Hazardous Area Classification Explained: Zones, Gas Groups, Temperature Classes and Choosing Ex Equipment

Every instrument, junction box, beacon and cable gland installed on a gas plant, refinery or tank farm must be suitable for the explosive atmosphere that could surround it. Getting this wrong is not a paperwork issue: an unsuitable enclosure, an unsealed entry or a hot surface can ignite a gas release. The process of hazardous area classification defines where flammable atmospheres may occur and how often; Ex equipment selection then matches the protection of each device to that classification, the gas group and the temperature class. This guide walks through the logic in the IEC 60079 series, how to read an Ex marking, and the installation and maintenance mistakes that most often invalidate protection on site.

Area Classification: Zones and How They Are Assigned

Area classification for gases and vapours is carried out to IEC 60079-10-1, and for combustible dusts to IEC 60079-10-2. Many operators also apply the Energy Institute Model Code of Safe Practice Part 15 or API RP 505 as the method for determining extents. The classification considers each source of release (flanges, pump seals, vents, sample points), its grade of release, ventilation and dilution, and the properties of the material.

Equipment Protection Levels (EPLs) express the level of protection of the equipment itself: “a” very high, “b” high, “c” enhanced, with G for gas and D for dust. The EPL approach also allows a risk-based assessment to deviate from the default zone-to-EPL mapping, but most projects apply the default.

North American installations may use the Class/Division system (NEC Article 500), where Class I Division 1 roughly corresponds to Zones 0 and 1 and Division 2 to Zone 2, or the zone system under NEC Article 505. The two systems are not directly interchangeable, so equipment certified only for one requires a check against the project’s specified system.

Gas Groups and Temperature Classes

Gas groups

Group I covers mines susceptible to firedamp. Group II covers surface industries and is subdivided by how easily the gas ignites and how readily a flame passes through a narrow gap:

• IIA: least easily ignited, for example propane and methane.
• IIB: for example ethylene and hydrogen sulphide.
• IIC: most easily ignited, for example hydrogen, acetylene and carbon disulphide.

Equipment marked IIC is suitable for IIB and IIA atmospheres; IIB is suitable for IIA. Dust groups are IIIA (combustible flyings), IIIB (non-conductive dust) and IIIC (conductive dust).

Temperature classes

The temperature class sets the maximum surface temperature of the equipment, which must be below the autoignition temperature of the gas present.

Autoignition temperatures vary between data sources, so use the values in IEC 60079-20-1 or the project’s classification schedule. A T4 or better rating covers most hydrocarbon process areas, but the governing gas and its temperature class must come from the classification drawing, not from habit.

Protection Concepts

Intrinsic safety is the only concept that permits live maintenance in the hazardous area without a gas-free certificate, within the limits of site procedures, which is why it dominates instrumentation.

Reading an Ex Marking

Take the marking “Ex db IIC T6 Gb” as an example:

• Ex: equipment certified for explosive atmospheres.
• db: flameproof enclosure at protection level “b”.
• IIC: suitable for gas groups IIC, IIB and IIA.
• T6: maximum surface temperature 85 °C.
• Gb: suitable for Zone 1 and Zone 2.

A full marking also carries the certificate number (for example an IECEx or ATEX certificate), the ATEX equipment group and category such as II 2 G under the ATEX directive, and the ambient temperature range. If no range is marked, the standard range is -20 °C to +40 °C. A certificate number ending in “X” means specific conditions of use apply, which must be read and followed; “U” denotes an Ex component that is not complete equipment.

ATEX is the EU legal framework; IECEx is an international certification scheme based on the same IEC standards. GCC operators commonly specify IECEx, ATEX or both, so check the project specification before ordering.

Intrinsically Safe Loops

An IS loop consists of field apparatus, interconnecting cable and associated apparatus (a zener barrier or galvanic isolator) in the safe area. Entity parameters must satisfy:

1. Uo ≤ Ui, Io ≤ Ii and Po ≤ Pi between associated apparatus and field device.
2. Co ≥ Ci plus cable capacitance, and Lo ≥ Li plus cable inductance (or the cable L/R ratio within limits).
3. Gas group of the associated apparatus at least that of the area.

