This document discusses hazardous area classification and control of ignition sources. It defines hazardous areas as any place where an explosive atmosphere may occur, and outlines the process to classify these areas into zones based on the likelihood and duration of an explosive atmosphere. The document provides guidance on identifying potential ignition sources and selecting appropriate equipment for use in the classified zones to ensure safety. It emphasizes the need to control ignition sources through design measures, work procedures, and proper equipment selection according to the zone classification.

1. Hazardous Area Classification and
Control of Ignition Sources
This Technical Measures Document refers to the classification of plant into hazardous areas, and
the systematic identification and control of ignition sources
The relevant Level 2 Criteria are 5.2.1.3(29)c, 5.2.1.11(63)f, 5.2.1.13 and5.2.4.2(93)a.
Design of plant, pipework and general plant layout is considered in Technical Measures
Documents on Plant Layout, Design Codes - Plant, Design Codes - Pipework, Plant Modification /
Change Procedures, Maintenance Procedures.
The Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) provide for
the first time a specific legal requirement to carry out a hazardous area study, and document the
conclusions, in the form of zones.
General Principles
Hazardous Area Classification for Flammable Gases and
Vapours
Area classification may be carried out by direct analogy with typical installations described in
established codes, or by more quantitative methods that require a more detailed knowledge of the
plant. The starting point is to identify sources of release of flammable gas or vapour. These may
arise from constant activities; from time to time in normal operation; or as the result of some
unplanned event. In addition, inside process equipment may be a hazardous area, if both
gas/vapour and air are present, though there is no actual release.
Catastrophic failures, such as vessel or line rupture are not considered by an area classification
study. A hazard identification process such as a Preliminary Hazard Analysis (PHA) or a Hazard
and Operability Study (HAZOP) should consider these abnormal events.
The most commonly used standard in the UK for determining area extent and classification is BS
EN 60079 part 101
, which has broad applicability. The current version makes clear the direct link
between the amounts of flammable vapour that may be released, the ventilation at that location,
and the zone number. It contains a simplistic calculation relating the size of zone to a rate of
release of gas or vapour, but it is not helpful for liquid releases, where the rate of vaporisation
controls the size of the hazardous area.
Other sources of advice, which describe more sophisticated approaches, are the Institute of
Petroleum Model Code of Practice (Area Classification Code for Petroleum Installations, 2002),
and the Institution of Gas Engineers Safety Recommendations SR25, (2001). The IP code is for
use by refinery and petrochemical type operations. The IGE code addresses specifically

2. transmission, distribution and storage facilities for natural gas, rather than gas utilisation plant, but
some of the information will be relevant to larger scale users.
Zoning
Hazardous areas are defined in DSEAR as "any place in which an explosive atmosphere may
occur in quantities such as to require special precautions to protect the safety of workers". In this
context, 'special precautions' is best taken as relating to the construction, installation and use of
apparatus, as given in BS EN 60079 -101
.
Area classification is a method of analysing and classifying the environment where explosive gas
atmospheres may occur. The main purpose is to facilitate the proper selection and installation of
apparatus to be used safely in that environment, taking into account the properties of the
flammable materials that will be present. DSEAR specifically extends the original scope of this
analysis, to take into account non-electrical sources of ignition, and mobile equipment that
creates an ignition risk.
Hazardous areas are classified into zones based on an assessment of the frequency of the
occurrence and duration of an explosive gas atmosphere, as follows:
• Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods;
• Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation;
• Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation
and, if it occurs, will only exist for a short time.
Various sources have tried to place time limits on to these zones, but none have been officially
adopted. The most common values used are:
• Zone 0: Explosive atmosphere for more than 1000h/yr
• Zone 1: Explosive atmosphere for more than 10, but less than 1000 h/yr
• Zone 2: Explosive atmosphere for less than 10h/yr, but still sufficiently likely as to require
controls over ignition sources.
Where people wish to quantify the zone definitions, these values are the most appropriate, but for
the majority of situations a purely qualitative approach is adequate.
When the hazardous areas of a plant have been classified, the remainder will be defined as non-
hazardous, sometimes referred to as 'safe areas'.
The zone definitions take no account of the consequences of a release. If this aspect is important,
it may be addressed by upgrading the specification of equipment or controls over activities
allowed within the zone. The alternative of specifying the extent of zones more conservatively is
not generally recommended, as it leads to more difficulties with equipment selection, and
illogicalities in respect of control over health effects from vapours assumed to be present. Where

