Hazardous-area control panels: zones and Class/Division, Ex d, Ex e, Ex p and Ex i protection, ATEX and IECEx

A hazardous-area control panel is a panel built so that it cannot ignite the flammable gas, vapour or dust that may surround it. The plant's area classification sets the zone (Zone 0, 1 or 2 for gas, Zone 20, 21 or 22 for dust; Class and Division in North America), the gas or dust group and the temperature class. The panel's type of protection follows from those: flameproof (Ex d), increased safety (Ex e), pressurised (Ex p), intrinsic safety for the field circuits (Ex i), dust protection by enclosure (Ex t), or a combination such as Ex de. Certified parts inside a panel do not by themselves make the finished panel suitable for the zone: whoever places the assembled panel on the market also has to assess the ignition hazards that combining the parts adds.

What makes an area hazardous

An explosion needs a flammable substance, oxygen from the air and an ignition source at the same time. Electrical equipment can supply the ignition source through sparks, arcs and hot surfaces, so each protection type either stops the ignition source from becoming active, contains an explosion inside the enclosure, or keeps the explosive atmosphere away from the ignition source.

Area classification decides where an explosive atmosphere can occur, how often and for how long. For gases and vapours the international standard is IEC 60079-10-1 (Explosive atmospheres, Part 10-1: Classification of areas, Explosive gas atmospheres, edition 3.0, December 2020); dust is covered by IEC 60079-10-2. Pepperl+Fuchs notes that the release conditions are part of the classification, and that ventilation can keep the concentration below the lower flammable limit (LFL). Wikipedia describes classification as joint work of operations, maintenance, safety and electrical and instrument engineers, using process diagrams and safety data sheets; the result is often an area classification plan, which may list each substance with its group and temperature rating.

In the European Union, the ATEX workplace directive 1999/92/EC makes the employer responsible for classifying places into zones and for keeping an explosion protection document up to date. The panel builder works from that classification and does not set it.

Zones and the Class/Division system

Directive 1999/92/EC defines the zones and the equipment category allowed in each; the IEC system uses equipment protection levels (EPL) for the same purpose.

Wikipedia quotes a rule of thumb for gas zones: more than 1000 hours a year for Zone 0, 10 to 1000 hours for Zone 1 and under 10 hours for Zone 2. The directive itself defines the zones in words, not hours.

In the United States, NFPA 70 (the National Electrical Code) Article 500, Hazardous (Classified) Locations, Class I, II, and III, Divisions 1 and 2, uses classes and divisions. Article 505, Zone 0, 1, and 2 Locations, and Article 506, Zone 20, 21, and 22 Locations for Combustible Dusts or Ignitable Fibers/Flyings, bring in the zone method (titles as in the 2017 edition). OSHA's rule 29 CFR 1910.307 allows the zone system as an alternative for Class I only; Class II and III locations use divisions.

The correspondence is approximate: Division 1 covers what the zone method splits into Zone 0 and Zone 1, and Division 2 corresponds to Zone 2. A Division approval does not by itself show zone suitability: OSHA requires Division-approved equipment used in a zone to be marked with the zone, gas group and temperature class too, and a Zone 0 or Zone 1 area may not border a Division 1 or 2 area.

Gas groups and temperature classes

The gas group reflects how easily the gas ignites and how readily a flame passes through a narrow gap; the temperature class limits the hottest surface of the equipment.

Equipment marked IIC is also suitable for IIB and IIA gases; equipment marked IIB is not suitable for IIC. For dust, IIIA covers combustible fibres and flyings, IIIB non-conductive dust and IIIC conductive dust.

The maximum surface temperature must stay below the ignition temperature of the gas present. Pepperl+Fuchs gives hydrogen as 560 °C, gasoline as 280 °C and carbon disulphide as 90 °C, which is why carbon disulphide needs T6 equipment. Unless the marking states another range, Ex equipment is rated for an ambient of −20 °C to +40 °C; a different range is marked with Ta or Tamb.

Protection types used for control panels

Flameproof enclosure, Ex d (IEC 60079-1). The enclosure withstands an explosion inside it and its flameproof joints stop the flame from reaching the outside atmosphere. Level db gives EPL Gb for Zone 1. R. STAHL describes flameproof enclosures as heavy, large and more expensive than Ex e, and as the usual choice for switchgear and control equipment. The user must not modify the flameproof joints. Cables enter through flameproof glands or sealing devices, through conduit with seals, or through seals into an Ex e terminal compartment. Whether a compound-filled (barrier) gland is needed is decided under IEC 60079-14; Hawke notes that the 2013 edition replaced the older selection chart (internal ignition source, gas group, zone, enclosure volume) with different criteria, so check the edition that applies to the project.

