Ex Equipment Selection Guide

The Selection Process

Selecting Ex equipment requires answering six questions in order:

1. What zone is the equipment in? → Determines minimum category (1, 2, or 3)
2. What gas or dust is present? → Determines gas group (IIA/IIB/IIC) or dust group (IIIA/IIIB/IIIC)
3. What is the auto-ignition temperature? → Determines temperature class (T1–T6) or maximum surface temperature
4. What does the equipment do? → Determines suitable protection methods (Ex d, Ex i, etc.)
5. What environment does it operate in? → Determines IP rating, materials, ambient temperature range (see installation requirements)
6. What are the electrical parameters? → Voltage, current, power, frequency

Each answer narrows the field until you have a shortlist of compliant options. Verify certification before purchasing. Then choose based on cost, availability, and suitability. Check the equipment marking to confirm compliance.

Step 1: Zone → Category

Match the zone to the required equipment category or EPL:

Gas/Vapor Zones

Dust Zones

Key rule: Higher-category equipment can always be used in lower-risk zones. Category 1 works everywhere; Category 3 only in Zone 2/22.

Step 2: Gas/Dust → Group

For Gas/Vapor

Identify the gas and find its group:

When in doubt, use IIC. It works for all gases, adds cost (~10–30% more than IIA), but eliminates risk if gas composition changes.

For Dust

Equipment marked III (no letter) covers all dust types (IIIA, IIIB, IIIC). Equipment marked IIIC covers IIIC only.

Step 3: Temperature → T-Class or Maximum Surface Temp

For Gas/Vapor

Find the gas's auto-ignition temperature (AIT) and select equipment with a T-class below that value:

Safety margin: Equipment T-class must be at least one class below the gas AIT. For example, if AIT = 200°C, use T3 (200°C) or colder, not T2 (300°C).

For Dust

Dust uses direct temperature marking (not T-classes). The equipment maximum surface temperature must be:

• ⅔ of the dust cloud MIT (minimum ignition temperature of dust cloud)
• 75°C below the layer ignition temperature (LIT) for a 5 mm layer

Use whichever is more restrictive.

Example: Wheat flour has MIT = 440°C, LIT (5 mm) = 220°C.

• Cloud limit: ⅔ × 440 = 293°C
• Layer limit: 220 - 75 = 145°C
• Use T145°C or lower (layer limit is controlling)

Step 4: Application → Protection Method

Choose the protection method based on what the equipment does and the power level:

Cost ranking (low to high): Ex nA (Zone 2 only) < Ex e < Ex ia/ib < Ex d < Ex m < Ex p

Weight ranking (light to heavy): Ex ia < Ex e < Ex m < Ex d < Ex p

Decision Flowchart

Choosing Between Ex d and Ex i

Is the power < 20 W?
├─ Yes → Consider Ex i (intrinsic safety)
│  └─ Is it a sensor, transmitter, or low-power control circuit?
│     ├─ Yes → Ex i is ideal
│     └─ No → Check if barriers/isolators fit the system architecture
│
└─ No → Use Ex d or Ex e
   └─ Does it have arcing/sparking parts (relays, switches, motors)?
      ├─ Yes → Ex d (flameproof)
      └─ No → Ex e (increased safety) is cheaper and lighter

Zone 0 Equipment

Zone 0 requires EPL Ga. Options:
├─ Ex ia (intrinsic safety, 2-fault tolerance)
├─ Ex ma (encapsulation, 2-fault protection)
└─ Special Ex d or Ex px designs (rare, very expensive)

→ 95% of Zone 0 installations use Ex ia.

Zone 2 Equipment

Zone 2 has relaxed requirements. Options:
├─ Ex nA (non-sparking, cheapest)
├─ Ex nC (enclosed, sparking allowed but enclosed)
├─ Ex nR (restricted breathing)
├─ Any higher-rated equipment (Ex d, Ex e, Ex i)

→ Use Ex nA for motors/lights when possible (cost savings).
→ Use Ex e for terminal boxes (good practice even though Ex nA would suffice).