3. occupiers choose to define extensive areas as Zone 1, the practical consequences could usefully
be discussed during site inspection.
As an example:
A proposal was made to zone an aircraft hanger as Zone 1, although the use of fuels handled
above their flash point would be a rare event. It proved difficult to obtain a floor-cleaning machine
certified for Zone 1 areas, though the floor needed sweeping regularly. The option of writing out
an exception to normal instructions to allow a non Ex-protected machine to be used regularly is
not recommended. Instead, a more realistic assessment of the zones is needed, and special
instructions issued for the rare event of using more volatile fuels.
A hazardous area extent and classification study involves due consideration and documentation
of the following:
• The flammable materials that may be present;
• The physical properties and characteristics of each of the flammable materials;
• The source of potential releases and how they can form explosive atmospheres;
• Prevailing operating temperatures and pressures;
• Presence, degree and availability of ventilation (forced and natural);
• Dispersion of released vapours to below flammable limits;
• The probability of each release scenario.
These factors enable appropriate selection of zone type and zone extent, and also of equipment.
The IP code gives a methodology for estimating release rates from small diameter holes with
pressurised sources, and shows how both the buoyancy and momentum of the release influence
the extent of a zone. It tabulates values for an LPG mixture, gasoline, natural gas, and refinery
hydrogen for pressures up to 100barg. Similarly the IGE code gives a methodology for natural
gas, relating the leak rate to the hole-size and the operating pressure. The tables of dispersion
distances to the zone boundary address in the main quite large diameter deliberate vents. There
is in practice little overlap between the codes.
The results of this work should be documented in Hazardous Area Classification data sheets,
supported by appropriate reference drawings showing the extent of the zones around (including
above and below where appropriate) the plant item.
Selection of Equipment
DSEAR sets out the link between zones, and the equipment that may be installed in that zone.
This applies to new or newly modified installations. The equipment categories are defined by the
ATEX equipment directive, set out in UK law as the Equipment and Protective Systems for Use in
Potentially Explosive Atmospheres Regulations 1996. Standards set out different protection
concepts, with further subdivisions for some types of equipment according to gas group and

4. temperature classification. Most of the electrical standards have been developed over many
years and are now set at international level, while standards for non-electrical equipment are only
just becoming available from CEN.
The DSEAR ACOP describes the provisions concerning existing equipment.
There are different technical means (protection concepts) of building equipment to the different
categories. These, the standard current in mid 2003, and the letter giving the type of protection
are listed below.
Zone 0 Zone 1 Zone 2
Category 1 Category 2 Category 3
'ia' intrinsically safe
EN 50020, 2002
'd' - Flameproof enclosure
EN 50018 2000
Electrical
Type 'n' - EN 50021 1999
Non electrical
EN 13463-1, 2001
Ex s - Special protection if specifically certified for
Zone 0
'p' - Pressurised
EN 50016 2002
'q' - Powder filling
EN 50017, 1998
'o' - Oil immersion
EN 50015, 1998
'e' - Increased safety
EN 50019, 2000
'ib' - Intrinsic safety
EN 50020, 2002
'm' - Encapsulation
EN 50028, 1987
's' - Special protection
Correct selection of electrical equipment for hazardous areas requires the following information:
• Classification of the hazardous area (as in zones shown in the table above);
• Temperature class or ignition temperature of the gas or vapour involved according to the table
below:
Temperature
Classification
Maximum Surface Temperature,
°C
Ignition Temperature of gas or vapour,
°C

5. T1 450 >450
T2 300 >300
T3 200 >200
T4 135 >135
T5 100 >100
T6 85 >85
If several different flammable materials may be present within a particular area, the material that
gives the highest classification dictates the overall area classification. The IP code considers
specifically the issue of hydrogen containing process streams as commonly found on refinery
plants. Consideration should be shown for flammable material that may be generated due to
interaction between chemical species.
Ignition Sources - Identification and Control
Ignition sources may be:
• Flames;
• Direct fired space and process heating;
• Use of cigarettes/matches etc;
• Cutting and welding flames;
• Hot surfaces;
• Heated process vessels such as dryers and furnaces;
• Hot process vessels;
• Space heating equipment;
• Mechanical machinery;
• Electrical equipment and lights
• Spontaneous heating;
• Friction heating or sparks;
• Impact sparks;
• Sparks from electrical equipment;
• Stray currents from electrical equipment
• Electrostatic discharge sparks:
• Lightning strikes.
• Electromagnetic radiation of different wavelengths
• Vehicles, unless specially designed or modified are likely to contain a range of potential ignition
sources
Sources of ignition should be effectively controlled in all hazardous areas by a combination of
design measures, and systems of work:

