Hazardous Area Classification (HAC): Zones & Equipment Explained

If you answered “no” to any of the following questions, you may want to consider re-evaluating your Hazardous Area Classification. 

A hazardous area classification (HAC) classifies an operational area based primarily on the likelihood of a flammable atmosphere being present, a flammable atmosphere being the combination of a fuel and an oxidant, typically air.

Once the extent of that atmosphere is known, any equipment inside it has to be built so it does not become the ignition source. A HAC identifies the flammable materials, weighs the ventilation, and accounts for the other factors that can create an explosive atmosphere. That goal is universal. How you reach it is not.

If you carry safety accountability across sites in more than one country, that is where a quiet problem lives. Your European sites zone to the DSEAR regulations and ATEX. Your US sites classify to NFPA. Each one can pass its local audit while the same hazard is controlled to two different standards, with no single view of it at group level. Compliant in every jurisdiction is not the same as safe across the estate. This guide takes the zones first, because that is what most people come here for, then sets the two systems side by side so you can see where they diverge, then the material properties that decide whether a gas, vapour, or dust can form a flammable atmosphere at all.

Building a suitable HAC system requires the identification of hazardous locations where fuel and air can result in an explosive atmosphere. Often this occurs inside equipment, but it applies equally to releases from equipment, either intentional or during a malfunction.

UK and European regulations provide specific definitions of hazardous areas and classify them through a risk-based approach. Think of it in terms of risk. The risk of an ignited event is a combination of the likelihood of a flammable atmosphere being present and the simultaneous likelihood of an ignition source being present.

Sometimes the flammable atmosphere is present continuously, such as above a liquid surface held above its flashpoint. In those cases it makes sense to ensure that any potential ignition source in that atmosphere has an extremely low likelihood of becoming an effective ignition source, because most of the risk reduction has to come from controlling ignition. On the other hand, if the flammable atmosphere occurs only infrequently, for example when a flanged connection leaks, the risk is reduced mainly by the atmosphere being absent almost all of the time, so the ignition source does not need to meet such a demanding standard.

For the ignited event we are trying to avoid, both the flammable atmosphere and the effective ignition source have to be present at the same time.

Equipment present within a flammable atmosphere therefore has to be special from the point of view of acting as an ignition source. It must go through a special ATEX (from the French for explosive atmospheres, Atmosphères Explosibles) certification process, which lowers the probability of the equipment completing the fire triangle of fuel, oxygen, and an ignition source. By making sure equipment does not act as an ignition source in a hazardous area, you reduce the risk of a fire or explosion.

Leadership affects process safety by making sure everyone in the organisation knows the hazards and their possible consequences.

The challenge for management is to keep process risk visible on the corporate agenda. This is tough when operational pressures push for short-term gains.

The hours-per-year figures are the practical bands widely used to interpret the zones, not a fixed clause in the standard.

The gas and vapour zones are easiest to read against real equipment. Zone 0 is the continuous case, such as the vapour space above a flammable liquid held above its flashpoint. Zone 1 is the normal-operation case, such as the area around a filling point, a sample point, or a vent that releases when the process runs. Zone 2 is the occasional case, such as the area exposed to a leak or spill that is not expected in normal running.

Combustible dust follows the same logic with a different set of numbers. Zone 20 is a cloud present continuously or for long periods, such as the inside of a mill, mixer, or silo. Zone 21 is a cloud likely to form in normal operation, for example around an open charging point. Zone 22 is a cloud that is unlikely and short-lived if it appears, such as dust disturbed only by a fault or during cleaning.

The zone decides which ATEX equipment category you are allowed to install in that area, which is where the classification starts costing money. Zone 0 requires Category 1 equipment, Zone 1 requires Category 2, and Zone 2 requires Category 3, with a G suffix for gas and vapour and a D suffix for dust. Equipment built for a more demanding zone can be used in a less demanding one, so Category 1 kit is valid in Zone 0, 1 and 2, but never the reverse. A zone on a drawing only protects anyone if the equipment installed there matches the category it calls for.

HAC in the United States shares the same fundamental goal as the UK, assessing and managing potentially explosive atmospheres, but the terminology and language differ to reflect the US regulatory system.

In the USA there are two primary guiding documents for the classification process: NFPA 497 and NFPA 499. NFPA 497 is the reference for situations involving flammable liquids, gases, or vapours, while NFPA 499 covers materials that can generate combustible dusts or clouds. If a facility handles materials that fall under these NFPA categories, it is obliged to follow the NFPA codes.

In the US the regulatory framework for flammable gases, vapours, powders, and dusts is governed by the National Fire Protection Association (NFPA). The process includes:

• NFPA codes application: if a facility handles materials with the potential for hazardous atmospheres, the NFPA codes are applied to determine whether the area should be classified as hazardous.
• NFPA 70 (National Electrical Code): if the area is classified as hazardous under NFPA 497 or 499, NFPA 70 then specifies the requirements for electrical installations within it.

The US process is prescriptive and considers several factors beyond a simple yes or no:

• Process size: the scale and scope of the process are evaluated. This is also considered in the UK.
• Potential weak points: vulnerable areas in the process are identified. In the UK this sits within the DSEAR risk assessment rather than the HAC.
• Material characteristics: the ignition energy requirements of the material are assessed. In the UK this also sits within the DSEAR risk assessment.
• Classification zone determination: a classification zone is determined following NFPA recommendations. This is, in effect, the example approach used in the UK.
• Electrical code compliance: within the zone, NFPA 70 is applied so that all electrical installations meet the required standard. In the UK the zone helps specify the correct Equipment Protection Level for equipment in that zone.

