ATEX marking:
how to select Ex equipment
[GUIDE + WORKBOOK]

Special marking of Ex equipment: what is the legal basis?

Manufacturers of equipment for explosive atmospheres must comply with the requirements of the ATEX Directive (2014/34/EU), the provisions of which were introduced into our legislation by the Ordinance of the Minister of Development of June 6, 2016 on requirements for equipment and protective systems intended for use in potentially explosive atmospheres (Journal of Laws 2016 item 817). This regulation implies, among other things, the obligation to include special markings on nameplates, in the declaration of conformity and in the type test certificate (if issued), which are referred to as the Ex marking.

An example of an Ex marking with an explanation of the various designations:

1. equipment group
2. category of equipment
3. confirmation that the equipment is designed as explosion-proof
4. type of explosion protection
5. explosion subgroup of dust/gas/vapors for which the equipment is intended
6. temperature class (gases and vapors) and/or maximum surface temperature (dust)
7. equipment protection level (EPL)

Ex marking: the meaning behind the symbols

The individual elements that make up the explosion-proof marking of equipment can be divided into two groups:

1. symbols necessary for the correct selection of equipment for operation in a hazardous area; they specify in which conditions the equipment can be used
2. symbol informing what safeguards have been used in the equipment to eliminate sources of ignition; often have impact on the cost of purchase, installation and operation of the equipment

The purpose of this article is to help you select equipment for operation in a given hazardous area (point 1) in a safe manner, and thus in compliance with applicable regulations. Since the explanation of the dependencies and consequences of the second point is an extensive topic we will describe it in a separate article.

Examples of Ex marking 

Below is an example of the explosion-proof marking of the HARDO OptiLine luminaire. This luminaire can operate in explosive atmospheres generated both by flammable gases and vapors of flammable liquids and by flammable dusts. For this reason, we have presented the designations in two lines – separately for gases and vapors and separately for dusts.

What to focus on when selecting equipment for a hazardous area

Below you will find tables with explanations of each element of the explosion-proof marking. The green color indicates that a given value is derived from the above designation, which you will find on the nameplate, declaration of conformity and in the type test certificate (commonly referred to as ATEX certificate). These markings, as we mentioned earlier, tell us about the conditions under which a given device can be used safely. They are therefore crucial to the correct selection of an Ex device in terms of safety and regulatory compliance. They are what we will focus on in the following section.

Equipment group

Equipment category

Depending on whether we are dealing with a piece of equipment operating in a mining plant or in a non-mining plant (see equipment groups above), equipment is assigned different categories. These indicate under which conditions (Group I) or in which explosion hazard zone (Group II) the equipment can be used.

Categories for equipment group I

Categories for equipment Group II

Which category a manufacturer can qualify a device for depends on the level of safety guaranteed by the device. 

Devices can be designed for operation in gas and/or dust zones. Therefore, in the designation on the nameplate next to the category of the device, the letters D and/or G are added, where:

• D – dust atmosphere
• G – gas (or flammable vapor) 

NOTE 1: In some cases, equipment may have different categories in different places, such as another in its interior and another outside. An example would be a fan that has a 2D category inside (this means that there may be a zone 22 or 21 in the duct), while outside it has a 3D category (it may be installed in zone 22). As a result, the designation of such a fan will look like this: 2/3D.  

NOTE 2: in intrinsically safe circuits, we may have to deal with a so-called associated device (intrinsically safe barrier or separator). This means that this device is designed to work with an intrinsically safe device installed in a hazardous area, and itself, depending on the design, may be located in another zone or in a safe space.

