Hazardous areas and explosion-proof equipment: zones and Ex certification

Hazardous area classification is the process of mapping where flammable gas, vapour or dust may be present in dangerous amounts, so that electrical equipment and other ignition sources there can be controlled. For gases and vapours the areas are divided into Zone 0, Zone 1 and Zone 2 by how often an explosive atmosphere appears, and electrical equipment inside them must carry an Ex certification that matches the zone.

"Explosion-proof" is the everyday word for equipment built to work safely in these areas. Strictly it names one method, the flameproof enclosure (Ex d), so this guide says "Ex equipment" for the whole family. It also covers the codes on a nameplate and the difference between ATEX and IECEx. For the shutdown functions that protect the same areas, read ESD, SIS and interlock.

An explosion needs three things at once: fuel mixed with air in the right proportion, oxygen, and a source of ignition. At a fuel terminal the fuel is gasoline or LPG vapour and the air is everywhere, so the one element engineers can control is the ignition source.

Area classification does that job on paper first. A process safety engineer studies every point where vapour can escape, such as vents, pump seals, loading arm couplings, sample points and drains, and draws zones around them. IEC 60079-10-1 sets out the method for gases and vapours. Dust has its own zones, 20, 21 and 22, under IEC 60079-10-2. The drawing then becomes a design input that tells electrical and instrument engineers which equipment may go where.

Classification does not remove the vapour, and it does not say an area is dangerous all the time. It says how likely an explosive atmosphere is, and that likelihood sets how strict the equipment rules become. The site owner carries the responsibility, and the study must be reopened when the product, the process or the layout changes.

Products matter. Gasoline vapour can form an explosive atmosphere at normal ambient temperature, while diesel has a higher flash point and usually does not unless it is heated or sprayed as a mist. The study should state this for each product the site handles.

Equipment with a higher protection level may be used in a lower-risk zone, so a Gb device is fine in Zone 2. The reverse is not allowed.

Some older installations designed to American practice use Class I, Division 1 and Division 2 instead of zones. Division 1 roughly corresponds to Zones 0 and 1, and Division 2 to Zone 2. When you meet both systems on one site, ask for the classification to be restated in zones before you compare certificates.

Engineers do not guess zones. They list each source of release, ask how often and how much it releases, and then ask how well air carries the vapour away.

IEC 60079-10-1 sorts releases into three grades. Continuous grade: vapour is released continuously or for long periods, like the liquid surface in a fixed-roof tank. Primary grade: release is expected occasionally in normal operation, for example a vent that breathes during filling or a coupling that is disconnected. Secondary grade: release is not expected in normal operation and would be short, such as at a pump seal or flange that leaks only when something fails.

Ventilation then adjusts the result. In the open air with some wind, vapour disperses quickly and the zone shrinks. In a closed pump house with little air change, the same release produces a larger zone, and weak ventilation can raise the zone type. Vapour density matters too. LPG and gasoline vapour are heavier than air and collect in pits, trenches and low points, so the classification often reaches further at ground level than the equipment itself suggests.

That is why classification drawings show a plan and a section. Zones have height, and a cable trench that leaves a zone can carry vapour out with it.

Equipment for hazardous areas either keeps an ignition from starting or keeps an explosion contained. IEC 60079-0 sets the general rules, and each method has its own part of the series. The method is shown by a letter in the Ex code.

Pick the method for the job. Motors and switchgear commonly sit in flameproof or increased safety enclosures. Transmitters and switches on a 4-20 mA loop often use intrinsic safety. Analyser cabinets and large panels can be pressurised. Intrinsic safety, Ex ia, is the usual way to put an instrument in Zone 0, and the Citia bulletin on an early warning system describes ex-proof sensors suitable for Zone 0 at storage areas.

The temperature classes above come from IEC 60079-0. A lower number allows a hotter surface, so T4 is stricter than T3 and T6 is the strictest.

ATEX and IECEx both certify equipment for explosive atmospheres, and both rest on the IEC 60079 standards. They are different instruments. ATEX is European Union law, named from the French "atmosphères explosibles". Directive 2014/34/EU covers equipment placed on the EU market, and Directive 1999/92/EC covers the duties of employers. IECEx is a voluntary international scheme under the IEC, with certificates in a public online database.

An Indonesian site is not under the ATEX directives, because those apply in the EU. Project specifications may call for the IEC 60079 series, IECEx certificates or ATEX certificates, so ask which one the owner accepts. The national standards body, BSN, issues SNI documents that adopt parts of the IEC 60079 series. Confirm the current list with BSN before you quote a standard number in a tender.

Citia's field instrumentation service prepares the datasheets and I/O lists where the Ex marking, gas group and temperature class of each instrument are recorded. The site installation crews work under permit to work and lockout-tagout, including grounding and cable termination.