This is where it bites if you run sites on both sides of the Atlantic. The UK and EU method is risk-based. You weigh how likely a release is and how long it lasts, then zone accordingly. The US method is more prescriptive. You match the process to a classification diagram and apply the code. Point the two at the same reactor and they will not always draw the same picture. A prescriptive method, applied without the judgement the risk-based one forces, tends to zone conservatively, which is one reason internal, diagram-led studies so often over-zone. For an EHS Director that is not a footnote. It means a plant in Texas and a plant in Rotterdam can both be signed off as compliant while the same hazard sits behind two different standards, and no one at group level can see the gap.

The outcome of a HAC is generally a drawing. It is a visual representation of the hazardous zones across a process or facility, placed against the actual sources of release. The zones on it are the Zone 0, 1 and 2 (and Zone 20, 21 and 22) classifications set out above.

These zones are not static. Their extent depends on the material, the ventilation, the process conditions, and the operations running at the time, so a change to any of those can change the drawing. The purpose of the drawing is to specify where equipment goes and which category it must meet, given the likelihood of both a flammable atmosphere and an ignition source being present at that spot.

Regulations set the high-level requirements for HAC, but the detail on how to engineer equipment and assign it to categories lives in the standards. In the UK and Europe, BS EN 60079-10-1 and BS EN 60079-10-2 provide the methods for classifying areas for gases and vapours, and for dusts. These standards give the detailed information and procedures for managing hazardous areas safely.

A flammable atmosphere occurs when the concentration of gas or vapour falls within a specific range known as the flammability limits. These are the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL). Too little air and the mixture is too rich to burn, above the UEL. Too much air, from efficient ventilation, and it sits below the LEL.

To manage a flammable atmosphere you need to understand the properties of the substances involved:

• Flashpoint: the temperature at which a substance gives off enough vapour to form a flammable atmosphere that will flash if ignited.
• Flammability limits: the vapour concentration range within which a substance can form a flammable atmosphere.
• Autoignition temperature: the minimum temperature at which a substance ignites without an external ignition source.
• Vapour density: substances with a lower vapour density tend to rise and create buoyant flammable atmospheres, hydrogen being the classic example. Substances with a higher vapour density, such as many industrial solvents, tend to sink and form flammable atmospheres closer to the ground.

Flammability limits and temperature class can change as temperature rises. Substance properties, and the flashpoint in particular, are not fixed, so you have to account for the range of operating conditions when you assess a flammable substance.

Flammable atmospheres are not limited to gases and vapours. Powders and dusts bring their own challenges. To form a flammable atmosphere with a powder, several conditions have to be met at once:

• Fuel capability: the powder must be able to act as a fuel and react with oxygen. Already-oxidised substances, such as chalk or sand, cannot form combustible clouds.
• Explosible concentration: the concentration must fall within the Minimum Explosible Concentration (MEC).
• Particle size distribution: to stay suspended in air, the particles generally need to be smaller than 500 microns. Particle size drives both the ability to form a cloud and the combustion behaviour.
• Ignition energy: an energy source above the powder’s minimum ignition energy is required.

Because of the nature of combustible dust, this data is difficult, and often impossible, to calculate, which is why your specific powders have to be tested. Particle size distribution and similar factors decide whether a given powder will ignite, and they can differ significantly between suppliers and sources. Many powders from different sources show widely different minimum ignition energies. This is much less of an issue with gases and vapours, where the data can usually be obtained from the literature. Powder and dust testing tells you how likely a flammable atmosphere is to form and how easily it can be ignited.

Plenty of labs can run these tests. An accredited result is a different thing, because it holds up in a regulator’s office or a courtroom. We hold that accreditation, and in the UK we are the only laboratory that holds it for the dust explosion test battery and the DSC and ARC thermal tests alongside it (UKAS 9248). The results come back in days, not the months a queue elsewhere can mean.

If you own more than one location, do not file this under done. Pull the hazardous area drawings for every site and ask a harder question than whether you passed the audit.

• Are the same hazards zoned to the same standard across the estate, or does each site classify in its own dialect?
• Where classification was done internally, was it risk-based, or did it default to blanket zoning and an equipment bill no one has revisited?
• When was each drawing last walked against the actual floor, and would it survive a release you have not had yet?
• If your worst site inherited your best site’s classification tomorrow, what would change?

If some of those answers are uncomfortable, that is not a team failing. A plant changes faster than anyone has the capacity to re-walk every drawing, which is exactly how the floor and the paper drift apart.

Compliance tells you that you cleared the bar on the day you were checked. It does not tell you that your Texas plant and your Rotterdam plant are as safe as each other this morning. Closing that gap is the work, and it is the part no audit does for you.

If you want a second set of eyes, we run the tests that show how your materials behave, including flashpoint testing, flammability limits, autoignition temperature testing, and the full combustible dust and dust explosion testing battery, and we reconcile classification across sites so one standard holds everywhere.

Send us one site, or one drawing you are not sure about, and we will tell you straight whether it is right-sized, defensible, and consistent with the rest of your estate. Process safety is all we do, so you are dealing with people who have walked plants like yours, not a generalist with a safety line item. Where a powder has to be tested to settle it, we run the dust explosion and thermal battery to ISO 17025 (UKAS 9248) and turn it round in days, not months, so the result stands up in a regulator’s office. And because we work to DSEAR and ATEX and to NFPA from the UK, the US, and India, one partner can hold a single standard across every site you answer for.

If you already know a site is over-zoned or overdue a review, book a DSEAR assessment and we will right-size the one costing you most. If you would rather pressure-test your thinking first, get in touch for a conversation with a consultant, not a salesperson.

Read more about what can and should trigger a DSEAR review. 

Learn more about HAC in our FREE on-demand webinar.