Explosion protection

Signalling devices in potentially explosive zones

Preventing explosions – explosion protection

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:

  • Primary explosion protection
    Primary explosion protection prevents potentially explosive atmospheres from forming, by ensuring adequate ventilation, for example.
  • Secondary explosion protection
    If the formation of potentially explosive atmospheres cannot be prevented by means of primary explosion protection, then secondary explosion measures must be taken to exclude sources of ignition. WERMA supplies safe signalling devices for this purpose, which cannot become sources of ignition.
  • Tertiary explosion protection
    Tertiary explosion protection is employed if the operator of a system cannot completely avoid the presence of potential sources of ignition. In such cases, the effects on the surrounding area must be reduced to such a degree that they are not dangerous.

Duties of the operator/installer

Explosionsschutz

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 – zone division

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:

  • Zone 0
    Areas in which a potentially explosive atmosphere made up of gases, vapours or mists is present constantly, for longer periods, or frequently.
  • Zone 1
    Areas in which a potentially explosive atmosphere made up of gases, vapours or mists is occasionally present.
  • Zone 2
    Areas in which normally no potentially explosive atmospheres made up of gases, vapours or mists are present; and if they should occur, then only seldom and for a short period of time.

Areas that are potentially explosive due to combustible dust:

  • Zone 20
    Areas in which a potentially explosive dusty atmosphere is present constantly, for longer periods, or frequently.
  • Zone 21
    Areas in which a potentially explosive dusty atmosphere is occasionally present.
  • Zone 22
    Areas in which normally no potentially explosive dusty atmospheres are present; and if they should occur, then only seldom and for a short period of time.

Explosion groups of gases, vapours and dusts

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
IIIB Non-conductive dust 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.

Surface temperature

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.

Device categories and EPL protection level

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.

Substance group Gas Dust
Device category 1G 2G 3G 1D 2D 3D
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’s duties

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.

Labelling explosion-protected equipment

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.