The ATEX Directive covers explosions from flammable gas/vapors and combustible dust/fibers (which, contrary to common belief, can lead to hazardous explosions).
The following are classifications for zones that can produce explosive atmospheres.
The following zones are each defined as a place in which an explosive atmosphere consisting of a mixture of air or dangerous substances in the form of gas, vapor, or mist...
Zone 0 – is present continuously or for long periods or frequently.
Zone 1 – is likely to occur in normal operation occasionally.
Zone 2 – is not likely to occur in normal operation, and if it does occur, will persist for a short period only.
These are defined as a place in which an explosive atmosphere is in the form of a cloud of combustible dust in the air.
Zone 20 – is present continuously, or for long periods or frequently.
Zone 21 – is likely to occur in normal operation occasionally.
Zone 22 – is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Effective ignition source
Effective ignition source is a term defined in the European ATEX directive as an event that, in combination with sufficient oxygen and fuel, can cause an explosion. Methane, hydrogen, and coal dust are good examples of possible fuels.
Effective ignition sources are:
Some frequencies of electromagnetic waves (Light waves)
Ultrasound (Any sound waves of higher frequency than what humans can hear; generally considered to be from ~20Hz to ~20kHz)
Electrical switches (Toggling an electrical switch (particularly turning it off) can cause arcing inside the switch)
Open flames (This may range from a lit cigarette to welding activity)
Hot gasses (This can include a gas that just has hot particulates in it)
Mechanically generated impact spark (For example, a hammer blow on a rusty steel surface compared to a hammer blow on a flint stone. The speed and impact angle (between surface and hammer) are important; a 90-degree blow on a surface is relatively harmless)
Mechanically generated friction sparks (The combination of materials and speed determine the effectiveness of the ignition source. For example, 4.5 m/s steel-steel friction with a force greater than 2 kN is an effective ignition source. The combination of aluminum and rust is also notoriously dangerous. More than one red-hot spark is often necessary in order to have an effective ignition source)
Electric sparks (For example, a bad electrical connection or a faulty pressure transmitter)
Electrostatic discharge (Static electricity can be generated by air sliding over a wing, or a non-conductive liquid flowing through a filter screen)
Exothermic reactions (A chemical reaction that expels heat from the involved substances, into the surrounding area)
Adiabatic compression (When air is pushed through a narrow passage quickly, causing the passage's surface to heat up)