6. • Using electrical equipment and instrumentation classified for the zone in which it is located. New
mechanical equipment will need to be selected in the same way. (See above);
• Earthing of all plant/ equipment (see Technical Measures Document onEarthing)
• Elimination of surfaces above auto-ignition temperatures of flammable materials being
handled/stored (see above);
• Provision of lightning protection
• Correct selection of vehicles/internal combustion engines that have to work in the zoned areas (see
Technical Measures Document on Permit to Work Systems);
• Correct selection of equipment to avoid high intensity electromagnetic radiation sources, e.g.
limitations on the power input to fibre optic systems, avoidance of high intensity lasers or sources of
infrared radiation
• Prohibition of smoking/use of matches/lighters
• Controls over the use of normal vehicles
• Controls over activities that create intermittent hazardous areas, e.g. tanker loading/unloading
• Control of maintenance activities that may cause sparks/hot surfaces/naked flames through a
Permit to Work System
• Precautions to control the risk from pyrophoric scale, usually associated with formation of ferrous
sulphide inside process equipment
Direct Fired Heaters, Hot Oil Systems and
Processes Operating Above Auto-Ignition
Temperatures
A range of petrochemical and refinery processes use direct fired heaters, e.g. steam crackers for
ethylene production. Clearly, if the fuel supply to the heater or the pipework carrying the process
fluid leaks close to the furnace, any leak must be expected to find a source of ignition, either
directly at the flames, or by a surface heated by a flame. In these circumstances, hazardous area
classification, and appropriate selection of ATEX equipment is not suitable as a basis of safety for
preventing fire and explosion risks.
Instead, safety should be achieved by a combination of a high standard of integrity of fuel and
process pipelines, together with a means of rapid detection and isolation of any pipes that do fail.
The consequences of the failure of a pipe carrying process materials within the furnace should be
considered in any HAZOP study.
Other processes (such as hot oil heating circuits) may handle products above their auto-ignition
temperature. Any such processes should be specifically identified in a safety case. Again, area
classification is not a suitable means of controlling the ignition risks, and the same considerations
apply, as with fired heaters.

7. Lightning Protection
Protection against lightning involves installation of a surge protection device between each non-
earth bonded core of the cable and the local structure. Further guidance can be found in BS
6651:19991
- (Code of practice for protection of structures against lightning). Ignitions caused by
lightning cannot be eliminated entirely, particularly with floating roof tanks, where vapour is
usually present around the rim seal. In these circumstances, measures to mitigate the
consequences of a fire should be provided.
Vehicles
Most normal vehicles contain a wide range of ignition sources. These will include electrical
circuits; the inlet and exhaust of any internal combustion engine; electrostatic build up;
overheating brakes, and other moving parts. Site rules should be clear where normal road
vehicles may be taken, and areas where they must be excluded.
Standard EN 17551
sets out the requirements for diesel powdered ATEX category 2 or 3 lift
trucks. Electric powered vehicles can also be built using a combination of this standard and the
normal electrical standards. No specification is available for vehicles with spark ignition engines,
and it is unlikely that such an engine could be built economically. Vehicles certified to ATEX
requirements are however expensive, and for many applications an unprotected type has to be
extensively rebuilt. Consequently, many employers are likely to try and justify not zoning storage
compounds, where lift trucks handle flammable liquids or gases in containers. In some stores,
perhaps with limited use of a vehicle, this may be acceptable. Discussions have been held with
the British Chemical Distributors and Traders Association, with the objective of clarifying when
storage areas should be classified as zone 2. The conclusions from this exercise will be made
available in due course. Discussions are also ongoing, about vehicles with gas detection
systems, designed to shut the engine and isolate other sources of ignition in the event of a gas
release. At present these are sold without any claim for ATEX compliance, but with the
suggestion they may be useful in cases of remote risk.
For the purposes of COMAH, an assessment is needed of the risk that an ignition within a
storage compound will produce a major accident, either directly or because a fire or explosion
spreads to involve other materials. If this is possible, it is more appropriate to provide controls to
prevent the spread, rather than simply apply more conservative zoning, and more restrictive rules
on the equipment used in the store.
Where specialist vehicles (e.g. cranes) are needed during maintenance operations, proper
controls and plant isolation may allow the normal zones to be suspended. Typically these will
involve written instructions, as specified in DSEAR schedule 1, or a formal permit to work system.
Many sites will have operations of filling and emptying road tankers with flammable materials.
Controls will be needed to prevent or minimise the release of gas or vapour but controls over

