Fire Alarm Systems in Hazardous Areas (Intrinsic Safety & BS 5839-1)
Overview
Fire detection and alarm systems installed in hazardous areas must be designed so that the equipment and wiring cannot ignite a flammable atmosphere.
Hazardous atmospheres may exist where:
- flammable gases
- vapours
- mists
- combustible dusts
are present.
BS 5839-1 does not directly prescribe specific explosion-protection techniques such as intrinsic safety, but it requires that fire alarm systems installed in such environments comply with the relevant hazardous area electrical standards, most notably the BS EN 60079 series.
In practice, most fire detection equipment installed in hazardous zones uses intrinsically safe (IS) circuits.
Relevant Requirements in BS 5839-1
Clause 9 – Systems in Explosive Gas or Dust Atmospheres
BS 5839-1 states that where fire alarm equipment or wiring is installed in areas where explosive atmospheres may exist, special protection measures are required to prevent ignition of the atmosphere by the system.
The standard further requires that:
- systems protecting such areas must be installed in accordance with BS EN 60079-14, and
- they must be maintained in accordance with BS EN IEC 60079-17.
These standards govern:
- equipment selection
- circuit protection methods
- installation techniques
- inspection and maintenance procedures.
Hazardous Area Standards Referenced
BS EN 60079-14
Electrical installations in explosive atmospheres – Design, selection and erection
This standard covers:
- classification of hazardous zones
- equipment certification
- protection techniques
- wiring methods
- earthing and bonding requirements.
BS EN IEC 60079-17
Inspection and maintenance of electrical installations in hazardous areas
This standard defines:
- inspection regimes
- maintenance procedures
- periodic testing requirements
- certification documentation.
Explosion Protection Techniques
Various protection methods may be used when installing fire alarm equipment in hazardous environments.
Common techniques include:
| Protection Type | Description |
|---|---|
| Intrinsic Safety (Ex i) | Limits energy so ignition cannot occur |
| Flameproof enclosure (Ex d) | Explosion contained within enclosure |
| Increased safety (Ex e) | Prevents arcs, sparks and excessive temperatures |
| Pressurisation (Ex p) | Maintains protective gas pressure in enclosure |
Among these, intrinsic safety is the most widely used method for fire detection systems.
Intrinsically Safe Fire Detection Systems
Principle of Operation
Intrinsic safety works by limiting the electrical energy available in the circuit so that:
- sparks
- arcs
- thermal effects
cannot ignite the surrounding explosive atmosphere.
This is achieved using barriers or galvanic isolators that restrict voltage and current entering the hazardous area.
Typical System Architecture
A typical intrinsically safe fire detection circuit consists of:
Fire Alarm Control Panel
│
Interface / Zone Module
│
Intrinsic Safety Barrier or Galvanic Isolator
│
Intrinsically Safe Circuit
│
Ex-rated Detector / MCP in Hazardous Zone
The barrier ensures that the electrical energy entering the hazardous area remains below ignition thresholds.
Hazardous Area Zones
Hazardous locations are classified based on the likelihood of explosive atmospheres being present.
Gas / Vapour Zones
| Zone | Description |
|---|---|
| Zone 0 | Explosive atmosphere continuously present |
| Zone 1 | Explosive atmosphere likely during normal operation |
| Zone 2 | Explosive atmosphere unlikely, short duration only |
Dust Zones
| Zone | Description |
|---|---|
| Zone 20 | Combustible dust continuously present |
| Zone 21 | Dust clouds likely during operation |
| Zone 22 | Dust clouds unlikely or short duration |
Intrinsic safety is commonly used in Zone 0, Zone 1, Zone 20 and Zone 21 areas.
Fire Alarm Devices for Hazardous Areas
Typical intrinsically safe fire alarm devices include:
- smoke detectors
- heat detectors
- manual call points
- sounders
- interface modules
All equipment used in hazardous zones must be certified for the appropriate ATEX / IECEx classification.
Design Considerations
When designing fire alarm systems for hazardous areas, the following must be considered:
Hazardous area classification
A hazardous area assessment must determine:
- zone classification
- gas group
- temperature class.
Barrier compatibility
Intrinsic safety barriers must be compatible with:
- the fire alarm equipment
- the field devices.
Loop design
Many intrinsically safe circuits are implemented as:
- radial circuits, rather than loops
- due to intrinsic safety limitations on circuit energy.
However, some modern addressable systems support intrinsically safe loops using specialised interface modules.
Cable requirements
Cabling may require:
- segregation from non-IS circuits
- specific earthing arrangements
- identification as intrinsically safe circuits.
Maintenance Requirements
Systems installed in hazardous areas must be maintained in accordance with:
- BS EN IEC 60079-17
Maintenance considerations include:
- inspection of Ex equipment certification
- barrier integrity checks
- cable gland inspection
- verification of earthing and bonding.
Practical Applications
Fire alarm equipment rated for hazardous environments is commonly used in:
- fuel storage facilities
- oil and gas plants
- chemical processing plants
- paint spray booths
- battery charging areas
- grain silos
- pharmaceutical manufacturing facilities.
Key Points
- BS 5839-1 requires special protection measures where fire alarm equipment is installed in explosive atmospheres.
- Installation and maintenance must comply with the BS EN 60079 series.
- The standard does not mandate intrinsic safety specifically, but it is the most common protection method used in fire detection systems.