Ex Triangle In order to prevent an explosion the aspects of the potential hazard must be defined in both type and degree, this allows us to implement measures to prevent the explosion.
The ignition triangle depicts the 3 parts which are required to be present for an explosion to occur.

  1. By definition oxygen (air) is present as if it were not were not the hazardous area standards would not strictly speaking apply1
  2. The probability of fuel being present2 i.e. Hazardous area zone should be assessed
  3. Ignition source parameters should be considered for both hot surface and Spark.

Putting aside the Air/Oxygen aspect the first stage of any assessment is to assess the probability of a fuel being present i.e the area to be assessed is classified into ZONEs as defined in EN 60079‑10.
The explosive nature of the fuel3 then needs to be defined, these parameters are not connected and must be independently evaluated for each fuel or combination of fuels likely to be present.

  • Gases - the autoignition temperature (group)4
  • Gases - the spark energy required for ignition
  • Dusts - the auto ignition temperature
  • Dusts - Particle size or ratio of particles less than 0.5mm
  • Dusts - electrical, conducting or insulationg

Due to the nature of the environment Mining and Surface industries are treated differently, and equipment is classified into 2 groups within Atex (and adopted by the UK) relating to the explosive atmosphere for which it is to be used, Group I for mining and Group II for surface use with an additional Group III for surface Dust.


  1. The EN standards generally relate to NTP (Normal Temperature & Pressure) unless specifically detailed, conditions such as enriched or depleted Oxygen are outwith their scope although it is feasible that a Test house could issue certification outside those conditions based on their own criteria.. 

  2. Fuel needs to be potentially present at an explosive mix i.e. between LEL & UEL, not just a time assessment to define the zones. levesl 

  3. Multiple gases are usually straightforward and based on the worst-case scenario i.e. gases with lowest ignition temperature and ignition energy except in a few cases where the gas source is a mix and therefore the may be calculated on that mix Hybrid environments i.e. gas and dust must be assessed as a single entity and usually will be a special case regarding certification. 

  4. Ignition parameters are determined by test and will vary for an individual fuel depending on the source of those figures.