According to the ATEX Directive, in areas where hazardous explosive atmospheres created by dust may occur, zones must be classified in 22, 21 or 20 depending on the probability of an explosive atmosphere to occur or on its persistence if it does.
Equipments intended to be used in one of these three zones require a marking of a different category.
Every possible effective source of ignition must be prevented as strictly as possible.
For example, zone 22 requires that no sources of ignition occur during normal operation, zone 21 requires that no sources of ignition occur even under conditions of occasional malfunction, zone 20 requires that no sources of ignition occur , also under the rare condition of one or two malfunctions at the same time.
It is not difficult to comply with the requirements set by the Directive up to the zone 21.
All you need is a risk assessment to define possible sources of ignition and neutralize risk by applying proper sensors to prevent the source from becoming effective.
For example, the most common sources of ignition can be prevented by applying speed/rotation and temperature control systems and belt alignment systems as well as ensuring the electric continuity of all components.
In the zone 20 it is also necessary to foresee the possible malfunction of the above mentioned systems.
So, in order to prevent a possible explosion, both the malfunction and the missing alarm due to the malfunction of the systems must be avoided.
Operations in the zone 20 can follow two different approach: to accept the possibility that an explosion may occur and act to abate risks for the health of the workers, or, maybe the less expensive option, to install intrinsically safe or redundant systems in order to eliminate risks caused by two malfunctions occurring at the same time.
In case of failure of the intrinsically safe sensors, the system will neutralize possible sources of ignition, that is, to stop operations of the plant.
Plant shutdown is due not only to the detection of the source of ignition, but also to the malfunction of the control equipment and the connected devices.
The redundant sensors, as a safety system, will be formed by two parallel replica borse and independent systems.
One alarm from only one of the two systems will prevent the effectiveness of the source of ignition.
As a result the equipment will consist of a dual control system and a control PLC with a intrinsically safe remote control (now on sale), or a couple of two parallel PLCs.
A thorough assessment of the safety system must be made to avoid the risk of explosion caused by the malfunction of one of its components.
However, because the requirements of the Directive for zone 20 are met with only the protection from a dual malfunction, only one redundant system is required.