The marking of the companion device will include the category and type of protection of the device with which it can cooperate. Whereby the said category will be in round brackets, while the type of protection will be in square brackets (see table below). If the associated device is intended for:

1. installation outside the hazardous area, only the category and type of protection of the device with which it can operate (in parentheses) will be in the marking
2. installation in a hazardous area – then in the marking, in addition to the category and type of protection of the device with which it can cooperate (point 1 above) will be its own category and type of protection placed outside the parentheses

Example of Ex marking for a connection of an associated Ex device for intrinsically safe circuits 

Visual example 1

Visual example 2

Examples of equipment categories:

Designation	Explanation	Where to use
Ex II 1 D	Group II equipment for use outside mines, category 1 dust	Installation in zone 20
Ex II (1) G D	Group II equipment, associated equipment, for use with category 1 dust and gas equipment	Mounting outside Ex zones, works with device mounted in zone 0 or 20
Ex II 2 (1) G	Group II appliance, associated appliance category 2, for use with category 1 gas appliance	Mounted in zone 1, cooperates with device mounted in zone 0 (e.g. cooperating device mounted in zone 1)
Ex II (2) G (1) G	Group II appliance, associated equipment, for use with category 1 gas and category 2 gas appliance	Mounting outside the Ex-zone, works with device mounted in zone 1 and device mounted in zone 0
II 1/2 G	Group II equipment, part of equipment category 1, part of category 2 gas	Mounting at zone boundaries, part of the unit operates in zone 0, part in zone 1 (e.g. sensor mounted in tank wall between zones 0 and 1)
II 3/3 D	Group II, category 3 device inside and outside the device	Mounting at zone boundaries, inside the unit zone 22, outside zone 22 (e.g. fan with zone 22 inside, mounted in zone 22)
II 2/- G	Group II, category 2 device inside	Mounting at zone boundaries, inside the unit zone 1, outside no Ex-zone (e.g. fan with zone 1 inside mounted outside Ex-zone)
II -/3 D	Group II, category 3 device outdoors	Non-explosive atmosphere equipment, Zone 22 installation (e.g. fan, non-explosive atmosphere switch mounted in Zone 22)
II 3 G II 2 D	Group II, Category 3 gas and Category 2 dust equipment	Installation in zone 2 or zone 21
GD	Protective system for use in gas/vapour/dust atmospheres	Installation in zone 0, 1, 2 or in zones 20, 21, 22

How to determine the required category of the equipment?

To know what category the equipment should be in, you need to check in which explosion hazard zone it will operate. 

According to the ATEX USER Directive (1999/92/EC), electrical and non-electrical equipment with an appropriate category should be used in explosive atmospheres, selected according to the type of hazard zone:

–      in Zone 0, only equipment category 1G

–       in Zone 0, only equipment category 1G lub 2G

–       in Zone 2, equipment category 1G, 2G or 3G

–       in Zone 20, only equipment category 1D

–       w strefie 21, equipment category 1D or 2D

–       w strefie 22, equipment category 1D, 2D or 3D

Zones are determined on the basis of applicable national standards taking into account the following aspects:

1. For gas zones

• substance properties
• possible sources of emissions that can cause an explosive atmosphere
• degree of emission (continuous, first or second)
• velocity of air flow over the emission source
• degree of dilution 
• dostępność/niezawodność wentylacji
• przeszkody fizyczne

2. For dust zones

• substance properties
• possible sources of emissions that can cause an explosive atmosphere
• size of dust particles (granularity)
• specific density of the dust
• humidity of the dust
• pressure in the equipment containing the dust
• location and size of the opening through which emission occurs
• emission efficiency
• presence of exhaust ventilation
• any mechanical obstructions
• cleaning operations applied (effectiveness of cleaning)

The classification is to indicate the areas within the installation where an explosive atmosphere may occur. Depending on the time or frequency of its occurrence, we can designate the following zones:

The classification of explosion hazard zones can be found in the explosion risk assessment or explosion hazard assessment documents. According to the ATEX USER Directive (1999/92/EC), the employer is required to draft an explosion risk assessment along with the designation of explosion hazard zones. In practice, this task is often outsourced to external expert companies. As the WOLFF GROUP, over 30 years in the market we have performed several hundred explosion risk assessments for industrial plants from all major industries.

Explosivity subgroup for gases and vapors of flammable liquids

Flammable gases and vapors of flammable liquids are assigned explosion subgroups IIA, IIB lub IIC. They are determined, among other things, by testing the Maximum Experimental Safe Gap (MESG) or Minimum Ignition Current (MIC).

How to determine the explosivity subgroup for gases and liquid vapors?