8. ignition sources are also needed. Hazardous areas may be considered to exist during the transfer
operation, but should not be present once the transfer is complete. Safe systems of work are
needed to ensure safety where such 'transient' zones exist.
Factors for Assessor of a Safety Case to Consider
• Is a full set of plans identifying hazardous areas available? For a large site they need not all be
provided in the report, but those examples relevant to the representative set of major accidents upon
which the ALARP demonstration is based must be included.
• Have all flammable substances present have been considered during area classification, including
raw materials, intermediates and by products, final product and effluents? Commonly these will be
grouped for the purposes of any area classification study.
• Locations where a large release is possible and the extent of hazardous areas has been minimised
by the use of mechanical ventilation should be identified, e.g. gas turbine power generation units,
compressor houses. Some reference to design codes, and commissioning checks to ensure the ventilation
achieves the design aim, should be provided. The consequences of a loss of power to the system should
be included in any section looking at other consequences of power loss.
• Have appropriate standards been used for selection of equipment in hazardous areas? Existing
plant will not meet the formula in DSEAR, but older standards distinguished between electrical
equipment suitable for zones 0, 1 and 2. Does the report identify old electrical equipment still in service
in a hazardous area, and what assessment has been made to ensure it remains safe for use?
• Is there a reference to the impact upon extent and classification of hazardous areas in the section
describing plant modification (see Technical Measures Document on Plant Modification / Change
Procedures); passive items like new walls and buildings can influence this if they obstruct natural
ventilation of adjacent plant
• Have all ignition sources been considered? A check list is provided in the DSEAR ACOP on
control and mitigation measures, and BS EN 1127 part 11
(Explosive atmospheres. Explosion prevention
and protection. Basic concepts and methodology).
Factors that could be considered during an on site
inspection
• If there are any large areas of zone 1 on the drawings, is there evidence that by design and
operation controls, the sources of release and consequently the location and extent of hazardous areas
have been minimised?
• Do any zone 2 areas extend to places where the occupier has inadequate control over activities that
could create an ignition source, or is there any suggestion that the zone boundaries have been arbitrarily
adjusted to avoid this?
• Has ignition protected electrical equipment been installed and maintained by suitably trained staff.
• Are the risks from static discharges controlled properly? Earthing of plant, drums and tankers is
the most basic requirement; other precautions are described in the references

9. • What control measures over ignition sources are adopted in hazardous areas during maintenance;
where ignition sources must be introduced, typical precautions include the use of supplementary
ventilation, portable gas detectors, and inerting of sections of plant. A local project onElectrical
Equipment in Flammable Atmospheres was undertaken and a report of the project's conclusions
completed.
Dust Explosions
The COMAH Regulations do not apply to any material if the only risk created is that of a dust
explosion. However, many toxic materials are handled in fine powder form, and a serious dust
explosion could cause a major accident. A dust explosion involving a non-toxic dust like
polyethylene would not result in a major accident as defined in the regulations, unless it also led
to loss of containment of a COMAH substance. A dust explosion could then be an initiator of a
major accident. Measures to prevent major accidents should address all potential initiators.
DSEAR requires that hazardous area classification for flammable dusts should be undertaken in
the same manner as that for flammable gases and vapours. Zoning as described above may be
applied, replacing 'gas atmosphere' with 'dust/air mixtures'. The zone numbers used are 20, 21
and 22, corresponding to 0,1 and 2 used for gases/vapours
The only relevant standard to help people zone their plant is BS EN 50281 part 3, 20021
, which is
an adaptation of the IEC equivalent.
Where toxic dusts are processed, releases into the general atmosphere should be prevented, and
the extent of any zone 21 or 22 outside the containment system should be minimal or non-
existent. The inside of different parts of the plant may need to be zoned as 20, 21 or 22,
depending on the conditions at particular locations.
Classification of dusts relating to autoignition and minimum ignition current is undertaken similarly
to gases/vapours, but involves additional complications.
The explosibility of dusts is dependent upon a number of factors:
• chemical composition;
• particle size;
• oxygen concentration;
Where toxic dusts are handled, in most cases occupiers will need to carry out testing of the
product for its explosion properties. Companies able to undertaken such testing are listed in
the IChemE's book on the prevention of dust explosions. There is no legally defined test for an
explosible dust. However, for many years we have used a small-scale screening test, the vertical
tube test, described in HSG 1032
. The issues about representative samples of dust, and other
factors that might cause the results to vary are also discussed in this guidance. In general, dusts
with a particle size greater than 500 µm are unlikely to cause an explosion. For most chemical

