Page review information
Edition covered
IEC 60079 series — parts -0, -1, -10-1, -11, -14, -17
Content status
Review pending
Coverage
6 of 12+ parts; remaining parts flagged as gaps on this page. The US approval-boundary statements (ATEX/IECEx vs. NEC Art. 505 and OSHA NRTL listing) were found defective and corrected in August 2026 against public OSHA sources — 29 CFR 1910.307, 1910.399, and the NRTL programme FAQ. That correction has not been author-reviewed, so the page is back to Review pending; the earlier May 2026 review did not catch the defect.
Last content update
August 2026
Technical validation
Not yet completed
Primary reference
The official published standard — required for final decisions

Confirm the governing edition, jurisdiction, and project-specific requirements before applying anything on this page.

International · Hazardous Area

IEC 60079 — Explosive Atmospheres

Quick Start

Standard Overview

Field Value
Standard family IEC 60079 series
Editions in corpus 2011 – 2017 (per part)
Publisher International Electrotechnical Commission (IEC)
Scope Electrical equipment and installations in explosive atmospheres
Repository control-standards/rag/standards_intelligence/international/hazardous_area/iec_60079/ — 6 parts
Status in corpus Core parts complete; non-core parts gap-flagged below

Equipment Marking System

All Ex equipment carries a marking string defined in IEC 60079-0:

Ex [protection method(s)] [gas group] T[temp class] [EPL]

Example: Ex d IIB T4 Gb

Element Meaning
Ex Certified for explosive atmospheres
d Protection method — flameproof enclosure (one of several; see below)
IIB Gas group — ethylene and similar (IIA < IIB < IIC by hazard)
T4 Max surface temperature 135 °C (T1 highest = 450 °C, T6 lowest = 85 °C)
Gb Equipment Protection Level — Zone 1

Multiple protection methods may appear in one marking — for example Ex de IIC T5 Gc for a flameproof + increased-safety combination.

EPL → Zone mapping

EPL (gas) Zone Protection level
Ga Zone 0 Highest — safe with two independent faults
Gb Zone 1 High — safe with one fault
Gc Zone 2 Enhanced — safe in normal operation
Da / Db / Dc Zone 20 / 21 / 22 Equivalent for dust atmospheres

Gas groups

Group Hazard Typical gases
IIA Lowest Propane, methane, acetone
IIB Intermediate Ethylene, hydrogen sulfide
IIC Highest Hydrogen, acetylene

Equipment marked IIC is suitable for any gas group (it satisfies the most demanding case). Equipment marked IIA is only suitable for IIA gases — using it in IIB or IIC atmospheres is non-compliant.

Temperature classes

T-code Max surface temp Notes
T1 450 °C Very high autoignition gases only
T2 300 °C  
T3 200 °C Kerosene, fuel oil
T4 135 °C Most common in process industry
T5 100 °C Carbon disulfide (rare)
T6 85 °C Diethyl ether

Rule. Select equipment whose T-code maximum surface temperature is below the autoignition temperature of the substance at the installation. Best practice keeps a margin of at least 50 °C below autoignition.

Corpus Coverage

The reference library carries six core parts of the IEC 60079 series — the parts most-consulted by control and process-safety engineers. Other parts of the family are referenced in equipment markings but not held in depth.

Part Title Corpus
60079-0 General requirements (marking, EPL, gas groups, T-codes) ✓
60079-1 Flameproof enclosures (Ex d) ✓
60079-2 Pressurization (Ex p) Planned
60079-7 Increased safety (Ex e) Planned
60079-10-1 Area classification — gas ✓
60079-10-2 Area classification — dust Planned
60079-11 Intrinsic safety (Ex i) ✓
60079-14 Installation design and erection ✓
60079-15 Non-sparking (Ex nA) Planned
60079-17 Inspection and maintenance ✓
60079-18 Encapsulation (Ex m) Planned
60079-31 Dust ignition protection by enclosure (Ex t) Planned

For Ex e and Ex p installations, refer to the IEC 60079-7 and 60079-2 standards directly — this site does not yet hold detailed application guidance for those protection methods.