Galvanic isolators such as the MTL5541 repeater power supply or the Pepperl+Fuchs KFD2-UT2-Ex2 temperature converter avoid the dedicated low-impedance IS earth that zener barriers need. For fieldbus, a high-energy trunk with barriers such as the MTL 9370-FB2 fieldbus barrier feeding IS spurs is a common architecture. More options are in the isolators and barriers category.

Installation: Glands, Seals and Entries

Installation is covered by IEC 60079-14. The points that most often fail inspection are cable entries:

• Cable glands must be certified for the protection concept and gas group of the enclosure, and suitable for the cable type. Armoured cable glands such as the Hawke 501/421 must be selected for the cable construction and the entry type.
• IEC 60079-14 sets selection rules for Ex d entries, including when a barrier (compound) gland is required rather than an elastomeric seal.
• Unused entries must be closed with certified stopping plugs, not dust caps or the shipping plugs.
• In conduit systems, sealing fittings such as the Crouse-Hinds EYS conduit seal are installed within the distance from the enclosure required by the code, and must be poured with the correct compound.
• Adapters and reducers are limited in number and must be certified.

Oman Site Considerations

The ambient temperature range is part of the certificate, and surface temperature class is defined at the maximum certified ambient. Equipment marked only for +40 °C is not suitable for an exposed location in an Omani summer, where air temperatures reach 45–50 °C and solar gain on a dark enclosure adds more. Specify a certified ambient of +55 °C or +60 °C where needed and confirm it on the certificate, not the brochure. Dust ingress raises enclosure IP demands, and coastal sites at Sohar, Duqm, Sur and Salalah need stainless steel or suitably coated enclosures, glands and fixings. Sour gas facilities also require attention to material compatibility of glands and fittings.

Common Mistakes That Invalidate Certification

• Painting over flamepaths or applying non-approved sealant to flameproof joints.
• Leaving unused entries open or closed with plastic transit plugs.
• Drilling extra entries or fitting non-certified components in an Ex d enclosure.
• Using glands not certified for the enclosure’s protection concept, or tightening them on the wrong cable diameter.
• Mixing IS and non-IS wiring in the same cable or terminal box without segregation.
• Replacing a field device with a different model without rechecking IS entity parameters.
• Missing or illegible nameplates, which make inspection impossible.

Inspection and Maintenance

IEC 60079-17 defines initial, periodic and sample inspections at visual, close and detailed grades. Periodic intervals are set by the plant’s risk assessment, and the standard indicates they should not normally exceed three years without expert justification. Repairs to Ex equipment follow IEC 60079-19. Personnel should be competent under schemes such as IECEx CoPC or equivalent operator requirements.

Ex Equipment Selection Checklist

• Zone and required EPL from the classification drawing.
• Gas group (or dust group) of the governing material.
• Temperature class versus autoignition temperature.
• Certified ambient temperature range, including solar gain.
• Protection concept suited to the device and maintenance approach.
• Certificate scheme required by the operator (IECEx, ATEX or both) and any “X” conditions.
• IP rating and corrosion resistance for the site.
• Gland, plug and adapter certification matching each enclosure.
• IS entity parameter calculation for each intrinsically safe loop.
• Inspection records and spare parts plan.

Sourcing Ex Equipment Through Seven Star LLC

Seven Star LLC supplies certified Ex equipment for hazardous areas in Oman and the GCC, including MEDC SM87 beacons, MEDC DB3 sounders, Crouse-Hinds EFSC control stations and R. STAHL 8570 plugs and sockets. Browse the Cooper Crouse-Hinds brand page and send the zone, gas group, temperature class and certification scheme with your enquiry. For detector selection in the same areas, see the gas detector selection guide.

Send your zone, gas group, temperature class and ambient range to Seven Star LLC to source correctly certified Ex equipment.