For gases and vapors of flammable liquids, the explosivity subgroup can be found in EN ISO/IEC 80079-20-1 Explosive atmospheres. Part 20-1: material properties for the classification of gases and vapors. Test methods and tabular data. It contains a table with results for more than two hundred substances. If necessary, you can also contact us to refer to our databases.

If the above fails, then to determine the explosivity subgroup according to standard, you need to:

1. Perform a laboratory test of the Minimum Incendiary Current (MIC).
2. Perform a laboratory test of the Maximum Experimental Safety Gap (MESG), aka the minimum extinguishing gap.
3. Determine the subgroup based on the similarity of chemical structure (so-called provisional classification).

As a rule, is sufficient to determine only one of the parameters (most often it is the MIC) to determine the explosivity subgroup. Only if the result is borderline should it be confirmed by determining the second parameter.

How to select equipment for a hazardous area created by gases and vapors of a given explosive subgroup ?

*MESG – Maximum Experimental Safe Gap – tzw. szczelina gasząca. the so-called extinguishing gap. A value (in millimeters), which is determined under laboratory conditions for gases and vapors.
**MIC – Minimum Ignition Current – the minimum current in a circuit capable of igniting a stoichiometric gas or vapor mixture. MIC is defined as the ratio of the ignition current of a substance to the ignition current of a methane-air mixture.

Explosivity subgroup for dusts

Combustible dusts are assigned explosion subgroups IIIA, IIIB and IIIC.

How to select equipment for a hazardous area created by combustible dust?

Surface temperature of the equipment as a potential ignition source

While in the case of most sources of ignition that can occur in the equipment the use of protection reduces the risk of its occurrence. However, temperature is different. The protection used does not eliminate it completely, but only limits it to a certain level. Which level?

The next position in the explosion-proof mark informs about this. For dust, the manufacturer gives a specific value for the maximum surface temperature, while for gases and vapors of flammable liquids the temperature class to which the surface temperature of the equipment is assigned.

Temperature class (gases and vapors)

The temperature class of the equipment is determined by its manufacturer. It is the maximum temperature that can occur on equipment surface under the most unfavorable operating conditions (e.g. highest ambient temperature, highest load on the equipment). If the device can reach different surface temperatures under different operating conditions, the manufacturer may specify different temperature classes in the rating plate, but this needs to be specified more precisely.

When selecting equipment for operation in a particular hazardous area, we must compare its temperature class with the temperature class of the gas and/or vapor that can create an explosive atmosphere in that area. The temperature class of the gas or vapor, in turn, depends on its self-ignition temperature. In other words, we must select the device so that the temperature on its surface expressed by its temperature class does not exceed the self-ignition temperature of the substance with which this device may come into contact.

Maximum surface temperature (dust)

The situation is slightly different for the maximum surface temperature of the equipment processing dust. In this case, the permissible temperature for equipment should be calculated according to the following table.

How do you determine the dust cloud temperature T_cl and the dust layer temperature T_5mm?

Both values are determined in notified laboratories by testing a specific dust from the area of your facility. To do this, you need to take a sample from the area where the dust is expected to have the most demanding flammability and explosivity parameters (along with testing ignition temperatures, a number of other parameters are most often tested, e.g., lower explosion limit, minimum ignition energy, maximum explosion pressure or K_st).

If your plant has an Explosion Protection Document (EPD) drawn up based on an Hazard Identification and Risk Assessment (HIRA), then both temperatures should be found right there. You can also use literature data, nevertheless this solution will be subject to some inaccuracy (sometimes considerable).

In case you have trouble finding these values, please contact us. We have an extensive database with parameters of combustible dust, which we will make available to you for free. We can also perform commercial tests in the laboratory on your behalf. The latter is the best solution, because the mentioned parameters are not physical constants and their values, depend on several factors such as the fineness of the particles of the tested dust, its moisture content or admixture content.

