What is an ATEX zone and how is it classified?
An ATEX zone is a location in a workshop or storage area where an explosive atmosphere may form, due to the presence of flammable gases, vapours or dusts mixed with air. The classification distinguishes zones where the explosive mixture is present permanently, occasionally, or only in the event of failure. For gases and vapours, we refer to zones 0, 1 and 2. For combustible dusts, we refer to zones 20, 21 and 22.
| Zone | Explosive atmosphere | Frequency and duration |
|---|---|---|
| 0 / 20 | Gas, vapour / dust | Present continuously or very frequently, for long periods |
| 1 / 21 | Gas, vapour / dust | Likely to form occasionally during normal operation |
| 2 / 22 | Gas, vapour / dust | Forms only rarely, and for a short duration only |
This classification determines the choice of any equipment installed in the zone, including fire detection.
Why is a standard fire detector not suitable in an ATEX zone?
A non-certified fire detector can itself become a source of ignition in an explosion-risk zone. An electrical contact, an internal spark, component overheating or a build-up of static electricity is sufficient to ignite an air/gas or air/dust mixture already present in the atmosphere.
The reasoning is reversed compared to a standard building: the aim is no longer just to detect the start of a fire, but also to ensure that the detection device itself is never the source of the explosion it is supposed to help prevent. This is why all electrical equipment installed in an ATEX zone—detector, junction box, siren or cabling—must have a recognised ignition protection mode, regardless of its detection performance.
What certifications must detectors and control panels installed in ATEX zones have?
Detectors and detection control panels installed in ATEX zones must bear the Ex marking and an equipment category corresponding to the target zone. This marking certifies a protection mode (intrinsic safety, flameproof enclosure, encapsulation) and a maximum surface temperature class compatible with the flammable product present.
| Target zone | Required equipment category | Group |
|---|---|---|
| 0 / 20 | Category 1 | II (surface installations) |
| 1 / 21 | Category 2 | II |
| 2 / 22 | Category 3 | II |
The temperature class (rated T1 to T6) must remain below the auto-ignition temperature of the product stored or handled. In practice, the fire detection control panel itself is often installed outside the ATEX zone, in a safe technical room, with only the detectors and cabling passing through the risk zone.
What precautions should be taken when installing in a workshop or flammable storage area?
The installation of fire detection in an ATEX zone is based on a prior zoning study, carried out before any equipment selection. This study determines the exact extent of each zone (0, 1, 2 or 20, 21, 22) around release sources, which directly determines the location and type of detectors that can be installed.
Common precautions include:
- the use of Ex-certified cable glands and junction boxes, with tightening and sealing in accordance with the manufacturer's instructions;
- the use of intrinsic safety barriers for circuits connecting the ATEX zone to a control panel installed outside the zone;
- careful earthing and equipotential bonding of all metallic elements;
- positioning detectors away from direct release sources when the room configuration allows, without compromising detection time.
No cabling modification or component replacement should be carried out without strictly complying with the Ex certificate of the installed assembly.
Who carries out ATEX zoning and who should install the detection system?
ATEX zoning is the responsibility of the employer, generally with the support of the prevention adviser, as part of the document describing the explosion risks specific to the activity. This document identifies release sources, delimits zones and serves as the basis for any electrical equipment selection, including fire detection.
The installation itself must be entrusted to a company able to demonstrate its competence in Ex equipment, based on standard NBN S 21-100-1 for the general design of the detection installation, adapted to the site-specific ATEX requirements. The use of BOSEC-certified equipment for the fire detection part does not exempt verification of the specific Ex certification of each component installed in the risk zone: the two certifications meet different requirements and are cumulative.
How is maintenance of fire detection in an ATEX zone carried out?
Maintenance of fire detection in an ATEX zone follows the frequencies of standard NBN S 21-100-2, with enhanced checking of the integrity of Ex protections at each visit. The technician checks the condition of cable glands, the absence of corrosion on flameproof enclosures, the tightening of boxes and the conformity of cabling against the original diagram.
Any intervention on a detector or box located in an ATEX zone must be carried out by a technician trained in explosive atmosphere risks, as opening a certified enclosure outside procedure may void its protection. The detection system maintenance register must specifically mention checks related to Ex certification, in addition to standard fire checks, to enable the emergency zone (hulpverleningszone) to verify the consistency of the installation during a periodic inspection.