Per-Part Depth

60079-0 — General Requirements

The general requirements part defines the marking system, Equipment Protection Levels (EPLs), gas groups, temperature classes, and the certification schemes that all other parts inherit. ATEX (EU) and IECEx (international) certification cover most equipment; the schemes are widely cross-recognised, and equipment carrying both marks is common.

That cross-recognition does not extend to the United States on its own. NEC Article 505 gives US practice the IEC-style Zone classification system; it does not make an IEC-scheme certificate a US equipment approval. See The certificate is not the approval below.

The margin rule is the most-applied piece of this part: pick a T-code whose max surface temperature is below the substance autoignition temperature, with at least a 50 °C margin. For ethylene (autoignition ~425 °C), T2 (300 °C) is the warmest acceptable equipment; cooler classes (T3, T4, T5, T6) are progressively more conservative.

60079-10-1 — Area Classification (Gas)

Area classification establishes which Zones exist, where they extend, and what gas group applies. Without it, no other selection decision can be made correctly.

Zone Condition Typical examples
Zone 0 Explosive atmosphere present continuously or for long periods Inside storage tanks; vapour space above process liquid
Zone 1 Explosive atmosphere likely in normal operation Within ~1.5 m of a Zone 0 opening; pump-seal areas; compressor rooms
Zone 2 Explosive atmosphere unlikely in normal operation; if present, only briefly Within ~3 m of a Zone 1 boundary; flanged-joint vicinity; analyser buildings

Release grades — from each release source, an atmosphere may be:

Multiple release sources can overlap, expanding zones or escalating their severity. Ventilation grade modifies the result: poor ventilation can convert a Zone 2 source into Zone 1, or extend Zone boundaries.

The classified-area drawing is the deliverable: Zone extents (plan and elevation), gas group(s), substance autoignition temperature, basis for classification, and revision history. It is required for the initial commissioning inspection (60079-14) and for periodic inspections (60079-17) — losing or failing to maintain it is a common compliance failure.

60079-1 — Flameproof Enclosures (Ex d)

Ex d is the workhorse for higher-power equipment with internal sparking — motors, large solenoid valves, junction boxes with switching contacts. The principle is fundamentally different from intrinsic safety: Ex d allows internal ignition to occur, then contains it. The enclosure withstands the explosion and quenches escaping gases through precision-machined flame paths (gap × length) so the gas cools below its autoignition temperature before reaching the outside atmosphere.

The flame path is the joint surface between mating parts of the enclosure. Three rules govern it:

  1. Gap width — must not exceed the maximum allowed for the gas group. IIC (hydrogen) demands the tightest tolerances; IIA the most generous.
  2. Flame path length — the minimum length to quench the flame before it reaches the atmosphere.
  3. Surface finish — flame paths must be machined metal-to-metal. Never apply paint, thread sealant, anti-seize, or any other coating to flame path surfaces — this changes the effective gap and invalidates the certification.

All openings in an Ex d enclosure must be sealed using either an Ex d certified cable gland (with integral flame path), an Ex d certified stopping plug for unused entries, or — in NEC conduit installations — a threaded conduit with a sealing fitting filled with sealing compound. Standard industrial cable glands are not Ex d rated.

Live work caution. Ex d covers must not be opened in a hazardous atmosphere while energised. De-energise first, and allow cooling before opening if the equipment generates internal heat.

60079-11 — Intrinsic Safety (Ex i)

Intrinsic safety prevents ignition by limiting the electrical energy in the hazardous-area circuit below the threshold required to ignite the gas. Unlike Ex d, IS allows live calibration and maintenance — the energy is too low to cause an ignition in the first place. This makes IS the default choice for field instrumentation.

Level Zone Fault tolerance Typical use
ia Zone 0, 1, 2 Two-fault — safe under any two component failures Most process instruments
ib Zone 1, 2 One-fault Less critical instruments
ic Zone 2 only No fault tolerance — safe in normal operation Zone 2 only

The associated apparatus in the safe area enforces the energy limits. Two types:

Entity concept. IS lets you mix certified components from different manufacturers as long as the entity parameters are compatible. The barrier exposes:

The field device exposes the matching internal parameters: Ui, Ii, Pi, Ci, Li.

