The ATEX directive concerning explosion safety is based on years of research and well understood knowledge about the origins of explosions. The most important causes are the occurrence of a spark, or a surface temperature in excess of the auto-ignition temperature of an explosive air-gas mixture. The accepted methods of protection are all based on removing one or more of the necessary condition for an explosion. The standard covering the general requirements to do this is EN50014 (in CENELEC countries).
The best known methods of protection are:
Ex d based on EN50018
This method keeps an ignited air-gas mixture inside the housing. Hot gasses are led to the outside though long narrow gaps is such a way that they are cooled below the point that a new ignition takes place outside the housing. This method places specific demands on the construction of the housing, especially on the resistance to internal explosions, dimensioning of the gaps, but also on the use. For instance the equipments may not be opened while energized. The advantage is the there are no particular requirements in the electronics in the housing, only for wirings that is led to the exterior. As a result the power supply will often need to be Ex d or Ex e as well, while signal inputs will be Ex i (see below).
Ex e based EN50019
This methods prevent sparking in passive equipment. In these circumstances this would be caused by disconnecting or short circuiting wiring. Therefore this methods attempts to prevent that from happening by placing specific requirements on terminals and connectors. As a result the method is mostly suitable for junction boxes and the like.
Ex i based EN50020
(for use in Zone 0 EN50284 also applies) This method is called intrinsic safety. Here the stored energy in the apparatus is limited to below the ignition energy of the air-gas mixture. The surface temperature is kept below the auto-ignition temperature of the air-gas mixture by limiting the supplied power.
Deviation of the methods in the standards are often necessary. The manufacturer then needs to prove that the alternative measures taken guaranty at least the same level of safety as when the standards would have been followed. This often requires testing by manufacturer and/or notified body/