— A Practical Guide for Engineers, EPCs, and Plant Operators

In oil and gas, chemical, and pharmaceutical facilities, the presence of flammable gases, vapours, or combustible dusts means that instrument selection is not just about performance—it is about safety. A poorly selected instrument in a hazardous area can become an ignition source, turning a minor fault into a catastrophic explosion.

This guide walks you through the fundamentals of hazardous area classification, the protection concepts available, and a step‑by‑step selection process for instruments used in Zone 0, Zone 1, and Zone 2 environments. Whether you are an EPC engineer specifying instruments for a new project or a plant engineer upgrading existing equipment, this guide will help you make safe, code‑compliant, and cost‑effective decisions.

1. Understanding Hazardous Area Classification

Hazardous area classification is a technique for assessing the probability of formation of a flammable atmosphere and its likely duration-22. The International Electrotechnical Commission (IEC) defines three zones for gases and vapours under IEC 60079‑10‑1-7:

For dust hazards, IEC 60079‑10‑2 defines equivalent zones: Zone 20 (continuous), Zone 21 (likely), and Zone 22 (unlikely)-7.

Key insight: Zone 0 is the most hazardous area, followed by Zone 1, and then Zone 2-42. It is significantly more expensive, complex, and time‑consuming to design and maintain equipment for Zone 0 than for Zone 1 or 2-42. Wherever possible, locate control panels and instruments outside Zone 0 areas—often moving them just a few feet can make the difference-42.

North American context: The Class/Division system follows the same logic—Division 1 effectively combines Zone 0 and Zone 1-2. API RP 505 provides guidelines for classifying locations at petroleum facilities as Class I, Zone 0, Zone 1, and Zone 2-.

2. Gas Groups and Temperature Classes

Beyond the zone, you must also consider the type of gas present and the temperature limits of your instrument.

Gas Groups (IEC 60079‑0)

Gases are classified into groups based on the ignition energy required-1:

Key rule: Equipment marked for Group IIC can also be used for IIA and IIB. Equipment marked for IIB can also be used for IIA—but not the reverse-1.

Temperature Class (T‑Class)

Temperature class sets the maximum allowable surface temperature of your device. This must stay below the auto‑ignition point of the hazardous substance-2:

3. Protection Concepts: Matching the Right Method to the Zone

Once you understand your area classification, match a protection concept to it-2. Each protection method has trade‑offs worth understanding before you commit-2.

Intrinsic Safety (Ex ia / Ex ib)

How it works: Limits electrical and thermal energy below the ignition threshold—the energy in the system is insufficient to ignite a hazardous atmosphere under any conditions--2.

Zone suitability:

• Ex ia: Suitable for Zone 0, Zone 1, and Zone 2-

• Ex ib: Suitable for Zone 1 and Zone 2-

Advantages: Simplifies field work, allows live calibration-2.
Trade‑off: Requires barriers or galvanic isolators in the safe area-2.

Critical note: While intrinsically safe pressure sensors marked Ex ia can be used in Zone 0, a number of additional conditions must be taken into account—not all intrinsically safe sensors are suitable for Zone 0. Suitability depends on the electrical connection and must be verified from the data sheet or EC type examination certificate-1.

Flameproof / Explosion‑proof (Ex d)

How it works: Contains ignition inside a robust housing and ensures hot gases cool before escaping-2. If ignition occurs inside the device, the housing prevents the flame or pressure wave from reaching the surrounding atmosphere-.

Zone suitability: Suitable for Zone 1 and Zone 2-1.

Advantages: Straightforward to understand and apply; does not require Zener barriers or galvanic isolators-.
Trade‑off: Complexity is in the mechanical domain—threaded flame paths, conduit seals, and hot‑work permits for routine tasks-2.

Increased Safety (Ex e)

How it works: Provides additional measures to prevent excessive temperatures, arcs, or sparks in normal operation.

Zone suitability: Suitable for Zone 1 and Zone 2.

Non‑incendive (Ex n / Ex nA)

How it works: Equipment that, in normal operation, is not capable of igniting a surrounding explosive atmosphere.

Zone suitability: Suitable for Zone 2 only-1-2.

Advantages: Reduced overhead in Zone 2 areas-2.

