# 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**.