What is an Ex-zone?
An Ex-zone (hazardous area zone) is a defined area in an installation or workplace where an explosive atmosphere of flammable gas, vapour, mist or dust may occur, classified by how often and for how long that atmosphere is present. Europe uses six zones: 0, 1 and 2 for gases, vapours and mists, and 20, 21 and 22 for dust. The classification comes from the ATEX workplace directive (1999/92/EC) and determines which equipment you may install there, how you install it and which working rules apply. For OT engineers, the Ex-zone is therefore a hard design boundary: a PLC, sensor, switch or tablet that works perfectly well outside the zone can become an ignition source inside it.
🧭 Which Ex-zones exist?
A zone describes the likelihood of an explosive atmosphere, not the severity of the consequences. The legal definitions are in Annex I of Directive 1999/92/EC.
| Gas zone | Dust zone | Legal definition | Typical example |
|---|---|---|---|
| Zone 0 | Zone 20 | Explosive atmosphere present continuously, for long periods or frequently | Vapour space of a solvent storage tank, inside a flour silo |
| Zone 1 | Zone 21 | Likely to occur occasionally in normal operation | Around a filling point or vent, at a powder bag-tipping station |
| Zone 2 | Zone 22 | Not likely to occur in normal operation and, if it does, only for a short period | Around flanges and pump seals in the open air, around a dust filter |
In practice people sometimes use a rule of thumb in hours per year: more than roughly 1,000 hours for zone 0, between 10 and 1,000 hours for zone 1 and fewer than 10 hours for zone 2. These figures come from other guidance documents and are not part of the European or IEC definition; they help the discussion but do not replace the assessment.
With dust, what lies on surfaces counts as well. A dust layer only a few millimetres thick can be stirred up by a small disturbance into a much larger cloud, so the directive treats layers and deposits as a source of explosive atmosphere. Good housekeeping in dust zones is therefore a safety measure, not a cosmetic one.
⚙️ How is an Ex-zone determined?
Zoning is a technical assessment that the employer must carry out or commission. Internationally, IEC 60079-10-1 (edition 3, 2020) describes the method for gas and IEC 60079-10-2 the method for dust; in the Netherlands these are elaborated in NPR 7910-1 (gases, vapours and mists, current version 2020 with a 2021 corrigendum) and NPR 7910-2 (dust). The assessment follows four questions:
- Which substance? - Flash point, explosive limits, density relative to air, gas group (IIA, IIB, IIC) and auto-ignition temperature. A heavy gas such as propane collects in pits and drains, whereas hydrogen rises.
- Where can it be released? - Each source of release gets a grade: continuous, primary (expected during normal operation) or secondary (only during abnormal conditions, such as a leaking gasket).
- How good is the ventilation? - The degree of dilution (high, medium, low) and the availability of ventilation (good, fair, poor) can make the zone more or less severe.
- How large is the area? - The extent follows from the release rate and the dilution. IEC 60079-10-1 also recognises a zone of negligible extent (NE), for example a small secondary leak in the open air that dilutes below the lower explosive limit almost immediately.
As a rule of thumb, a continuous source leads to zone 0, a primary source to zone 1 and a secondary source to zone 2. Poor ventilation makes the zone more severe, good ventilation less so. In complex situations the boundaries are backed up by dispersion calculations, sometimes supported by a HAZOP to capture abnormal process conditions.
🔧 Which equipment may you use in which Ex-zone?
The zone sets the minimum equipment category from the product directive 2014/34/EU, called the Equipment Protection Level (EPL) in IEC terms. A higher category may always be used in a less severe zone, never the other way round.
| Zone | Permitted ATEX category | Minimum EPL | Common protection types |
|---|---|---|---|
| 0 | 1G | Ga | Ex ia (intrinsically safe), Ex ma |
| 1 | 1G or 2G | Gb | Ex db, Ex eb, Ex ib, Ex pxb, Ex mb |
| 2 | 1G, 2G or 3G | Gc | Ex ec, Ex ic, Ex nR, Ex pzc |
| 20 | 1D | Da | Ex ia, Ex ta |
| 21 | 1D or 2D | Db | Ex tb, Ex ib, Ex pxb |
| 22 | 1D, 2D or 3D | Dc | Ex tc, Ex ic |
Besides the category, the gas group and the temperature class (T1 = 450 °C to T6 = 85 °C maximum surface temperature) must suit the substances present. For dust, the maximum surface temperature is stated directly in degrees on the rating plate, and it must stay a safe margin below the ignition temperature of the dust layer. The protection types themselves are defined in the IEC 60079 series.
An IP rating such as IP54 or IP65 only says something about the ingress of dust and water. It is not an Ex protection type: an ordinary IP65 enclosure is not permitted in zone 2 or 22 without an Ex certificate.
🏭 What does an Ex-zone mean for OT and IT/OT convergence?
Digitalisation brings more and more electronics into the process plant. Every new transmitter, camera, access point or edge gateway must therefore be checked against the zone drawing first. Typical choices:
- Control cabinets - A control cabinet with a PLC and O is preferably placed outside the zone. Inside zone 1 it can be done with a pressurised enclosure (Ex p) or a flameproof enclosure (Ex d), but that is expensive and needs extra maintenance.
- Measurement loops - 4-20 mA and HART instruments in zone 0 and 1 are usually intrinsically safe, with a barrier or isolator in the safe area and a loop calculation according to IEC 60079-14.
