Fire Detection in Hazardous and Explosive (ATEX) Areas
Some workplaces contain atmospheres where a flammable gas, vapour or dust may be present — fuel stores, certain process plants, paint and solvent areas, some agricultural and milling operations. In these areas ordinary electrical equipment is not a neutral bystander: it can be the very thing that ignites an explosion. Fire detection here is a specialist discipline where getting the equipment wrong is dangerous in itself. This guide covers the principles an engineer needs so they know when to stop and call for specialist input.
The defining point is that in a hazardous area the detector and its wiring must never become the source of ignition, which changes everything about equipment selection.
Who this is for
This is for competent fire alarm engineers who may encounter classified hazardous areas within the premises they work in. The experience level assumed is competent engineer. Use it to recognise the limits of ordinary practice; the classification of the area and the design of any detection within it are specialist matters governed by the relevant regulations and carried out by competent specialists, coordinated with BS 5839-1 where a building fire alarm is involved.
What a hazardous area means
A hazardous area is one where a flammable or explosive atmosphere may be present, whether from gases, vapours or combustible dusts. The significance for detection is that ordinary electrical equipment could act as an ignition source, so any equipment installed there must be suitable for the classified zone — typically intrinsically safe or otherwise appropriately protected, with the correct barriers. This is not a refinement of normal practice but a different regime, governed by hazardous-area classification and the applicable regulations, and coordinated with BS 5839-1 only where the building's fire alarm interacts with the area.
Why equipment selection is everything
In an explosive atmosphere the selection of equipment is the safety measure. Standard detectors and cabling are not acceptable, because they are not designed to avoid becoming an ignition source under fault or normal conditions. Instead, equipment is chosen for the classified zone using recognised protection methods, with intrinsically safe barriers or other protection as the design requires. The consequence for the engineer is simple and strict: you cannot fit a general-purpose detector, or replace a certified one with ordinary stock, in a classified area. Doing so can defeat the very protection that keeps the area safe.
Classification comes first
Detection design in these areas is always downstream of classification. Deciding whether an area is hazardous, and which zone applies, is done by competent specialists as part of managing the risk of explosive atmospheres under the relevant regulations — it is not the fire alarm engineer's call. That classification defines the zones and the equipment categories required, and the detection then works within it. If you are ever unsure whether an area is classified, the correct response is to treat it as hazardous until a specialist confirms otherwise, and to raise it with the responsible person rather than proceeding on assumption.
Servicing without introducing risk
Servicing in or near a classified area is mostly about not making things worse. Confirm the installed equipment matches the area classification and is undamaged, that intrinsically safe barriers, glands and seals are intact and correct, and that no unsuitable equipment has been introduced since. From field experience, the dangerous findings are damaged protection, well-meaning like-for-like swaps from general stock, and process changes that have altered the classification without the detection being reviewed. Use only equipment certified for the zone, never substitute from ordinary stores, and escalate anything beyond your competence.
Common points to check
Recurring issues include unsuitable or uncertified equipment fitted in a classified area, damaged barriers, glands or seals, and a process change that may have altered the classification. Confirming that only zone-appropriate, undamaged equipment is present is the essential check, alongside knowing when to stop.
When not to rely on this alone
When not to use this article: do not use it to classify an area or design detection for a hazardous zone. Those are specialist tasks under the applicable regulations, carried out by competent specialists. This article helps you recognise the situation and its limits, not resolve it yourself.
Relevant standards
Where a building fire alarm interfaces with a hazardous area, it follows BS 5839-1, a code of practice; the classification and protection of the area itself are governed by the regulations for dangerous substances and explosive atmospheres, on which HSE provides guidance. The general legal duty for fire precautions in most non-domestic premises sits under the Regulatory Reform (Fire Safety) Order 2005. Separate the legal duty from the recommended methods, work to current editions, and defer to specialist classification.
Professional disclaimer
This is an educational and workflow resource for competent engineers and does not replace the current British Standards, the applicable regulations, the hazardous-area classification, or competent specialist judgement. Hazardous-area work is specialist; verify all arrangements against current documentation and specialist advice.
Related documentation
Use this with the current BS 5839-1, the hazardous-area classification for the premises, and the applicable regulations and HSE guidance. Record equipment against the classification, and escalate any uncertainty about classification or equipment suitability to a specialist.