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**.

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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.

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This standard covers:

* classification of hazardous zones

* equipment certification

* protection techniques

* wiring methods

* earthing and bonding requirements.

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This standard defines:

* inspection regimes

* maintenance procedures

* periodic testing requirements

* certification documentation.

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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**.

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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.

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A typical intrinsically safe fire detection circuit consists of:

The barrier ensures that the electrical energy entering the hazardous area remains below ignition thresholds.

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Hazardous locations are classified based on the likelihood of explosive atmospheres being present.

| Zone | Description |

| ------ | --------------------------------------------------- |

| Zone 0 | Explosive atmosphere continuously present |

| Zone 1 | Explosive atmosphere likely during normal operation |

| Zone 2 | Explosive atmosphere unlikely, short duration only |

| 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.

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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**.

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When designing fire alarm systems for hazardous areas, the following must be considered:

A **hazardous area assessment** must determine:

* zone classification

* gas group

* temperature class.

Intrinsic safety barriers must be compatible with:

* the fire alarm equipment

* the field devices.

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**.

Cabling may require:

* segregation from non-IS circuits

* specific earthing arrangements

* identification as intrinsically safe circuits.

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Systems installed in hazardous areas must be maintained in accordance with:

Maintenance considerations include:

* inspection of Ex equipment certification

* barrier integrity checks

* cable gland inspection

* verification of earthing and bonding.

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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.

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* 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**.