Sheet Metal Hazardous Location Enclosure Guide 2026钣金危险场所外壳设计指南（2026）

A Houston-based skid OEM brought us a six-figure rush job last year. They had commissioned a new panel line in southern China, sent drawings marked "ATEX/IECEx Zone 1," and were six weeks from shipment when the certification body's first article report came back with a single redline: flame-path gap on the door joint was 0.22 mm — 50% over the IEC 60079-1 limit. The first 80 enclosures were scrap. The fabrication had been done by a capable sheet metal shop. The problem was that the shop had never been asked to hold a 0.1 mm-class flame-path tolerance on a production run, and nobody had told them they needed to. They had quoted the job against the IP and tolerance spec on the drawing — neither of which mentioned Ex d flame path.

This guide is written for the person who has to make sure that story doesn't happen on their program. It walks through what hazardous location enclosure certification actually requires from the sheet metal fabricator — not from the certification body, not from the system integrator — and how the design decisions on the drawing drive the manufacturing decisions on the shop floor. There is a 6 × 8 protection-concept matrix in Section 3, a zone / division comparison in Section 2, and a 12-point DFM checklist in Section 5 that you can lift into an internal review.

Why Hazardous Location Standards Exist (And Why They Don't Agree)

An enclosure in a refinery, a grain silo, and a battery-storage hall faces the same fundamental risk: a flammable atmosphere meeting an electrical spark. Three frameworks solve that risk, and they do not agree with each other.

The North American system, codified in NFPA 70 (NEC) Articles 500–506, dates to the early 20th century and leans on heavy mechanical containment — thick-walled cast or fabricated housings with machined flame paths. The European and IEC system takes a probabilistic approach: classify zones by how often the explosive atmosphere is present, then match a protection concept to the zone. ATEX is the EU legal directive (2014/34/EU); IECEx is the international certification scheme that lets one test report travel across participating countries. A single skid shipped from Houston to Rotterdam can need both.

The two systems are not interchangeable. A Zone 2 enclosure is not automatically a Class I Div 2 enclosure; a Zone 1 enclosure is not automatically a Class I Div 1 enclosure. The mapping is approximate (Division 1 ≈ Zone 0 + 1, Division 2 ≈ Zone 2), and the test methods differ. If your program needs both — and most OEM projects shipping globally do — the right move is to design the enclosure so that it can be submitted to two parallel test campaigns. We cover how to do that in Section 7.

Zone vs Division: The Table Every Spec Should Start With

Before you pick a protection method, you need to know the zone or division of the installation point. That classification comes from the end user's hazardous area classification drawing, not from the enclosure designer. But the designer has to read it correctly, because every design decision that follows depends on it.

One quick rule that saves arguments in procurement conversations: Division 1 ≈ Zone 0 + Zone 1; Division 2 ≈ Zone 2. The mapping is approximate — the IEC system is more granular, and the gas group letter (IIA / IIB / IIC) and temperature class (T1–T6) cross over too. For a deeper look at how NEMA and IP ratings layer onto this picture, see our NEMA vs IP enclosure ratings guide.

The 6 Protection Concepts You'll Actually Specify

"Explosion-proof" is not a single thing. It is a family of techniques, each marked with a two-letter code, and each placing different demands on the sheet metal fabricator. Pick the wrong one for the zone and the certification body will reject the submittal; pick the right one and fail the tolerance on the flame path, and the certification body will reject the first article. The matrix below covers the six you will see on a real OEM RFQ.

Ex d — Flameproof (the heavy-wall workhorse)

Ex d is the concept that the Houston skid should have specified (and did specify, on paper). The enclosure is designed to contain an internal explosion. If a spark ignites the gas inside the box, the box holds the pressure, and the flame that escapes through the joint is cooled below the auto-ignition temperature of the surrounding atmosphere. The cooling happens across the "flame path" — a precisely machined gap between two mating surfaces. IEC 60079-1 specifies the maximum gap and minimum engaged length as a function of internal volume and gas group; for a typical 0.5–2 L enclosure in Group IIB, that means a gap of around 0.15 mm and an engaged length of around 12.5 mm.

Those numbers are production tolerances, not inspection tolerances. A fabricator that holds ± 0.1 mm on a brake-formed panel is not holding a 0.15 mm flame path; they are holding a 0.15 ± 0.05 mm flame path, and the upper bound blows the certification. Ex d fabrication requires machined joint surfaces, dial-indicator verification at the bench, and a documented FAI program that measures every joint on every first article.

