Why area classification comes BEFORE equipment selection

Many engineers approach Ex installations by first selecting equipment (“we’ll use Ex d motors and Ex i instruments”) and only afterward thinking about where to install them. This is backwards. The CORRECT sequence is :

1. Identify all sources of release (every flange, pump seal, vent, sampling point, equipment opening that can release flammable substances)
2. Classify each source (continuous / primary / secondary grade)
3. Compute the extent of the hazardous area around each source (based on gas properties, release rate, ventilation conditions)
4. Draw the resulting Zone 0 / 1 / 2 map of the facility
5. Select Ex equipment per zone classification

The result is the Area Classification Document — typically a plot plan with colored zones, ranges of extent, plus a comprehensive register of sources of release with their characteristics. This document is a legal requirement in ATEX-regulated facilities and must be approved by a competent person.

The decision logic

For each Source of Release, IEC 60079-10-1 follows this decision tree :

What grade is the source ?
├── Continuous  →  produces Zone 0 in its vicinity
├── Primary     →  produces Zone 1 in its vicinity
└── Secondary   →  produces Zone 2 in its vicinity

What's the ventilation ?
├── High  +  Good availability  →  zone EXTENT very limited (or "negligible extent")
├── Medium + Good availability  →  zone EXTENT moderate (default case)
└── Low   or  Poor availability →  zone EXTENT large, may upgrade to higher zone

For example : a bolted flange (secondary source) outdoors with good wind (High Ventilation, Good Availability) produces a Zone 2 of “negligible extent” — effectively no zone classification needed. Same flange in an enclosed indoor room with Poor ventilation might produce a large Zone 1.

This is why placing equipment outdoors with natural ventilation is cheaper than placing it indoors — zone classifications collapse and Ex requirements diminish.

Practical methods (Annexes)

The standard provides multiple quantitative methods :

Annex B : direct lookup tables for common cases (recommended for typical equipment). Quick.

Annex C : example calculations with formulas. Step-by-step. More flexible but slower.

Annex D : computational fluid dynamics (CFD) for complex geometries. Most rigorous but expensive.

Annex E : examples (informative). Shows worked classifications for typical plant configurations.

Most plants use Annex B + C as baseline, falling back to CFD only for difficult cases (e.g., complex enclosed spaces with multiple sources).

Common pitfalls

1. Forgetting indoor / outdoor distinction. Outdoor with natural ventilation is dramatically different from indoor. A leak in an enclosed building can accumulate to LEL much faster than the same leak outdoors.

2. Underestimating release rate. A pump seal can release tens of g/s of vapor when failed, dwarfing the typical permeation calculation. Use realistic release rates, not theoretical minima.

3. Ignoring buoyancy. Hydrogen rises fast (low density), accumulates near ceilings — Zone 1 may need to extend upward 1-2 m even with good ventilation. Acetylene similar. Propane is heavier than air, accumulates at floor level — Zone may extend downward but not upward.

4. Static classification on dynamic plants. A plant under construction (welding hot work) has different zone requirements than the same plant in normal operation. Permit-to-work systems must address this.

5. No periodic review. Area classification must be updated when : new equipment installed, process changes, new substances introduced, ventilation system modified.

Difference vs IEC 60079-10-2 (dust)

The companion IEC 60079-10-2 treats dust atmospheres similarly but with adapted concepts :

• Zone 20 (continuous dust cloud, like inside a flour silo)
• Zone 21 (likely dust cloud in normal operation)
• Zone 22 (unlikely dust cloud, only on upset)

Dust classification adds the dust layer concept (dust accumulating on hot surfaces can ignite even without a cloud) — substantially different physics than gas. Don’t apply gas methodology naively to dust.

API RP 505 — the US alternative

In the US, API Recommended Practice 505 is sometimes used in oil & gas instead of IEC 60079-10-1. It uses Class I Division 1 / Division 2 framework (NEC heritage). Conceptually similar (Division 1 ≈ Zones 0+1 combined, Division 2 ≈ Zone 2) but with different details. Modern US oil & gas facilities increasingly adopt the Zone system from IEC 60079-10-1 for international portability, but legacy facilities and some niche sectors still use Division.

Where this fits in the broader Ex workflow

Area Classification feeds everywhere in the Ex lifecycle :

IEC 60079-10-1 / -10-2  →  Zone map  →  Equipment selection per IEC 60079-0
                                       Installation design per IEC 60079-14
                                       Inspection regime per IEC 60079-17
                                       Workplace Explosion Protection Document
                                       (ATEX 99/92/EC requirement)

Without a current, approved Area Classification Document, none of the downstream activities can be justified. It is the start point of every Ex installation project.