The four-step framework
Selecting an ATEX enclosure is a systematic process, not a product choice. The selection sequence matters: each step constrains the options at the next. Skipping steps or going in the wrong order leads to common errors, over-specification that costs money, or under-specification that creates a compliance and safety risk.
The four steps are:
- Zone, what zone is the enclosure located in? This determines the minimum equipment category and EPL required.
- Protection concept, what electrical equipment is inside the enclosure? This determines which protection method applies.
- Material and IP rating, what is the environment? This determines material selection and ingress protection class.
- Cable entries, how many cables enter, and what type and size? Entries must be specified at the same time as the enclosure.
Step 1: Establish the zone
Zone classification is the starting point for all ATEX specification. You must have area classification drawings before selecting any equipment. If drawings are not available, commissioning them is the first task.
From the zone, determine the minimum equipment category:
| Zone | Minimum category | Minimum EPL |
|---|---|---|
| Zone 0 (gas) | Category 1G | Ga |
| Zone 1 (gas) | Category 2G | Gb |
| Zone 2 (gas) | Category 3G | Gc |
| Zone 20 (dust) | Category 1D | Da |
| Zone 21 (dust) | Category 2D | Db |
| Zone 22 (dust) | Category 3D | Dc |
You may always use a higher category than the zone requires, Category 2 equipment in a Zone 2 location is compliant and may be preferred for future flexibility if the zone classification is reviewed upwards.
The gas group and temperature class on the enclosure certificate must also match the hazardous substance. A T4 (max surface 135 °C) enclosure is not suitable for a zone with a substance whose ignition temperature is below 135 °C, for example ethyl nitrite (ignition temperature 90 °C) requires T6 (max surface 85 °C).
Step 2: Choose the protection concept
The protection concept depends on what is inside the enclosure. This is the decision most engineers get wrong, particularly the temptation to use Ex e for equipment that may spark.
| What's inside the enclosure? | Protection concept | Notes |
|---|---|---|
| Terminals only (no switching) | Ex eb (Zone 1) or Ex ec (Zone 2) | Commonest application for terminal boxes |
| Switching contacts (contactors, relays) | Ex db or Ex db/eb | Ex e cannot be used for sparking components |
| Instrument loop (4–20 mA sensor) | Ex ia or Ex ib system | The enclosure may be Ex eb; the instrument is IS |
| Motor connections (windings) | Ex db/eb | Terminal compartment Ex eb; winding compartment Ex db |
| Large panel with non-Ex internal equipment | Ex s (overpressure system) | Requires purge controller and clean air supply |
| Dust zone terminal box | Ex tb (Zone 21) or Ex tc (Zone 22) | IP6X required for Zones 20 and 21 |
Step 3: Select IP rating and material
IP rating and material selection are driven by the installation environment, not the zone classification alone.
IP rating guidance:
- Ex eb minimum is IP54; in practice, IP65 or IP66 for any outdoor or industrial location
- IP65, low pressure jets; outdoor in sheltered locations, road equipment
- IP66, powerful jets; standard for outdoor industrial and marine deck
- IP67, immersion; below-deck marine, offshore splash zones, underground
- IP68, continuous submersion; pump chambers, underwater installations
- IP69K, high-pressure hot water; food processing, vehicle wash
Material selection guidance:
| Environment | Recommended material | Reason |
|---|---|---|
| Chemical plant (aggressive atmosphere) | GRP or 316L stainless | GRP resists most process chemicals; 316L resists chlorides |
| Offshore topsides (salt spray) | GRP or 316L stainless | Both resist chloride corrosion; GRP is lighter |
| Food processing (washdown) | 316L stainless | Hygienic; withstands cleaning chemicals |
| General outdoor industrial | GRP or 304 stainless | Good balance of cost and performance |
| Zone 0 (antistatic required) | Antistatic GRP or die cast | Standard GRP can build electrostatic charge; must be antistatic grade for IIC |
| High temperature environment | 316L stainless | GRP loses strength above 80–90 °C continuous |
Step 4: Specify cable entries
Cable entries are often specified as an afterthought, but they are a critical part of the ATEX system. Under-specified or incorrectly fitted glands are the most common cause of non-compliance in ATEX installations.
For each cable entry, specify:
- Cable type, SWA (steel wire armour), unarmoured, or braided; the gland type depends on this
- Cable outer diameter range, glands have a specified grip range; the cable OD must fall within it
- Protection concept, the gland must be certified to the same or higher protection concept as the enclosure
- IP rating, the gland must maintain the enclosure IP rating
- Cable gland material, brass is standard; nickel-plated brass or 316 stainless for marine/corrosive; polyamide for chemical resistance or weight
Unused entries must be fitted with certified blanking plugs. Count the entries carefully before ordering, modifying an Ex d enclosure after manufacture to add entries requires re-certification; Ex eb enclosures have more flexibility but still require certified glands.
Common specification mistakes
- Specifying Ex e for switching equipment. A contactor cannot be installed in an Ex e enclosure. Use Ex d or Ex d/e combined.
- Ignoring gas group. IIA certification is not sufficient for IIC gas. Always check the gas group of the substance and specify at least that group.
- Under-specifying IP rating. IP54 is the legal minimum for Ex eb; specifying IP54 in an outdoor coastal location is a maintenance problem waiting to happen.
- Forgetting blanking plugs. Unused entries without certified blanks invalidate the IP rating and the ATEX certification.
- Specifying material for appearance, not environment. Stainless steel in a high-chloride marine environment without surface treatment corrodes; GRP is often the better choice.
- Not specifying entries at time of order. Adding entries to an Ex db enclosure after manufacture requires re-certification. Specify all entries, including spares, before the enclosure is made.