10. products it is preferable to test dust taken from the process, but if the particle size distribution
varies, it is common to test material that passes a 63-micron sieve, and take this as the worst
case.
Ignition due to a hot surface is possible, but the temperature needed to ignite a dust layer
depends on layer thickness and contact time. For COMAH sites with toxic dusts, the most likely
hazard would arise in drying processes, if substantial quantities were held for extended periods
hot enough to start self heating or smouldering combustion.
Status of Guidance
Existing codes of practice provide information with respect to good practice for hazardous area
classification. The standards detailing selection of appropriate electrical apparatus have been
updated to take into consideration ventilation effects.
European equipment standards may become 'harmonised' when a reference to them is published
in the Official Journal of the European Community. A list of ATEX harmonised standards can be
checked on the EU web site :
Equipment built to such a harmonised standard may assume automatic conformity with those
essential safety requirements of relevant directives that are covered by the standard. The EPS
regulations describe the conformity assessment procedures that apply to different types of
equipment.
Reference Documents
• HS(G)512
Storage of flammable liquids in containers, HSE, 1998.
Appendix 3 describes the requirements for hazardous area classification. The use of BS EN 60079-10:
20031
, and the Institute of Petroleum Code 'Area Classification Code for Petroleum Installations: Model
Code of practice in the Petroleum Industry' Part 15 are recommended. It suggests all drum stores should
be zone 2, to a height 1m above the stack. The same advice appears in HSG 166 and HSG 113 on
ignition protected lift trucks. Discussions with industry on the relaxation of this in particular
circumstances are ongoing.
• HS(G)712
Chemical warehousing: the storage of packaged dangerous substances, HSE, 1998.
This contains very limited information on hazardous area classification or control of ignition sources
• HS(G)1032
Safe handling of combustible dusts: precautions against explosions, HSE, 2nd Edition,
2003
• HS(G)1132
Lift trucks in potentially flammable atmospheres. The contents of this have been
overtaken to some degree by DSEAR, and the EPS regulations.
• HS(G)1402
Safe use and handling of flammable liquids, HSE, 1996.
Appendix 3 describes the requirements for hazardous area classification. The use of BS EN 60079-
141
and the Institute of Petroleum Code 'Area Classification Code for Petroleum Installations: Model

