gas |
2G | Gb | 1 | |
3G | Gc | 2 | |
1D | Da | 20 | Dust |
2D | Db | 21 | |
3D | Dc | 22 | |
M1 | Ma | Mining | Coal Dust, Methane(CH4) |
M2 | Mb |
5. Atmosphere Groups
The choice of the equipment is to be made according to the zone it will be installed into, the present substances and their ignition temperatures. Electric equipment of group II are divided into subgroups IIA, IIB and IIC for gas and IIIA, IIIB and IIIC for dust. Group C is more restrictive than group B, that in turns in more restrictive than group A in terms of danger and safety regulation. Therefore, equipment designed for group IIC or IIIC can be used for application IIB or IIIB but not vice versa.
Gas Group | Representative gas | Dust Group | Representative Dust | |
IIA | Propane(C3H8) | IIA | Combustible shavings | |
IIB | Ethylene(C2H4) | IIB | Non-conductive | |
IIC | Hydrogen(H2), Acetylene(C2H2) | IIC | Conductive |
6. Temperature Class
The ignition temperature of the substances constituting the explosive atmosphere is another fundamental aspect to be considered. This value, defined as the lowest temperature at which an explosive mixture ignites spontaneously, must be higher than the maximum superficial temperature of the different components. For gas, electric components are divided into 6 temperature classes from T1 to T6 according to the limits reported in table
Temperature class | Equipment maximum surface temperature | Flammable substance ignition temperature |
T1 | 450℃ | >450℃ |
T2 | 300℃ | >300℃ |
T3 | 200℃ | >200℃ |
T4 | 135℃ | >135℃ |
T5 | 100℃ | >100℃ |
T6 | 85℃ | >85℃ |
Component classified as T3 may be used in zones, which require T1 or T2, but a component classified as T4 must not be used in zones, which require T5. In the following table some examples of most commonly used substances in ATEX sectors are reported.
Gas | Group | Temperature class | Ignition temperature |
Hydrogen(H2) | IIC | T1 | 560℃ |
Propane(C3H8) | IIA | T1 | 470℃ |
Ethylene(C2H4) | IIB | T2 | 425℃ |
Acetylene(C2H2) | IIC | T2 | 305℃ |
Kerosene | IIA | T3 | 280℃ |
Diethyl ether | IIB | T4 | 160℃ |
Carbon disulphide | IIC | T6 | 95℃ |
For combustible dusts a distinction is mad between clouds and layers of dusts.In case of cloud of dust with ignition temperature, regulation states that the highest superficial temperature must be T≤2/3 ignition temperature . In case of layer of dust instead, the highest permissible temperature is = T5mm −75 ℃ (there are special table to evaluate the highest temperature for layers of dust thicker than 5mm).
Dust | Typical ignition temperature [°C] | |
Gas | Cloud | Layer |
Aluminum | 560 | 450 |
Ground coal | 420 | 230 |
Cellulose | 520 | 410 |
Flour | 380 | 320 |
Wood | 450 | 220 |
Phenolic resin | 530 | 450 |
PVC | 700 | 450 |
Suger | 490 | 460 |
7. Ingress Protection Code
A further information about the protection level of the electrical equipment is given by the IP code, defined in IEC standard 60529: it is defined by the code “IP XY” where the first digit X denotes the protection from solid particles, while the second digit Y denotes the protection from liquid penetration. For example M.A.M. products are classified IP 66 meaning that they are completely protected from dusts and powerful water jets (see tables below).
IP | First digit meaning – solid particle |
0 | no protection |
1 | effective against solid particles with dimension > 50 mm |
2 | effective against solid particles with dimension > 12 mm |
3 | effective against solid particles with dimension > 2.5 mm |
4 | effective against solid particles with dimension > 1 mm |
5 | dust protected (no harmful deposit) |
6 | completely dust protected |
IP | Second digit meaning – liquid penetration |
0 | no protection |
1 | protected from vertically dripping drops |
2 | protected from dripping drops titled up to 15° |
3 | protected from dripping drops titled up to 60° |
4 | protected from splashing water from any direction |
5 | protected from water jet |
6 | protected from powerful water jet |
7 | protected from temporary immersion |
8 | protected from continuous immersion |