Increased safety, Ex e (IEC 60079-7). Arcs, sparks and excessive temperatures are prevented by design: generous dimensions, mechanical strength and secure terminations. Level eb (EPL Gb) is limited to terminals and connections, motors and lights; level ec (EPL Gc) allows more complex electronics for Zone 2. Pepperl+Fuchs lists terminal boxes and the connection enclosure of Ex de control panels among its uses. R. STAHL notes that its surface temperature is limited by design and by a maximum number of components. Cable entries must keep at least IP54.

Combination, Ex de. R. STAHL notes that switchgear produces ignition sources in normal operation, so Ex e alone is not enough, and that Ex d is usually combined with Ex e. Sparking parts stay in flameproof enclosures while wiring and terminals sit in an increased-safety compartment, with sealed entries between the two.

Pressurised enclosure, Ex p (IEC 60079-2, NFPA 496). Air or an inert gas keeps the inside of the enclosure at a higher pressure than the surrounding atmosphere, after a purge that clears any gas already inside (five volume exchanges for simple layouts under IEC 60079-2, four under NFPA 496). Pepperl+Fuchs gives three levels:

• pxb (Type X in North America): installed in Zone 1 or 21, with ignition-capable equipment inside. Switching power on and off must be automatic, driven by the safety devices.
• pyb (Type Y): installed in Zone 1 or 21, with equipment rated for Zone 2 or 22 inside.
• pzc (Type Z): installed in Zone 2 or 22, with ignition-capable equipment inside; a person may be part of the response to loss of pressure.

Pressurisation lets standard equipment, touch-panel interfaces included, be used inside, allows affordable cooling, and does not limit enclosure size in principle. It needs clean, dry, oil-free instrument air from a compressor whose intake is in a non-hazardous area, and a purge can need roughly 20 to 40 CFM. Pepperl+Fuchs states that loss of pressure must raise an alarm, that in pxb the unprotected equipment inside must also be shut down, and that in Zone 1 and Zone 2 the enclosure is purged before power is applied. Batteries, which cannot be switched off, and circuits such as Ethernet, USB or 4–20 mA signals need their own protection or safe switching.

Intrinsic safety, Ex i (IEC 60079-11). The energy in a circuit is limited so that neither a spark nor a hot surface can ignite the atmosphere. Level ia stays safe with two faults (Zone 0), ib with one fault (Zone 1) and ic in normal operation (Zone 2). A zener barrier or galvanic isolator in a safe-area panel limits the energy sent to the field device. Each loop is checked with the entity parameters. In Macromatic's notation, the field device's maximum voltage and current (Vmax, Imax) must be at least the barrier's open-circuit voltage and short-circuit current (Voc, Isc), and the device's capacitance and inductance plus those of the cable must not exceed what the barrier allows (Ca, La). Intrinsically safe terminals stay at least 50 mm from other terminals, the cables are marked, and the user documents the verification of each loop. Ex i suits instruments and signals, not motors or lighting.

Dust protection by enclosure, Ex t (IEC 60079-31). The enclosure keeps dust out and limits its surface temperature. Levels ta, tb and tc give EPL Da, Db and Dc; for conductive dust (IIIC) every level requires IP6X.

Choosing an approach for a panel

• Can the panel stand outside the hazardous area? A panel in a control room or other safe area can be a standard panel. The circuits that run into the hazardous area then use Ex i barriers or isolators, or connect to field equipment certified for the zone.
• Does it contain sparking or hot equipment? Switching devices produce ignition sources in normal operation, and standard controllers, power supplies and HMIs carry no Ex marking. In the hazardous area such equipment needs a flameproof enclosure, an Ex de arrangement or a pressurised enclosure, unless the device itself is certified for the zone (some operator terminals are marked for Class I Division 2 and Zone 2; see what is an HMI).
• How large is it and how much heat does it make? R. STAHL describes flameproof enclosures as heavy, large and more expensive. Pressurisation does not limit size in principle and allows cooling, at the cost of a protective gas supply, purge time and pressure monitoring.
• Does it hold only terminals? Then an Ex e (eb) terminal or junction box is an option, since eb covers terminals and connections.
• How will it be maintained and cabled? Opening the door of a pressurised panel loses the overpressure, and the enclosure has to be purged again before the equipment inside is powered. Every cable entry needs a gland suited to the protection type, and unused entries are closed with blanking elements that keep the IP rating and can be removed only with a tool.