Compatibility checks:

Uo ≤ Ui      Io ≤ Ii      Po ≤ Pi
Ci + Ccable ≤ Co        Li + Lcable ≤ Lo

Cable capacitance (typically a few hundred pF/m) and inductance (typically under 1 µH/m) must be added to the field device’s internal values before the comparison.

FISCO (Fieldbus Intrinsically Safe Concept) extends this to FOUNDATION Fieldbus and PROFIBUS PA installations — multiple field devices on one IS segment fed by a single FISCO-certified power conditioner, eliminating per-device entity calculation on that segment.

60079-14 — Installation Design

A correctly certified piece of equipment installed incorrectly loses its protection. Part -14 governs how Ex equipment is wired, connected, and commissioned.

Equipment selection check (perform on every item):

  1. EPL matches the Zone (Ga ≥ Zone 0; Gb ≥ Zone 1; Gc ≥ Zone 2).
  2. Gas group rating matches or exceeds the substance group (IIC equipment satisfies any gas group).
  3. T-code max surface temperature is below the substance autoignition temperature with the 50 °C margin.
  4. Certificate is valid (not withdrawn) — verify against the IECEx database or ATEX notified-body record.

Cable selection. Armoured cable (SWA/AWA) is typical for general Ex wiring; mineral-insulated cable is used for high-temperature or fire-survival circuits; screened instrumentation cable is used for IS, with the screen earthed at one point only (typically the safe-area side) to avoid earth loops.

Cable segregation. IS wiring must be physically separated from non-IS wiring — different conduits, different trays, or a metallic barrier. Mixing the two in a multicore cable or a shared tray can inject energy into the IS circuit that exceeds entity-parameter limits, defeating the protection.

Equipotential bonding. All metallic structures in the hazardous area connect to a single bonding network — vessels, pipework, steelwork, cable trays, equipment enclosures. Bonding resistance between any two bonded points should be <10 Ω (many sites require <1 Ω for critical systems). The IS earth (zener-barrier installations) is a dedicated ≤1 Ω bond to a known clean earth, separate from the general protective earth.

Initial verification before energising. Equipment certificates verified, glands and stopping plugs in place, flame paths undamaged (Ex d) or entity parameters within limits (Ex i), bonding continuity verified, classified-area drawing matches installed equipment. A commissioning certificate is issued at completion — required by 60079-14.

60079-17 — Inspection and Maintenance

Hazardous-area equipment degrades over time — flame paths corrode, cable glands loosen, IS earth resistance drifts, equipment is modified without re-certification. Part -17 defines the inspection regime that keeps an installation compliant between commissioning and decommissioning.

Grade Scope Typical frequency
Visual External condition — damage, corrosion, nameplate legibility, gland tightness Continuous to monthly
Close Visual checks plus covers removed to verify flame paths, IS parameters, internal connections Annually
Detailed Close checks plus full re-verification against current certificate, dimensional checks, entity recalculation Every 3–5 years; after incidents

Competent person — inspections must be performed by someone with documented knowledge of the protection methods present, the applicable installation standards, the hazardous substances, and practical experience. National schemes (CompEx in the UK, EEHA in Australia, equivalents elsewhere) document the qualification.

Common findings classified as immediate hazard that take equipment out of service immediately: non-certified cable gland in use, modified enclosure with extra holes, corroded flame path with gap out of tolerance, withdrawn certificate. Records of every inspection — date, inspector qualification, grade applied, findings, actions, next due date — are required for regulatory audits, insurance reviews, and incident investigation.

Worked Example — IS Loop Entity Check

Scenario. A 4–20 mA pressure transmitter on an ethylene tank, classified Zone 1, gas group IIB, with the area drawing requiring at least T3 equipment (substance autoignition margin). The transmitter is wired through a 100 m armoured screened twisted-pair cable to a zener barrier in the safe-area control room.

Selected components.