Ex e — Increased Safety

Ex e takes the opposite approach. Instead of containing an internal explosion, it prevents ignition sources from forming in the first place. No switching contacts that can spark, no hot surfaces above the temperature class, creepage distances and clearances doubled over the IEC standard industrial spec, IP54 or better. The sheet metal work is less demanding than Ex d, but the wiring and component layout work inside the box is more demanding. Ex e and Ex d are sometimes combined as "Ex de" on the nameplate — the enclosure is flameproof, but the terminals inside are increased-safety. That is the most common marking we see on chemical and refinery skid enclosures.

Ex i — Intrinsic Safety

Ex i is the odd one out. The protection lives in the circuit, not the enclosure. Power and energy inside the enclosure are limited to values below the ignition threshold of the surrounding gas — typically < 1.5 V and < 100 mA for the most restrictive ia classification. The enclosure itself can be a standard industrial box; what matters is the barrier device, the cable parameters, and the grounding. From a sheet metal perspective, Ex i is the easiest concept to fabricate but the hardest to integrate, because every active component in the box has to be reviewed against the entity parameters. See our EMI shielding guide for the EMC side of the same integration.

Ex p — Pressurized

Ex p uses clean instrument air or inert gas to maintain a positive pressure inside the enclosure, keeping the hazardous atmosphere out. A purge controller runs a pre-energization purge (typically 5× the internal volume for non-hydrogen gases, 10× for hydrogen) and then monitors the pressure during operation. The sheet metal work is standard industrial — the complexity is in the purge skid, the interlock to the disconnect, and the alarm path. The IEC 60079-2 standard defines three sub-classes: px (Zone 1), py (Zone 1 with some relaxation), pz (Zone 2 only). Pressurized enclosures are common in analyzer houses and control rooms where the equipment inside is too large or too complex to certify as Ex d or Ex e.

Ex t — Dust by Enclosure

Ex t is the dust-zone counterpart to the gas-zone concepts above. The protection method is straightforward: keep dust out (IP6X), limit surface temperature below the dust layer ignition temperature, and avoid flat horizontal surfaces where dust can accumulate. From a sheet metal perspective, this means tighter gasketing, smoother exterior geometry (no upward-facing flanges), and a finish that does not trap dust. For a process-level look at gasketing and IP performance on outdoor enclosures, see our outdoor IP enclosure guide.

Ex n — Non-incendive

Ex n is a Zone 2-only family — the least restrictive of the six. Non-sparking (nA), restricted breathing (nR), and enclosed-break (nC) are the common sub-types. Ex n lets you use standard industrial enclosures and standard industrial components, provided the energy levels stay below the ignition threshold under normal operation. It is the cheapest of the six and the one we see most often on Zone 2 chemical skid enclosures. The trade-off is that any active component (relay contact, switch, motor starter) has to be reviewed for non-incendive performance, which pushes complexity into the integration rather than the fabrication.

Reading the Nameplate Marking

A typical ATEX/IECEx nameplate looks like a string of hieroglyphics until you read it left to right. The format is fixed by IEC 60079-0: the Ex symbol, the protection concept letter, the gas or dust group, the temperature class, and the equipment protection level (EPL).

Example: Ex db eb IIB T4 Gb. That reads: "Flameproof enclosure (db) with increased-safety terminals (eb), for gas group IIB, temperature class T4 (surface ≤ 135 °C), equipment protection level Gb (suitable for Zone 1)."

If your panel nameplate shows "Ex d IIC T4 Gb," the enclosure must hold a tighter flame-path gap than the same concept in Group IIB (because IIC is the most restrictive gas group — hydrogen, acetylene). It must also limit surface temperature to ≤ 135 °C, which drives the thermal management work inside the box. We have written a separate guide on thermal management for the second half of that problem.

Sheet Metal DFM — 12 Drawings That Don't Get Built

This is the section we wish our Houston customer had read before they released the drawing. Over a 12-year log of OEM enclosure programs, the failures that reach us at the fabrication stage fall into a recognizable set. We have grouped them into 12 DFM rules that — if followed at the drawing stage — would have caught roughly 80% of the rejected submittals we have seen.