ATEX, IECEx and who certifies what

ATEX product directive 2014/34/EU covers equipment and protective systems for potentially explosive atmospheres and has applied since 20 April 2016. Only notified bodies may issue certificates under it, and the CE marking is affixed after the conformity assessment. Article 13 sets the procedure by category: categories 1 and 2 electrical equipment need EU-type examination by a notified body combined with a production check, while category 3 equipment uses internal production control by the manufacturer. Unit verification (Annex IX) is a further option: a notified body examines and tests the individual item and issues a certificate of conformity, and the Commission's guidelines say it may be applied to unique products and limited series.

The European Commission's ATEX Guidelines (sixth edition, January 2026) treat an assembly placed on the market as a single functional unit as equipment in its own right, and its manufacturer takes responsibility for compliance. The assembler may rely on parts that are already compliant but must assess the ignition hazards that combining them adds. Connecting separately supplied ATEX equipment on site counts as an installation instead, and the guidelines admit that the line between an installation and an assembly is not always clear: a unique part of an installation built off-site under the end user's responsibility, directly or through a contractor, is usually not treated as a product under the Directive.

IECEx is one of the IEC's conformity assessment systems. In its Certified Equipment Scheme the manufacturer applies to an IECEx certification body (ExCB), samples are tested and a factory inspection checks production, followed by periodic audits; the outputs are a test report (ExTR), a Quality Assessment Report (QAR) and a Certificate of Conformity (CoC), which can be checked in the IECEx online certificate system. For a defined number of items from a single production run, IECEx also issues Unit Verification Certificates, which need an ExTR but no QAR. Other IECEx schemes certify service facilities that design, install, inspect or repair Ex equipment (against IEC 60079-14, -17 and -19) and the competence of persons.

The two schemes are separate. Under ATEX only notified bodies issue certificates, and the European Commission states that voluntary certificates are not a recognised means of proving compliance. An IECEx certificate is issued under IECEx rules, not under the Directive, so on its own it does not show ATEX compliance in the EU; outside the EU, check what the country of installation accepts.

When reading a certificate or marking, two suffixes matter. U marks an Ex component certificate: the part is not intended to be used alone and needs further evaluation before it is built into Ex equipment. X means the certificate lists specific conditions of use, which must be read with the certificate and followed at installation.

What the end user should specify in an RFQ

The RFQ should state:

• The area classification drawing and the zone at the panel location, for gas and for dust.
• Gas group(s) and temperature class, or the substances and their ignition temperatures; dust group if any.
• The certification required (ATEX, IECEx, North American Class/Division or Zone, or a national scheme) and the EPL or ATEX category.
• The site ambient range, if it goes outside −20 °C to +40 °C, and the IP rating.
• The equipment to go inside, with its heat load, and anything that must stay live after a trip.
• Cable entries: number, cable types and sizes, armour, gland type and thread; unused entries closed with blanking elements that keep the IP rating.
• For Ex p: the protective gas (instrument air or inert gas), its quality and available flow, where the supply is taken from, and the action on loss of pressure (alarm or automatic shutdown).
• For Ex i: the loop list with field device entity parameters and cable data.
• The documents expected at delivery: certificates, marking, instructions, any X conditions and the intrinsic safety verification.

TDL project references

TDL's hazardous-area control panels page lists two references. At Dinh Co GPP, CC01 turbo expander (2019), TDL migrated the SLC 500 control system to a redundant ControlLogix platform; the project page states that the Ex-p and Ex-e control and marshalling panels were fabricated and certified for Zone 2 installation by a DNV (DNV GL) notified body. At Phu My Gas Distribution Station, heater E-401B (2021), TDL upgraded the heater controller; the local control panel is flameproof and dust-protected, marked Ex db IIB T6/T5 Gb and Ex tb IIIC Db, IP66, for Zone 1/2 and Zone 21/22, with Ex-certified cable glands. Read with the tables above, that marking means flameproof protection for gas group IIB at EPL Gb, and dust protection by enclosure for conductive dust at EPL Db. At Phu My Fertilizer Plant, the natural gas compressor control system migration that TDL carried out for the EPC contractor included intrinsically safe barriers for field devices.

For panel design and factory testing in general see control panel engineering; for the safety system side, see what is ICSS and the oil and gas page.