Component Marking Entity parameters
Field transmitter Ex ia IIB T4 Gb Ui = 30 V, Ii = 100 mA, Ci = 15 nF, Li = 50 µH
Zener barrier (associated apparatus) [Ex ia] IIC Uo = 28 V, Io = 93 mA, Co = 83 nF, Lo = 5.7 mH
Cable (100 m) Standard IS-rated screened TP 200 pF/m, 0.7 µH/m

Step 1 — marking compatibility.

Step 2 — cable contribution.

Step 3 — entity checks.

Check Calculation Result
Uo ≤ Ui 28 V ≤ 30 V ✓
Io ≤ Ii 93 mA ≤ 100 mA ✓
Ci + Ccable ≤ Co 15 nF + 20 nF = 35 nF ≤ 83 nF ✓
Li + Lcable ≤ Lo 50 µH + 70 µH = 120 µH ≤ 5 700 µH ✓

All four entity checks pass. Po ≤ Pi must also be verified from the certificates (typically driven by the Uo×Io product). The barrier–transmitter–cable combination is a compliant IS loop.

Step 4 — earthing.

Step 5 — documentation deliverables.

Common Mistakes

  1. Choosing equipment by gas-group letter alphabetically — “IIC must be best, pick that”. IIC equipment satisfies any gas group, but it is more expensive and Ex d / Ex e equipment in IIC has stricter installation requirements. Match the equipment group to the actual hazard. Specifying IIC where IIB suffices wastes budget and complicates installation.
  2. Trusting the nameplate without checking the certificate. Certificates can be withdrawn by the certifying body without recall of installed equipment — the nameplate persists. Verify against the IECEx online certificate database or the relevant ATEX notified-body record at commissioning and during detailed inspections (60079-17).
  3. Mixing certified and uncertified components in an IS loop. The entity-check math fails immediately if any uncertified component touches the loop — there are no entity parameters for an uncertified surge protector or terminal block. The whole loop becomes non-compliant.
  4. Routing IS and non-IS cables in the same multicore cable or shared tray without segregation. Energy from the non-IS circuit can couple into the IS circuit and exceed the entity-parameter limits, defeating the protection. Per 60079-14, segregate physically — different conduits, different trays, or a grounded metallic barrier.
  5. Using zener barriers with inadequate IS earth (>1 Ω). Zener barriers depend on a low-impedance path to clear faults. If the IS earth has drifted high (typical cause: corrosion at the bonding lug, undersized bonding conductor, or shared with a noisy supplementary earth), barriers behave unpredictably under fault. If the IS earth cannot be reliably maintained ≤1 Ω, switch to galvanic isolators.
  6. Field-modifying an Ex enclosure (extra cable entries, larger openings, drilled mounting holes) without re-certification. The certificate covers the equipment as built. Any modification — even what seems harmless, like an extra cable gland hole — invalidates the certification. The corpus’s 60079-17 inspection table classifies a modified enclosure as an immediate hazard: take it out of service, re-certify or replace.

Practical Checklist

Area Classification

Equipment Selection

Installation (IEC 60079-14)

Periodic Inspection (IEC 60079-17)

Lifecycle Application

Stage IEC 60079 activity
Standards Selection Decide IEC 60079 (Zone) vs. NEC Art. 500 (Class/Division) for the project; identify substances and confirm gas groups; preliminary area classification feasibility
Detailed Design Area classification drawing per 60079-10-1; protection-method choice per equipment (Ex d for sparking power devices, Ex i for instruments); IS loop entity calculations; cable and bonding scheme per 60079-14
Build Equipment register populated; certificates archived; cable glands, stopping plugs, and flame-path surfaces preserved; IS earth and equipotential bonding installed
Installation 60079-14 installation rules at the site; cable segregation enforced; bonding continuity verified; IS earth measured
Pre-Commissioning Initial verification per 60079-14: certificate validity, gland integrity, flame-path condition, IS entity-check sign-off, classified-area drawing matches installed equipment
Commissioning Commissioning certificate issued; first periodic-inspection schedule defined per 60079-17 (visual / close / detailed) and competent person assigned

Relationship to NEC

NEC has two parallel approaches to hazardous areas — IEC 60079 maps to one, parallels the other.