1. Flame-path joints on Ex d must be machined, not just brake-formed. A bent flange is straight to ± 0.2 mm at best. The certification body wants ± 0.05 mm on the gap. Specify a milled or wire-EDM joint surface.
2. Weld spatter within 6 mm of a flame path or gasket surface invalidates Ex d and most Ex e ratings. Move welds, or specify post-weld cleanup with a documented procedure.
3. Powder coating is an insulator. Mask the flame-path joint, all bonding stud surfaces, all gasket grooves, and the inside of any threaded conduit entry. Most Ex failures on coated enclosures happen here. Conduit or individual gland is a separate decision from the coating mask — our conduit or cable gland decision guide covers where each is required by code.
4. Wall thickness minimums are not uniform across protection concepts. Ex d in 316L typically wants ≥ 2.0 mm for small enclosures; Ex e can drop to 1.5 mm; Ex t depends on impact rating. State the minimum on the drawing; do not leave it to the fabricator.
5. Cable entries must use Ex-rated glands — and unused entries must be closed with Ex-certified blanking plugs. A standard cable gland on an Ex d enclosure is a certification void. Body material, seal material and thread selection for those glands is walked through in our cable gland material, seal and thread guide.
6. Breather drains are not optional on Ex e enclosures in humid environments. Specify the certified breather by part number on the drawing, not "install breather as required."
7. Hinges, latches, and access hardware must meet the same protection concept as the enclosure. An Ex d box with a non-Ex quarter-turn latch fails. Specify Ex d latches.
8. Internal mounting brackets must not bridge flame-path surfaces. A bracket that crosses the door-joint flame path blocks the joint from closing and breaks the certification.
9. Material certificates (EN 10204 3.1) must accompany every lot of 316L or aluminum used on an Ex program. The certification body will request them at audit. Build the certificate request into the PO.
10. Bend radii on flame-path flanges must be specified — they cannot be left to the brake operator. A sharp bend on a flame-path flange moves the metal out of the certified geometry.
11. Door flatness after welding must be ≤ 0.2 mm across the seal width. A welded door that is bowed 0.5 mm will not seal at the flame path. Specify weld sequence, or specify a post-weld flattening op.
12. Every Ex program needs a documented FAI on the assembly, not just on the part. The part-level FAI confirms dimensions; the assembly FAI confirms joint gaps, torque values, and bond resistances on the production hardware.

Rules 1, 2, 3, 7, and 12 account for most of the rejections we have seen on real programs. For the broader DFM framework that these rules sit inside — bend allowances, tolerance stacks, tool access — see our sheet metal DFM design guide, and for the welding side of rules 2 and 11, our welding guide.

Material Selection: 316L vs Aluminum vs CRS

Material choice for a hazardous area enclosure is driven by three constraints: the corrosion environment, the certification body's expectations, and the weight budget.

For offshore or chemical sites, 316L is the conservative default and is what most certification bodies expect to see on the drawing. For weight-sensitive cabinets (battery enclosures on vehicles, portable analyzer skids), aluminum is the right answer despite the thickness penalty, because 1 kg of weight saved on the box pays for several kilograms of structural reinforcement. For indoor dry-zone applications, powder-coated mild steel is the cheapest path and remains widely used in Chinese OEM enclosures.

The thickness-to-volume relationship matters more than absolute material choice. A 0.5 L enclosure in 316L can sit at 2.0 mm wall; a 10 L enclosure in the same material will want 2.5–3.0 mm to handle internal pressure on the flame path during an internal ignition event. We have a deeper comparison of carbon vs stainless decisions in our stainless vs carbon steel guide.

Dual Certification Pathway (ATEX + UL)

The fastest path to dual certification is to design for it from the start, even if the first build is single-region. Here is the workflow that has worked on the last dozen programs we have run.

1. Pick the highest-common-denominator protection concept. Ex d IIB T4 Gb is the conservative choice that maps to UL Class I Div 1 Groups C, D and Class II Div 1 Groups E, F, G. Designing to the most restrictive zone you might ship to means the lower-zone builds are easy.
2. Pick the highest-common-denominator material. 316L is acceptable to every major certification body. Powder-coated mild steel is acceptable to ATEX but raises questions with some UL auditors on long-term corrosion.
3. Run FAI on the part to the tighter of the two tolerances. ATEX flame-path tolerance (typically ± 0.05 mm) is tighter than UL's typical ± 0.1 mm. Hold the ATEX number for both audits.
4. Generate one documentation pack. Drawings, BOM, weld maps, FAI reports, material certificates, torque charts — all of it can be shared across both submissions.
5. Submit to both bodies in parallel. ATEX/IECEx through TÜV, DEKRA, or SGS; UL through UL or CSA. Expect 8–14 weeks for ATEX/IECEx and 10–16 weeks for UL on a populated enclosure.
6. Plan for the audit visit, not just the file review. Both bodies will send an auditor to the fabrication site for the first program. The auditor walks the floor, checks the calibration of the flame-path measurement tool, reviews the weld procedure qualification records. A fabricator that has never been audited should expect two days of preparation work per body.

The savings come from not maintaining two drawings. The cost comes from two parallel test campaigns and two audit visits. On a populated enclosure in the 200–500 piece annual volume range, dual certification typically adds 18–25% to the unit cost over a single-region build, against 60–80% if you try to retrofit certification onto a single-region design after the fact.