Therefore, dust can have different combustible and explosive properties at different points in an installation. In extreme cases, the same dust from one point of the installation can be chemically safe (e.g., high moisture levels), while dust from another location can pose a high risk of explosion (e.g., dust from a dryer or mill). Below is an example of the parameters of wood dust in different parts of the installation:

Because of the above, we should consider where these parameters will be highest, i.e. where in our installation the dust is finest and driest, and sample from there. Most often we recommend taking a sample from dust collectors. The amount of dust needed for full testing is approximately 3.5 kg (~7.7 lbs).

Practical exercises

Task 1

You are in search of a light fixture to be used in a chemical industrial plant. It will operate in explosion hazard zone 2 caused by hydrogen. You know that this gas belongs to explosion subgroup IIC and temperature class T1. Which device can work in these explosion hazard conditions?

Which light fixture(s) is/are appropriate?

Answer:

Since we are dealing with gaseous explosive atmospheres, we focus only on the second line of the designation (one that refers to gases and liquid vapors). We know that the minimum requirements that the device must allow operation in the following conditions:

• gaseous explosive atmosphere,
• Zone 2
• explosion subgroup IIC
• temperature class T1
• ATEX type examination certificate is not required (Zone 2)

Light fixture 1

II 3 G  Ex ec IIC T4 Gc

Fixture 2

II 2 G  Ex eb IIC T2 Gb

Fixture 3

II 3 G  Ex ec IIA T1 Gc

Task 2

The situation as in task 1. except that, in addition to hydrogen, hydrogen sulfide may also appear in the working area of the fixture. You will find the parameters of both gases in the table below. For the record, we are dealing with explosion hazard Zone 2 and an industrial plant other than a mining plant.

Which light fixture(s) is/are appropriate?

Answer:

Since we are dealing with a gaseous explosive atmosphere, we only focus on the second line of the marking, since it is the one that refers to gases and liquid vapors. This time we are dealing with two gases, and to select a device, we need to select the most demanding parameters. So, analyzing the situation described, we know that the minimum requirements that the device should meet must allow operation in the following conditions:

• gaseous explosive atmosphere,
• Zone 2
• explosion subgroup IIC (hydrogen)
• temperature class T3 (hydrogen sulfide)
• ATEX type examination certificate is not required (Zone 2)

Fixture 1

II 3 G  Ex ec IIC T4 Gc

Fixture 2

II 2 G  Ex eb IIC T2 Gb

Fixture 3

II 3 G  Ex ec IIA T1 Gc

Task 3

You want to select a lighting fixture for a facility in the food industry. The fixture will operate in a space where a 22 explosion hazard zone can occur due to flour and sugar dust. You have tested the explosivity of these dusts. Below you will find an excerpt from these tests:

Which light fixture(s) is/are appropriate?

Answer:

Since we are dealing with a dust explosive atmosphere, we focus only on the first line of the marking. By analyzing the situation, we know that the minimum requirements that the equipment should meet must allow operation in the following conditions:

• dust explosive atmosphere
• Zone 22
• explosion subgroup IIB
• maximum surface temperature of the device should be lower than 207°C
• ATEX type examination certificate is not required (Zone 22)

EXPLANATION: To calculate the maximum surface temperature of the device, we took the lower value of the ignition temperature of the dust layer (sugar – 430°C) and the ignition temperature of the dust cloud (sugar – 310°C). Then we substituted them into the formulas below and chose the lower of the results.

T_{5mm safe} = T_5mm – 75°C

T_{cl safe} = 2/3 • T_cl

T_cl = 310°C — 2/3 → 206 °C

T_5mm = 430°C — 430 – 75 → 355 °C

CONCLUSION: maximum temperature of the external surface of the equipment must not exceed 206°C.

Fixture 1

II 2 D  tb IIIC T70°C Db

Fixture 2

II 3 D  Ex tc IIIA T70°C Dc

Fixture 3

II 3 D  Ex tc IIIB T230°C Dc

A very important closing note:

Please note that when selecting equipment for operation in explosive atmospheres, we take into account their category, explosion sub-group and temperature class (equipment for gases and vapors of flammable liquids) or maximum surface temperature (equipment for dust). We are not at all interested in how the device is made (in a reinforced construction cover “e”, in a flameproof cover “d” or protected by a housing “t”). This information is important later, when operating, inspecting and maintaining the equipment.