11. Code of practice in the Petroleum Industry' Part 15 are recommended. This is aimed mainly at small
scale handling, with containers of 200 litres or less.
• HS(G)1662
Formula for health and safety: guidance for small and medium sized firms in the
chemical industry, HSE, 1997.
The guidance describes the requirements for hazardous area classification, and gives some typical
examples. These should now be seen as rather conservative. The use of BS EN 60079-14, BS EN 50281
and BS EN 605291
are recommended. This is basic level guidance, and COMAH reports should
normally reference more specific publications, such as the other HSG series books listed, and other
items in this list.
• HS(G)1762
The storage of flammable liquids in tanks, HSE, 1998.
Paragraphs 35 to 39 describe the requirements for hazardous area classification. This cross references
BS EN 60079-10: 20031
, and the Institute of Petroleum Code 'Area Classification Code for Petroleum
Installations: Institute of Petroleum Model Code of Safe Practice, part 15, area classification for
installations handling flammable fluids, 2nd edition 2002.
• HS(G)1862
The bulk transfer of dangerous liquids and gases between ship and shore, HSE, 1999.
Appendix 2 describes the requirements for hazardous area classification. The use of BS EN 60079-
101
and the Institute of Petroleum Code 'Area Classification Code for Petroleum Installations: Model
Code of practice in the Petroleum Industry' Part 15 are recommended. Contains useful information about
electrostatic hazards during unloading.
• LPGA COP 1 Bulk LPG storage at fixed installations. Part 1: Design, installation and operation of
vessels located above ground, LP Gas Association, 1998.
• LPGA codes have not previously drawn a clear distinction between hazardous areas, and
separation distances required for other reasons. These are currently under revision, and will specify
hazardous areas, that in most cases will be smaller than the separation distance. Current codes are listed
on the UKLPG website .
• Model Code of practice in the Petroleum Industry' Part 15 is recommended. The guidance also
recommends that zones be recorded in a plan to prevent sources of ignition being brought in.
Electrical Equipment
• Standards produced by Europe in the BS EN 50014 range are gradually being superseded by
international standards produced in the range BS IEC 600791
. Equipment built to older standards,
including purely national standards may remain in service, provided it is properly maintained. The IEC
range of standards also includes documents on selection, installation and maintenance of equipment for
use in explosive atmospheres.
Non-electrical equipment
• The first standard for explosion protected non-electrical equipment is BS EN 13463 part 11
. It
describes requirements for "Category 3" equipment. Further parts of this standard are well advanced and
will appear during 2004.

12. • BS EN 1127-1: 19981
Explosive atmospheres - Explosion prevention and protection - Part 1: Basic
concepts and methodology, British Standards Institution.
This gives additional general advice on the many of the issues covered in this TMD.
Electrostatic ignition risks
The most recent general source of advice was drafted by a European standards working group,
but was published in the UK as BS PD R044-001 and not as a full standard. It contains much
useful advice about limiting pumping speeds, electrostatic risks from clothing, and many detailed
operations. The two parts of the older BS 5958: 19911
Code of Practice for the control of
undesirable static electricity remain current, because they contain some useful information not
duplicated by the PD. The two parts are:
Part 1: 1991 General considerations;
Part 2: 1991 Recommendations for particular industrial situations
• BS EN 502811
. The different parts of this standard set out requirements for construction of
equipment for use in atmospheres containing explosive dusts; information about selection and
maintenance; and
• BS EN 50281-3: 20021
covers the classification of areas where combustible dusts are or may be
present.
• BS 6651:19991
. Code of practice for protection of structures against lightning, British Standards
Institution.
Section 9 provides guidance on lightning protection of structures with inherent explosive risks.
• BS 7430:19981
Code of practice for earthing, British Standards Institution.
Lightning protection. Section 23 provides guidance on lightning protection.
Further Reading Material
Cox, A.W., Lees, F.P. and Ang, M.L., 'Classification of Hazardous Locations', 1993.
This was a study led by a consortium of the chemical, electrical and mechanical engineering
institutes, and showed how the subject spanned the traditional divides. It was important in the
development of ideas, but provides no new methodology for users.
A Guide to Safety in Aerosol Manufacture, BAMA, Third Edition, 1999.
Dust Explosion Prevention and Protection: A Practical Guide, IChemE,ISBN 0852954107
A practitioner's handbook - Electrical installation and maintenance in potentially explosive
atmospheres , Publication No. 186, The Engineering Equipment and Materials Users
Association.
References

13. 1. For further information on the relevant British Standards, please access the British Standards web
site and use the search facility.
Page last updated: 22nd September 2004
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Hazardous Area Classification Zones (0, 1, 2)
for Gases, Vapours, Mists & Dusts
The dangers of installing electrical apparatus in areas where explosive mixtures of gases and air could occur
was first recognised in the mining industry early in the 20th century. Awareness spread quickly to the
chemical and petrochemical industries who recognised that, unlike the coal mining situation where the