Equipment by Industry

Oil & Gas (Offshore Platform)

• Main motors (pumps, compressors): Ex d IIC T3, Category 2 G
• Lighting: Ex d IIC T4 LED fixtures, IP66
• Junction boxes: Ex e IIC T6, stainless steel (marine environment)
• Transmitters: Ex ia IIC T4, 4–20 mA loops with barriers in safe area
• Control panels: Ex px IIB T4 (pressurized enclosures in Zone 1 modules)

Chemical Processing

• Agitator motors: Ex de IIB T3 (combined flameproof motor, increased safety terminal box)
• Process sensors: Ex ia IIC T4–T6 depending on solvent
• Analyzers: Ex d IIB T4 housings or Ex p purged cabinets
• Field instruments: Ex ib IIC T5 for Zone 1, Ex ia for critical loops

Pharmaceutical (Solvent Processing)

• Tablet coating pans: Ex d IIA T3 motors (alcohol vapor = IIA)
• Extraction equipment: Ex d IIB T4 (ethanol/IPA = IIA, but use IIB for margin)
• Instrumentation: Ex ia IIC T4 (some solvents are IIB)
• Lighting: Ex d IIA T4 LED, IP66 (washdown environment)

Grain Handling (Dust)

• Bucket elevator motors: Ex tb IIIB T135°C Db, IP6X
• Silo level sensors: Ex ia IIIB (some sensors certified for dust)
• Lighting: Ex tb IIIB T135°C Db, IP6X, LED
• Conveyor motors: Ex tb IIIB T120°C Db

Paint Spray Booth

• Exhaust fan motor: Ex d IIA T3 (solvent vapor)
• Lighting: Ex d IIA T4 LED (inside booth = Zone 1)
• Controls: Ex e IIA T6 push-button stations
• Interlocks: Ex ia IIA safety circuits

Step 5: Environment → IP Rating and Materials

IP Rating Selection

Material Selection

• Aluminum (AlSi10Mg, LM6): Light, good for most environments. Avoid in highly corrosive areas.
• Stainless steel (316L): Marine, offshore, chemical exposure. Heavier and more expensive than aluminum.
• Brass: Small components (cable glands, connectors). Good corrosion resistance.
• Plastic/GRP: Ex e, Ex tb enclosures. Lightweight, corrosion-proof. Not suitable for Ex d (can't withstand internal explosion).
• Coatings: Powder coat, epoxy, or marine-grade paint for aluminum. Avoid painting flameproof joints.

Common Selection Mistakes

1. Using Zone 2 Equipment in Zone 1

Category 3 (Gc) is only suitable for Zone 2. Installing it in Zone 1 violates ATEX/IECEx requirements and creates real ignition risk.

2. Gas Group Mismatch

Installing IIA-rated equipment in a hydrogen (IIC) atmosphere. The flameproof gaps are too wide, and ignition energy limits are too high. Explosion will propagate.

3. Temperature Class Error

Using T3 equipment (200°C max) with gasoline vapor (AIT ~230°C). The equipment surface could reach 200°C, which is too close to the ignition threshold. Use T4 (135°C) or colder.

4. IP Rating Underspecification

Specifying IP54 for outdoor coastal installations. Water ingress and salt corrosion will damage the equipment within months. Use IP66/67 minimum for marine environments.

5. Ex i System Design Errors

Exceeding cable capacitance limits. The barrier/isolator allows 100 nF, the field device has 50 nF internal capacitance, and the cable is 50 nF/100m. Installing 150m of cable (75 nF) exceeds the total budget (100 nF). Explosion risk if a fault occurs.

6. Ignoring Ambient Temperature Limits

Standard equipment is rated for -20°C to +40°C. Installing it in a desert (55°C ambient) or Arctic location (-40°C) without verifying extended temperature rating. The T-class is calculated at max ambient — exceeding that changes the surface temperature.

Selection Checklist

1. ☐ Confirm zone classification from area classification drawing
2. ☐ Identify all gases/dusts that may be present (check process data, safety data sheets)
3. ☐ Determine gas group: IIA, IIB, or IIC
4. ☐ Find auto-ignition temperature (AIT) for gas or MIT/LIT for dust
5. ☐ Select temperature class or maximum surface temperature
6. ☐ Match category/EPL to zone (Category 1 for Zone 0, Category 2 for Zone 1, Category 3 for Zone 2)
7. ☐ Choose protection method based on application (Ex d for motors, Ex i for sensors, etc.)
8. ☐ Specify IP rating for environment (IP66 for outdoor/offshore)
9. ☐ Check ambient temperature range (-20 to +40°C standard; extended if needed)
10. ☐ For Ex i: calculate entity parameters (Ui/Ii vs Uo/Io, Ci/Li vs Co/Lo + cable)
11. ☐ Verify electrical parameters (voltage, current, frequency) match supply
12. ☐ Obtain equipment certificate and verify it matches all requirements

Related Topics

• Zone Classification — Understanding zones 0/1/2 and 20/21/22
• Gas Groups — IIA, IIB, IIC explained
• Temperature Classes — T1–T6 and AIT tables
• Protection Methods — All Ex types in detail
• Explosion Proof vs Intrinsically Safe — Choosing between Ex d and Ex i
• Equipment Protection Levels — Ga/Gb/Gc and Da/Db/Dc explained
• Cable Glands — Selecting the right glands for your equipment