- Digital field networks - intrinsically safe fieldbuses such as PROFIBUS PA and FOUNDATION Fieldbus H1 usually use FISCO; Ethernet-APL brings Ethernet into zone 0 with the 2-WISE concept from IEC TS 60079-47.
- Wireless - WirelessHART and Wi-Fi equipment needs an Ex certificate; IEC 60079-0 limits the radiated power, to 2 W for gas group IIC.
- Drives - A variable frequency drive usually sits outside the zone, but the motor inside the zone must be certified for converter operation, because the extra heating caused by the drive can exceed the temperature class and bearing currents can cause sparking in the bearings.
- Mobile devices - Tablets, phones and laptops in an Ex-zone must be Ex certified, or may only be taken in after a gas-free declaration and under a work permit.
Conversely, IT/OT convergence also helps: remote maintenance, condition monitoring and better diagnostics from HART instruments reduce the number of times someone has to enter an Ex-zone with tools. Mind the terminology, though: an Ex-zone is not the same as a security zone from IEC 62443 (see Zone). Both exist side by side in a process plant, and a network drawing that mentions zones should make clear which of the two it means.
🛠️ How do you set up Ex-zones in a Dutch installation?
In the Netherlands, ATEX 1999/92/EC is transposed into the Working Conditions Decree (Arbobesluit), articles 3.5a to 3.5f, under the Arbowet. The Netherlands Labour Authority supervises compliance. A workable approach:
- Inventory the flammable substances and sources of release as part of the RI&E risk assessment.
- Classify the zones according to NPR 7910-1 and NPR 7910-2 and record them on floor plans and cross-sections, including the height of each zone.
- Draw up the explosion protection document (in Dutch: EVD) before work starts, and update it after every significant change.
- Mark the entry points with the yellow triangular warning sign bearing the letters EX, as required by Annex III of the directive.
- Select and install equipment per zone according to NEN-EN-IEC 60079-14, alongside NEN 1010 for the low-voltage installation, and have a competent person verify explosion safety before first use.
- Inspect and maintain according to NEN-EN-IEC 60079-17; without expert advice, the periodic inspection interval is at most three years. Work on the installation is done by authorised personnel in line with NEN 3140.
For mobile measuring equipment without an Ex version, the Netherlands Labour Authority published an adjusted enforcement approach on 2 March 2023: non-certified equipment is allowed by exception, but only in zone 2 or 22 and after an ATEX expert has assessed all ignition sources. If your company falls under the Seveso rules, zoning, functional safety and cybersecurity are directly connected: gas detection that shuts down a plant via the SIS is only reliable if the control system behind it is protected as well.
🔄 How does the zone system compare with the North American classification?
If you work with American suppliers, you will come across the North American Class/Division system from the National Electrical Code. The NEC also contains a zone system aligned with IEC.
| Europe and IEC | North America (Class/Division) | Note |
|---|---|---|
| Zone 0 | Class I, Division 1 | Division 1 covers zone 0 and zone 1 together |
| Zone 1 | Class I, Division 1 | Division 1 equipment is not automatically suitable for zone 0 |
| Zone 2 | Class I, Division 2 | Comparable level |
| Zone 20 and 21 | Class II, Division 1 | Combustible dust |
| Zone 22 | Class II, Division 2 | Combustible dust |
A UL or FM approval does not replace an ATEX certificate in the EU, and neither does the international IECEx certificate. This is a well-known pitfall when buying skids and machines from the US or Asia.
❓ Frequently asked questions
Who determines the Ex-zones in an installation?
The employer is responsible for the zone classification of an installation, under Directive 1999/92/EC and, in the Netherlands, the Arbobesluit. In practice an ATEX specialist carries out the zoning according to NPR 7910-1 and NPR 7910-2, and the employer records the result in the explosion protection document.
What is the difference between zone 1 and zone 2?
In zone 1 an explosive atmosphere is expected occasionally during normal operation, in zone 2 only during abnormal conditions and then for a short time. Zone 1 therefore requires category 2G (EPL Gb) equipment or better, while category 3G (EPL Gc) is sufficient in zone 2.
Can an ordinary PLC be installed in an Ex-zone?
An ordinary PLC without an Ex certificate may not be installed in an Ex-zone, not even in zone 2 or 22. The usual solution is to place the PLC outside the Ex-zone and bring the field signals in intrinsically safe or via Ethernet-APL; otherwise an Ex p or Ex d enclosure is needed.
Why is over-classifying zones a bad idea?
Over-classifying looks safe, but a larger Ex-zone means more expensive equipment, more inspections and more work permits without reducing the risk. A carefully justified zone classification focuses attention on the places where the hazard really is.
What is the difference between an Ex-zone and a zone in IEC 62443?
An Ex-zone indicates where an explosion hazard from gas or dust exists and belongs to physical safety under ATEX. A zone in IEC 62443 is a group of systems with the same cybersecurity requirements; the two concepts are unrelated, but they do occur together in the same plant.
How often must an installation in an Ex-zone be inspected?
An electrical installation in an Ex-zone is inspected before first use and periodically thereafter according to NEN-EN-IEC 60079-17. Without justified expert advice, the interval between periodic inspections may not exceed three years.
📌 In summary
An Ex-zone records where and how often an explosive atmosphere can occur, and thereby determines which equipment, installation methods and working rules apply.
There are three gas zones (0, 1, 2) and three dust zones (20, 21, 22). The employer classifies them according to IEC 60079-10-1/-2 or NPR 7910 and records them in the explosion protection document. For OT projects the zone drawing is the starting point: it determines whether a cabinet, instrument, network or tablet may be installed at all, and if so in which Ex version.