Hazardous areas are classified into zones based upon the frequency of the occurrence and duration of an explosive gas atmosphere.
i.e. this is the probability of a fuel source being present and are critical to the risk assessment of a site and the site owner has a legal obligation to conduct a risk assessment and zone the site accordingly.
Gas Zones: Defined in EN 60079-10-1
- Area in which an explosive gas atmosphere is present continuously or for long periods or frequently
ZONE 1 - Area in which an explosive gas atmosphere is likely to occur in normal operation occasionally
ZONE 2 - Area in which an explosive gas atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only
The zone represents the probabilty of an explosive mix existing in the area, (low, medium or high) and it must be remembered an explosion in Zone 2 has the same intensity of that in Zone 0 but the conditions for explosion are less likely to occur.
Dust Zone: Defined in EN 60079-10-2
- A place in which an explosive dust atmosphere, in the form of a cloud of dust in air, is present continuously, or for long periods or frequently.
ZONE 21 - A place in which an explosive dust atmosphere, in the form of a cloud of dust in air, is likely to occur in normal operation occasionally.
ZONE 22 - A place in which an explosive dust atmosphere, in the form of a cloud of dust in air, is not likely to occur in normal operation but, if it does occur, will persist for a short period only.
Specific time periods are not defined in the standards, the gas zones were introduced in the ICI guidelines which suggested for gas
Zone 0 >1000 hours, Zone 2 < 10 hours and Zone 1 in between.
These were used in the old BS standard but then dropped and not referenced in the Atex standards as the absolute figures do not sit well with statistical risk analysis techniques but are still commonly erroneously quoted.
Hybrid Mixture : - mixture of flammable substances in different physical states, with air
Where both gas and dust hazards can occur simultaneously, a Hybrid Mixture, the area would need to be zoned for both. This seems to be a rare occurrence where both gas and dust exist in the same area, more commonly it is the result of poor zone assessment.
It is strongly recommended that an area which is zoned as hybrid should be looked at in more detail to either to assess:
a. Must it be really be zoned for both, or is it the result of lazy zoning?
b. If both Gas and dust hazards can exist, can it be demonstrated they cannot occur at the same time?
A more detailed zone risk assessment is likely to be easier than finding suitable Hybrid certified equipment.
Contrary to common belief, equipment certified for a hybrid gas and dust atmosphere is very rare and generally requires special certification of equipment with an in-depth risk assessment of the effect of the dust on the gas certification and vice versa.
This is certainly critical for Ex d certification
where the flameproof concept is incompatible with Dust.
Should you require advice regarding existing hydrid zones and compliance please contact me.