14. release of firedamp is unpredictable in quantity and duration, the occurrence of flammable atmospheres was
due to mechanical and process failures or to deliberate situations created by personnel. This meant that the
presence and persistence of such situations could be predicted to a level much higher than was the case in
mining. For an ignition to take place, a source of ignition must coincide with an explosive atmosphere.
Zoning therefore applies to non-mining situations only. Industrial sites handling flammable materials are
divided up according to the probability of a flammable atmosphere occurring into three zones of risk, based
upon frequency and persistence.
What is a flammable atmosphere?
A flammable atmosphere must contain fuel, plus oxygen to allow the fuel to burn and release energy. The
only source of oxygen considered is air, which is approximatly one fifth oxygen. The fuel must be finely
divided form - the ultimate state of division is the individual molecule of fuel, such as occurs in flammable
gases and vapours. Thus, gases and vapours can, under ideal conditions, mix intimately with the oxygen
molecules and, if the mixture is within the flammable range, introducing an ignition source will cause the
entire mixture to burn rapidly, usually within a fraction of a second. If this occurs outdoors, the result is likley
to be a flash fire. If indoors, an explosion could result. Anyone undertaking a hazardous area classification
exercise should take a special care to identify and deal with any areas where an explosion risk is present.
Hazardous Area Zones for Gases, Vapours and Mists
Gas/Vapour/Mist Zones
Zone 0 Hazardous Area - Flammable atmosphere present continuosly or for long periods or frequently.
Typically 10-1000 hr/year
Zone 1 Hazardous Area - Flammable atmosphere likley in normal operation occasionally. Typically 10-1000
hr/year
Zone 2 Hazardous Area - Flammable atmosphere unlikley in normal operation and, if it occurs, will exist only
for a short time. Typically 1-10 hr/year
Zones for Flammable Dusts (IEC 60079-10-2:2009)
Dust Zones
Zone 20 Hazardous Area - Flammable atmosphere present continuously or for long periods or frequently -
Typically 1000 hr/year
Zone 21 Hazardous Area - Flammable atmosphere likley in normal operation occasionally - Typically 10-
1000 hr/year
Zone 22 Hazardous Area - Flammable atmosphere unlikley in normal operation and, if it occurs, will exist
only for a short time - Typically 10hr / year, max 1hr / release
Zone 0 Hazardous Area
Releases causing a flammable atmospheres that are part of the normal running of the plant may occur with
some regularity. Examples would be releases as a result of sampling operations or the opening of a manway
to allow more reagent to be added to a process. Zones 1 may occur on parts of plant where people may
work but are associated with smaller releases that create only small localised areas, so it is less likley that
personnel will have significant exposure to them. (Although the toxic nature of chemicals is completely
divorced from area classification, it is true that exposure to many flammables also has health implications).
An example of Zone 1 is the tank which was used as the Zone 0 example. When this tank is filling, the
atmosphere in its vapour space will be expelled. The zone extent, typically 3m from the vent, represents the
distance for the vapour to dilute to a fraction of its LEL.
Again as a rule of thumb, it can be said that if a potentially explosive atmosphere exists for between 10 and
1000 hours a year, it is classified Zone 1.
As a result of the difficulty of adequately assessing the effectivness of ventilation in indoor plant areas, many
of these are given a blanket Zone 1 classification, even when only secondary grade releases are present.
Large Zones 1 in outdoor plant should normally be avoided, however.

15. Outdoor Zones 1 are relatively rare compared to Zones 2 and usually small.
Indoor process areas with poor ventilation are often Zone 1 throughout.
Zone 2 Hazardous Area
By definition this is the least hazardous of zoned areas. On a normal outdoor process plant it is usually the
case that the great majority of areas will be Zone 2, because of its very nature, Zone 2 is the zone created
by the unlikley condition. It is not intended to cover conditions of catastrophic process failure which are so
unlikley as to be almost unforeseeable. Because it is the abnormal occurrence that results in Zone 2, it is
also likely to involve the largest quantity of flammable material released and thus the largest hazardous area
results. Typical of the Zone 2 situation is the area in bund of the vented tank. This takes account of possible
over-filling of the tank (an abnormal situation), resulting in a small quantity of liquid running down the side of
the tank and collecting in the bund. Note that the example in this particular code of practise does not cover
the situation where the entire contents of the tank are emptied into the bund: this is considered a
'catastrophic' failure of the control system.
Most outdoor hazardous areas are Zone 2
Indoors, zone 2 is only possible with adequate ventilation (say 12 air changes/hr), otherwise Zone 1 is
appropriate.
- See more at: http://www.cablejoints.co.uk/hazardous/hazardous-area-zones-
gases-vapours-mists-dusts#sthash.i5PQdQbT.dpuf