Close cooperation between all of the parties involved is necessary in order to ensure safety in potentially explosive zones. This makes it essential for manufacturers, operators, test institutes and supervisory authorities to work together.
Three types of explosion protection must be considered in order to prevent explosions:
The operator of a system, or the installer commissioned by the operator, must first make maximum use of all possible primary explosion protection measures. Any remaining potentially explosive areas must be assessed with regard to the potential risks. This involves dividing the area into zones, determining the explosion group and defining the maximum permissible surface temperature.
Potentially explosive areas are divided into zones in accordance with EC Directive 1999/92/EC.
To provide a basis for assessing the necessary extent of protection measures, the operator must divide the potentially explosive areas into zones according to the probability of a dangerous, potentially explosive atmosphere developing.
Areas that are potentially explosive due to combustible gases, vapours or mists:
Areas that are potentially explosive due to combustible dust:
The explosion group is determined according to the potentially explosive substances and their ignitability:
|Area||Explosion group||Combustible substances||Ignitability|
|Mining||I||Pit gas, (methane), coal dust|
|Gas||IIA||Acetone, petrol, methanol, propane, toluene||Less high|
|IIB||Ethylene, town gas||High|
|IIC||Hydrogen, acetylene, carbon disulphide||Very high|
|Dust||IIIA||Combustible lint||Less high|
|IIIC||Conductive dust||Very high|
All WERMA signalling devices have been tested for use in the highest explosion group IIC or IIIC, which means that they are suitable for all explosion groups in the relevant area.
The ignition temperature of the potentially explosive substances determines the maximum permissible surface temperature of the operating equipment. The explosion protected equipment must be designed such that its surface temperature cannot cause ignition to take place.
Ignition temperatures and temperature classes of potentially explosive gas and vapour atmospheres
|Temperature class||Ignition temperature of the potentially explosive gas/vapour atmosphere||Permissible surface temperature of the operating equipment|
|T1||≥ 450°C||≤ 450 °C|
|T2||≥ 450 … ≤ 300 °C||≤ 300 °C|
|T3||≥ 300 … ≤ 200 °C||≤ 200 °C|
|T4||≥ 200 … ≤ 135 °C||≤ 135 °C|
|T5||≥ 135 … ≤ 100 °C||≤ 100 °C|
|T6||≥ 100 … ≤ 85 °C||≤ 85 °C|
For dusts, there is no grouping according to temperature classes. Instead, the maximum permissible surface temperature in °C is given.
WERMA offers different signalling devices for the various temperature classes for gases and vapours, and for the maximum surface temperatures.
The ATEX guideline divides operating equipment in device group II into 6 device categories. The IEC standards and EN standards divide the devices into 6 EPLs (Equipment Protection Levels). Device categories and EPLs have the same meaning and indicate the zones in which the device may be used.
|EPL protection level||Ga||Gb||Gc||Da||Db||Dc|
|Suitable for zones||0,1,2||1,2||2||20,21,22||21,22 22|
The manufacturer of devices for potentially explosive areas is obliged to attach a label to the equipment, in accordance with EC Directive 94/9/EC, showing the operator the types of location in which the equipment may be used. It is also known informally as the ATEX guideline.
The operating equipment must meet all of the relevant requirements for applying the CE mark and be tested by a recognised, independent test institute. Device category 3 is not included here.
This is confirmed by the EC type testing certificate. The manufacturer must also have a suitable quality assurance system, which must be verified by an EC certificate.
This labelling is specified in the relevant series of standards as well as in EC Directive 94/9/EC.As a result of various changes made to the standards in recent years, the labelling has been altered several times. Since adjustments to the labelling require testing to be carried out by the test institute, they can only be updated progressively. Consequently, it is possible that devices do not correspond to the most recent labelling standards. Their suitability for use in explosion protected areas is not affected by this, however.
In the case of equipment used in areas with explosive gas and with explosive dust, there is a separate identification line in each case.