Dust

Zone 20
A zone in which an explosive atmosphere in the form of a cloud of flammable dust is constantly present, or for a long time, or frequently (EN 60079-10-2).
Zone 20 generally covers areas inside closed containers, pipes, and apparatuses in which flammable dust in the form of a cloud is present constantly or for a long time or often. When using equipment for Zone 20, ignition sources must be protected from an explosion, even if the malfunction is rare. For this reason, the equipment must meet the following requirement:
In case of failure of one type of protection or simultaneous occurrence of two faults, adequate explosion protection must be provided. According to directive 2014/34/EU, equipment for use in Zone 20 must meet the requirements of category 1D

Zone 21
A zone in which an explosive atmosphere in the form of a cloud of flammable dust in the air may occasionally occur during regular operation (EN 60079-10-2).
Zone 21 includes, among other things, mills, coal supplies or grain stores, and the area around gas stations. Here, for example, explosive dust clouds can occur due to the periodic emission of dust from the hole. The risk of danger due to dust deposits is often underestimated.
Explosive dust-air mixtures can be formed due to the formation of a glow nest or low-temperature carbonization gas and a result of deflagration of lowtemperature carbonization gas, gas, or dust swirling caused by a smoldering fire.
Ignition sources that occur during regular, trouble-free operation and sources that generally happen in the event of malfunctions must be reliably prevented.
The individual types of protection are described in the chapter “Electrical equipment for se in hazardous areas.” According to Directive 2014/34/EU, equipment for Zone 21 must meet the requirements of category 2D.

Zone 22
A zone in which an explosive atmosphere in the form of a cloud of flammable dust in the air is unlikely to occur during regular operation, and it persists only for a short period (EN 60079-10-2).
Under normal operating conditions, zone 22 is unlikely to form an explosive dust/air mixture, and an explosive atmosphere can only be expected in malfunctions, for example, due to whipped dust. During regular, uninterrupted operation, ignition sources must be reliably prevented. According to Directive 2014/34/EU, equipment for zone 22 must meet the requirements of category 3D.
Dust reacts differently depending on whether it is in a deposited dust layer or a suspended dust cloud. Layers of dust can begin to smolder on hot surfaces, while a cloud of dust ignited locally or on contact with a hot surface can explode immediately. Dust explosions are often the result of smoldering layers of dust that rise and can cause ignition. If such a layer is agitated, for example, by mechanical cleaning methods during transport or incompetence extinguishing attempts, this can lead to a dust explosion.
A gas or vapor/air explosion can also stir up dust, which often results in a gas explosion occurring first, followed by a dust explosion. In deep coal mines, methane/flame explosions often caused coal dust explosions, the consequences of which were more severe than those of the initial flame explosion.

Notes:
1. Layers, deposits, and piles of fuel must be treated like any other source that may form an explosive atmosphere.
2. Normal operation means using the installations within their design parameters.
3. Explosive atmosphere definitions comply with EU directives and EN standards.