4. Step‑by‑Step Instrument Selection Process

Step 1: Identify the Zone Classification

Start by confirming the hazardous area classification for your installation location. This is typically defined in the project's area classification drawings.

• Zone 0: Only Ex ia (Category 1G) instruments are permitted-2-

• Zone 1: Ex ia, Ex ib, Ex d, Ex e instruments are permitted-1

• Zone 2: Ex ia, Ex ib, Ex d, Ex e, Ex n instruments are permitted-1-3

Step 2: Identify the Gas Group and Temperature Class

Determine the gas group (IIA, IIB, or IIC) and the required temperature class (T1–T6) for your application-2.

Example: A Zone 1 area with hydrogen requires an instrument rated Ex d IIC T6 (flameproof, Group IIC, T6 temperature class).

Step 3: Select the Protection Concept

Choose the appropriate protection method based on the zone and your operational requirements:

Step 4: Verify Certification and Marking

Check that the instrument carries the correct certification for your region:

• Europe: ATEX certification (mandatory)

• International: IECEx certification (voluntary but widely accepted)

• North America: FM or CSA certification-2

Understanding the marking:

• Ex ia = Intrinsically safe for Zone 0, 1, 2-

• Ex d = Flameproof / explosion‑proof-

• Category 1G = Suitable for Zone 0 (gas)

• Category 2G = Suitable for Zone 1 (gas)

• Category 3G = Suitable for Zone 2 (gas)-1

Important: ATEX, IECEx, FM, CSA, and UKCA certifications are not interchangeable-2. If your facility has global sites, confirm which certifications apply before you write a specification-2.

Step 5: Document Your Protection Philosophy

Document your protection philosophy in your instrument index. When project engineers understand which protection method applies to each area classification, you eliminate one‑off specials. Consistent documentation also simplifies technician training, spare parts management, and regulatory audits-2.

5. Installation and Maintenance Considerations

Selecting the right instrument is only half the battle. Proper installation and maintenance are equally critical.

Installation Standards

• IEC 60079‑14 is the installation standard for electrical systems in hazardous areas, covering the installation and required initial inspection of Ex equipment-

• An initial inspection must be carried out when the equipment is first installed-

• IEC 60079‑17 requires that the interval between inspections shall not exceed three years without seeking expert advice-

Key Installation Requirements

• Cable glands must be certified for the same zone and protection method as the installation

• For armoured cables, the gland must properly terminate and earth the armour

• Unused cable entries must be sealed with certified blanking plugs

• Equipment installed in Zone 0 requires Ex ia certification and must be connected through an approved safety barrier

6. Common Mistakes to Avoid

7. Why Choose Anhui Tiankang for Hazardous Area Instrumentation?

Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial instruments for nearly five decades. Our hazardous area instruments are trusted by major oil companies and EPC contractors worldwide.

Comprehensive hazardous area product range:

• Pressure transmitters: Ex ia, Ex d, and Ex n configurations

• Temperature sensors: Thermocouples and RTDs with Ex ia/Ex d certification

• Level instruments: Radar level meters and level switches with Ex certification

• Instrumentation cables: Intrinsically safe cables with low capacitance and inductance

• Ex cable glands: Ex d and Ex e certified glands in brass and stainless steel

Full certifications:

• CCC Ex (China compulsory certification)

• ATEX (European market)

• IECEx (International)

• SIL (Functional safety)

• China Classification Society (CCS) for marine applications

CNAS‑accredited laboratory: Full performance testing ensures every instrument meets its specification before shipment.

Proven track record: Long‑term supplier to CNPC, Sinopec, CNOOC, and international EPC projects.

8. Conclusion

Selecting instruments for hazardous areas requires a systematic approach: identify the zone → determine the gas group and temperature class → choose the right protection concept → verify certification → install correctly. Getting it wrong can have catastrophic consequences; getting it right ensures safe, reliable, and compliant plant operation.

Quick reference:

• Zone 0 → Ex ia only

• Zone 1 → Ex d, Ex e, Ex ia/ib

• Zone 2 → Ex n, Ex d, Ex e, Ex ia/ib

Remember: The cost of an HSE incident will always exceed the time spent specifying correctly-2.

Contact Us

For hazardous area instrumentation selection advice, technical documentation, or project quotations, please contact:

Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/

Anhui Tiankang – Your partner for safe, reliable hazardous area instrumentation.