Cost & Lead Time Benchmarks

These numbers are engineering estimates from SanJun's 12-year quote log, accurate to within roughly ± 30%. They cover a populated control enclosure, dual-certified, in the 200–600 mm size range with stainless hardware and a documented FAI program.

The wide bands reflect the three variables that drive cost most: enclosure size, the number of cable entries and penetrations, and the population density of components inside. A 200 × 200 × 150 mm Ex e junction box with four cable glands is at the bottom of the Zone 2 band. A 600 × 800 × 300 mm Ex d analyzer house with 30 entries and a VFD inside is at the top of the Zone 1 band, and may push past it. For a deeper breakdown of what drives the numbers, our cost guide walks through the seven line items.

The SanJun Hazardous Area Workflow

For customers who ship globally, we run a five-step workflow that takes a one-off Ex enclosure from RFQ to volume production:

1. Zone mapping. Send us your hazardous area classification drawing. We confirm the protection concept and the certification body.
2. Concept review. We review your concept drawing against the 12 DFM rules in Section 5 and flag any item that will fail certification. Most programs need 2–4 iterations before the drawing is releasable.
3. Prototype build. First article in the certification-grade material, with full FAI documentation. The prototype goes through the certification body's test program.
4. Audit-ready production. We set up the fabrication cell with the certified joint inspection procedure, the documented weld sequence, the masking SOP, and the FAI checklist. The certification body's auditor visits the cell before the production run starts.
5. Volume build with lot traceability. Every enclosure carries a serial number, a material certificate, an FAI stamp, and a shipped-to-customer record. For more on how the prototype-to-volume handoff works on non-Ex programs, see our prototype-to-production guide.

The output of the workflow is a one-page Ex program pack — protection concept, certification body, material callout, joint inspection procedure, masking drawing, FAI checklist — that travels with every shipment and gets the customer's certification auditor off your back on the receiving end.

Frequently Asked Questions

Does an IP65 enclosure also meet ATEX or IECEx?

No. IP65 is an ingress protection rating against dust and water. ATEX/IECEx is a separate certification that addresses ignition risk from explosive gas or dust atmospheres. An enclosure can carry IP65 without any Ex marking, and an Ex enclosure is specified to a zone (e.g. Zone 1) rather than an IP number. The two ratings are independent and often combined on the same nameplate, but neither implies the other.

What is the typical flame-path gap and length on an Ex d enclosure?

For a typical Group IIB enclosure in the 0.5–2 L volume range, IEC 60079-1 specifies a maximum flame-path gap of around 0.15 mm and a minimum engaged length of around 12.5 mm. The exact values depend on internal volume, gas group (IIA / IIB / IIC) and the certification body's interpretation. The fabricator must hold these tolerances on every joint surface, not just on first article.

Can the same drawing be certified to both ATEX and UL?

In most cases, yes. The drawings, weld maps, bill of materials and FAI can be shared. The certification bodies run two parallel test programs, and the manufacturer pays two sets of fees. The savings come from not having to maintain two separate drawings; the cost comes from running two test campaigns. Expect 8–14 weeks for ATEX/IECEx and 10–16 weeks for UL on a populated enclosure, run in parallel.

What thickness does 316L stainless need to be for a Zone 1 enclosure?

Industry practice and several certification bodies treat 2.0 mm as a soft minimum for a small Zone 1 enclosure (under ~10 L internal volume) in 316L. For larger volumes, or for impact-prone locations, 2.5–3.0 mm is the more common choice. These thicknesses are engineering guidelines drawn from certification experience, not IEC mandates. Always confirm with your notified body before committing the drawing.

Does powder coating break Ex e or Ex d certification?

Yes, if applied over the flame path or bonding surfaces. Powder coat is an insulator. On an Ex d enclosure, the flame-path joint surfaces must be masked or post-machined after coating. On an Ex e enclosure, all bonding points, earthing studs and the inside of the gasket groove must be masked. Most certification bodies accept a documented mask-and-post-machine step in the SOP, but they will fail the audit if they find coating inside the certified joint.

Conclusion

A hazardous location enclosure is a sheet metal box that has to pass a certification test. The certification test is unforgiving — it checks flame-path gaps down to ± 0.05 mm, weld spatter on gasket surfaces, and insulation on bonding zones. The drawing has to anticipate every one of those failure modes, because the fabricator cannot fix them after the fact without re-certification.

If you are at the RFQ stage, send us your hazardous area classification drawing, your preferred certification body, and your annual volume. We will return a one-page Ex program pack — protection concept, material callout, joint inspection procedure, masking drawing, FAI checklist, and a not-to-exceed unit price band — within 24 hours.