Important concepts (ATEX)


This is the minimum temperature at which, under specified test conditions, a liquid gives off sufficient combustible gas or vapour to ignite momentarily on application of an effective ignition source.

Auto-ignition temperature

This is the minimum temperature required to ignite a gas or vapour (air or dust cloud) without a spark or flame being present.

For dust, 2 different ignition temperatures need to be controlled: the ignition temperature of a dust cloud and of a dust layer (5mm).

Lower and upper explosion limit (LEL & UEL)

These are limits (expressed in volume % of vapour or gas or gr of dust per m³) of the 'explosion range'. This is the range of the concentration of a flammable substance in the air, in which an explosion can occur.

If the value is lower than the LEL, the mixture is too poor. If the value is higher than the UEL, the mixture is too rich (with other words: there is not enough O² present)

Temperature class

The temperature class refers to the maximum surface temperature of equipment. 

Temperature class

Max. surface Temperature













The choice of the temperature class of max surface temperature must be defined so that:

For gas:

  • The chosen temperature class must be chosen in relation to the ignition temperature of the gas or vapour
  • For fans of Category 2, the temperature class may not exceed 80% of the ignition temperature. For category 3 fans, this is not the case

For dust:

  • The maximum surface temperature shall be equal or lower than the lowest value of:
    • 2/3 of the ignition temperature of a dust cloud
    • Ignition temperature of dust layer -75°C
  • For dust, one uses the maximum surface temperature as an absolute figure (not temperature class)

Explosion group

Groep II is divided into explosion groups.

A, B and C are subdivisions according to the Maximum Experimental Safe Gap (MESG), which shows the maximum dimension of an opening so that it still draws enough energy from a flame of the concerned gas or vapour to put it out. There is a direct relationship with the ignition energy. For fans, Almeco doesn't distinguish between IIA and IIB. We do have a seperate design for Hydrogen of group IIC.