Fire detection in
hazardous areas
ATEX and intrinsically safe devices
Fire alarm equipment in explosive atmospheres requires specialist certification. This guide explains ATEX, intrinsic safety, and when each approach is appropriate.
In environments where flammable gases, vapours, mists, or dusts are present, standard fire alarm equipment cannot be used. A conventional smoke detector or sounder contains electrical components that could, under fault conditions, produce a spark sufficient to ignite the surrounding atmosphere. In these environments, specialised explosion-protected equipment is required.
What is ATEX?
ATEX is the European framework for equipment used in potentially explosive atmospheres. The name derives from the French ATmosphères EXplosibles. In the UK, following departure from the EU, the equivalent domestic framework is UKEX — but ATEX-certified equipment continues to be widely used and accepted in the UK under transitional arrangements, and the two frameworks are essentially identical in technical requirements.
ATEX regulation covers both the classification of hazardous areas and the certification of equipment for use in those areas. This is a separate and specialist discipline from standard fire alarm design and installation — it sits at the intersection of fire engineering, electrical engineering, and process safety.
Area classification
Hazardous Area Zones
Before specifying fire alarm equipment, the hazardous area must be formally classified into zones based on the frequency and duration of the explosive atmosphere. This classification determines which category of equipment can be used.
| Zone | Hazard type | Definition |
|---|---|---|
| Zone 0 | Gas / Vapour | Explosive atmosphere present continuously or for long periods |
| Zone 1 | Gas / Vapour | Explosive atmosphere likely to occur in normal operation occasionally |
| Zone 2 | Gas / Vapour | Explosive atmosphere not likely in normal operation but possible in abnormal conditions |
| Zone 20 | Dust | Explosive dust cloud present continuously or for long periods |
| Zone 21 | Dust | Explosive dust cloud likely to occur in normal operation occasionally |
| Zone 22 | Dust | Explosive dust cloud not likely in normal operation but possible in abnormal conditions |
Typical hazardous area environments
Industries and environments where hazardous area fire alarm equipment is commonly required include oil and gas platforms and refineries, chemical and pharmaceutical manufacturing, paint spraying facilities, grain handling and flour milling (combustible dust), solvent-based manufacturing processes, fuel storage and dispensing areas, battery charging rooms (hydrogen evolution), and wastewater treatment plants.
Protection methods
Intrinsic Safety — the Most Common Method for Fire Alarm Devices
Intrinsic safety (designated Ex i or ia/ib) limits the electrical energy available within the equipment and its associated wiring to levels below that required to ignite the surrounding explosive atmosphere — even under fault conditions. Rather than containing a potential ignition source within a robust enclosure, intrinsic safety eliminates the ignition source entirely by limiting the energy in the circuit.
Fire alarm devices are low-power by nature — detectors, call points, and sounders operate on low voltages and currents that are compatible with intrinsically safe energy limits. This makes intrinsic safety the most practical protection method for conventional fire alarm field devices in hazardous areas.
| Detail | |
|---|---|
| How it works | Zener barriers or galvanic isolators in the safe area limit the voltage and current available to the hazardous area circuit. The field devices are designed to operate within these energy limits |
| Applicable zones | ia — suitable for Zone 0, 1, 2 (gas) and Zone 20, 21, 22 (dust). ib — suitable for Zone 1, 2 (gas) and Zone 21, 22 (dust) |
| Cable requirements | Intrinsically safe circuits must use cable that meets defined capacitance and inductance parameters — standard fire alarm cable may not be suitable and specialist cable may be required |
| Segregation | Intrinsically safe circuits must be kept physically and electrically segregated from non-intrinsically safe circuits throughout |
Flameproof and Other Protection Methods
Where higher-powered equipment is required — such as sounders and beacons that draw more current than intrinsic safety limits permit — other ATEX protection methods are used.
| Method | Designation | Principle | Typical application |
|---|---|---|---|
| Flameproof enclosure | Ex d | The equipment is enclosed in a robust housing that can contain an internal explosion and prevent it propagating to the surrounding atmosphere | High-power sounders and beacons in Zone 1 and 2 |
| Increased safety | Ex e | Additional measures are taken to prevent sparks and excessive temperatures under normal operation | Junction boxes, terminal enclosures, some sounders in Zone 2 |
| Encapsulation | Ex m | Components are encapsulated in a resin compound to exclude the explosive atmosphere | Some electronic components and sensor elements |
| Pressurised or purged enclosure | Ex p | The enclosure is maintained at a positive pressure with clean air or inert gas to exclude the explosive atmosphere | Larger equipment where other methods are not practical |
In practice
Key Considerations for Hazardous Area Fire Alarm Design
Hazardous area classification must come first
Before any equipment is specified, the hazardous areas must be formally classified by a competent person — typically a process safety or electrical engineer with ATEX expertise. Fire alarm designers cannot specify appropriate equipment without knowing the zone classification of each area. This classification also determines the required system category.
Equipment must be correctly certified
Every device used in a hazardous area must carry the appropriate ATEX or UKEX certification mark, with the correct Equipment Group, Category, and Temperature Class for the specific hazard present. Using uncertified equipment in a hazardous area is a serious legal offence under the Fire Safety Order and ATEX/UKEX regulations.
Specialist design and installation
Hazardous area fire alarm systems should be designed and installed by engineers with specific ATEX competence — not by general fire alarm contractors unless they can demonstrate this expertise. The consequences of incorrect installation in a hazardous area can be catastrophic.
Documentation
ATEX installations require thorough documentation — including the hazardous area classification drawing, equipment schedules with certification details, installation records, and inspection and maintenance records. This documentation must be maintained throughout the life of the system and kept alongside the fire alarm log book.
Maintenance by competent persons only
Maintenance of ATEX-certified fire alarm equipment must be carried out by engineers who can demonstrate specific competence in hazardous area electrical work — not simply general fire alarm servicing competence. The two are distinct disciplines and the qualifications required are different.
Competency
CompEx — the Recognised Competency Framework
In the UK, the recognised scheme for demonstrating competence in the design, installation, inspection, and maintenance of electrical equipment in explosive atmospheres is CompEx — the Competency for Ex scheme, administered by the Energy Institute and delivered through accredited training centres.
| Module | Scope |
|---|---|
| Ex01–Ex04 | Inspection and maintenance of existing electrical installations in hazardous areas — the modules most relevant to ongoing fire alarm servicing in ATEX environments |
| Ex05–Ex06 | Design and installation of new electrical equipment in hazardous areas — relevant where new fire alarm devices or circuits are being installed |
| Ex07–Ex08 | Inspection and maintenance in dust explosive atmospheres (Zones 20, 21, 22) — the dust equivalents of Ex01–Ex04 |
The right question to ask any contractor
When appointing a contractor to install, maintain, or inspect fire alarm equipment in a hazardous area, always ask which CompEx units their engineers hold — not simply whether they have “ATEX experience.” Experience is not the same as assessed competence. Holding Ex01–Ex04 covers inspection and maintenance but does not qualify an engineer for new installation work, which requires Ex05–Ex06. The specific modules held matter — a single CompEx certificate does not cover all aspects of hazardous area work.