NEC Article Scope Relationship to IEC 60079
Art. 500–503 Class / Division system (older US approach) Parallel methodology — see the IEC 60079 ↔ NEC Art. 500–505 crosswalk for equipment-level mapping
Art. 504 Intrinsically safe systems Aligns with IEC 60079-11; entity concept, IS earth requirement, and cable segregation map directly
Art. 505 Zone classification system Carries the IEC 60079 zones, protection concepts, and EPLs into US practice — a classification alignment, not an approval route

For a US installation choosing between paths, Art. 505 (Zone) suits greenfield work whose design and equipment population are already IEC-based; existing facilities often run mixed Art. 500 and Art. 505 sections. The choice settles the classification vocabulary, not the equipment approval.

The certificate is not the approval (US installations)

Five things get collapsed into one when people say “Article 505 accepts ATEX equipment”. Keep them apart:

  1. Area classification system — Zone 0/1/2 (Art. 505) or Class I Div 1/2 (Art. 500/501). OSHA’s 29 CFR 1910.307(a)–(b) permits either for Class I and requires the classification to be documented.
  2. Protection concept and EPL — Ex d, Ex ia, Ex e, Ex p, Ex m, Ex nA, and which zone each suits. This is what IEC 60079 defines.
  3. Certification scheme — ATEX (EU conformity), IECEx (international scheme), or a US NRTL certification. A certificate is evidence about the product.
  4. US approval / listing — whether the product may lawfully be installed in a US workplace. 29 CFR 1910.399 defines “approved” as acceptable to the authority enforcing OSHA Subpart S, with acceptability normally resting on acceptance, certification, listing, labelling, or a safety determination by an OSHA-recognised NRTL.
  5. AHJ acceptance — whether this installation, as built and documented, is accepted by the inspecting authority.

OSHA’s NRTL programme guidance is explicit on the gap between 3 and 4: the CE mark is unrelated to US product-safety requirements, and equipment intended for use in potentially explosive atmospheres in the United States must bear the specific mark of an NRTL that OSHA has recognised to certify that equipment type. 29 CFR 1910.307(c) requires equipment to be approved for the class of location and the ignitable properties of the specific gas or vapour; 1910.307(g) requires Class I Zone equipment to be listed and marked with class, zone, the symbol AEx, protection technique(s), gas group(s), and temperature class — for example Class I, Zone 0, AEx ia IIC T6.

The narrow non-NRTL routes in 1910.399 — equipment of a kind that no NRTL certifies at all, inspected instead by another Federal agency or a State/municipal/local authority enforcing the occupational-safety provisions of the NEC, and custom-made equipment determined safe by its manufacturer on retained test data — do not cover mainstream Ex-certified instruments and enclosures, and are not a general AHJ allowance for ATEX marking.

Practical reading: an Ex ia IIC T6 Ga certificate answers questions 2 and 3. It does not answer question 4. When specifying IEC-scheme product for a US site, verify the NRTL mark and its scope on the actual product, and settle the marking form and classification drawings with the AHJ before procurement. This site makes no claim about how commonly US AHJs accept IEC-scheme certificates.

Sources for this subsection (public, retrieved 3 August 2026, paraphrased): OSHA, NRTL Program — Frequently Asked Questions; 29 CFR 1910.307; 29 CFR 1910.399. NEC 2023 Article 505.9’s own marking provisions have not been checked against the licensed edition; where OSHA’s rule and the NEC differ, OSHA enforces its own.

← Hazardous area family

Trust Boundary — Engineering Judgment Required

This site is an independent educational reference for industrial automation standards. It paraphrases and organizes guidance in original language; it is not a substitute for the official published standards, professional engineering judgment, or legal review, and content has not been independently verified against current published editions.

Items marked Review pending await technical review against the identified edition. Items marked Planned have no reviewed source material behind them. Do not rely on this site for compliance determinations, safety-critical design decisions, or legal interpretation.