Hazardous areas are classified into zones based upon the frequency and potential duration of the occurrence and duration of an explosive mix. However, these are not just based on time as a full risk assessment is required. The methosd of assessment defined in EN 60079-20 belies the simple definition.
Gas Zones: Defined in EN 60079-10-1
ZONE 0 - 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 probability of an explosive mix existing in the area, i.e. a fuel source being present between LFL and UFL
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.
LFL - concentration of flammable gas or vapour in air below which an explosive gas atmosphere
does not form (see IEC 60079-20-1)
UFL - concentration of flammable gas or vapour in air above which an explosive gas atmosphere
does not form (see IEC 60079-20-1)
Note: These are also referred to as LEL (Lower Explosive Limit) and UEL (Upper Explosive Limit) in some standards and the terms are interchangeable.
The site owner has a legal obligation to conduct a risk assessment (DSEAR) and zone the site accordingly and the Zones should be clearly identified around the site, the method is down to the site but may consist of some or all of:
- Signs with written warnings
- Signs with plans showing boundaries
- Painted lines on the floor
- Detailed site plan - The zones are 3 dimensional and should be representede as such.
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 very old BS standard but then dropped and are not referenced in current standards as the absolute figures do not sit well with statistical risk analysis techniques. Despite this they 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 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 flamepath concept is fundamentally incompatible with Dust.
Should you require advice regarding existing hydrid zones and compliance please contact us.