Selection of Detection Equipment Types

When choosing the type of detector to be used in a particular area it is important to remember that the detector has to discriminate between fire and the normal environment existing within the building, i.e. smoking in hotel bedrooms, fumes from forklift trucks in warehouses, steam from bathrooms, kitchens etc.

Heat detectors
Heat detectors may be the Point type (which respond to temperatures surrounding one particular spot), or the Line type (which responds to temperature change along its line).

All Point type heat detectors should include a fixed temperature element operating at a pre-determined temperature. Some may also include a rate-of-rise element designed to operate in response to a rapid rise in temperature. Heat detectors are in general less sensitive than other types of detector and should therefore not be used where a small fire will cause unacceptable losses.

Smoke detectors
There are two principal methods of smoke detection: the ionisation chamber, and the optical scatter chamber. The detection method chosen will usually depend on the type of fire risk to be protected against.

In the ionisation chamber, an electric current flows between two electrodes and is reduced by smoke. Ionisation detectors are particularly sensitive to small particle smoke such as that produced in rapidly burning fires but are relatively insensitive to large particle smoke such as that produced by overheated PVC or smouldering polyurethane foam.

In the optical chamber, light is scattered, or in some cases absorbed by smoke. Optical detectors are more sensitive to large particles found in optically dense smoke, but are less sensitive to the small particle smoke. Today, optical smoke detectors are more widely used than ionisation types due to the growing use of flame retardant materials in building construction, decoration and furnishings.

Careful consideration must be given to any specific risks that might occur.  

Siting and spacing of detectors
In a building the greatest concentration of smoke and heat will generally collect at the highest parts of the enclosed areas and it is here therefore, that detectors should normally be sited.

Depending on the category of system chosen and the nature of the property to be protected, detection will typically be placed in other high risk areas such as kitchens, boiler rooms and plant rooms.

Obstructions
(i) Heat and smoke detectors should not be mounted within 500mm of any walls or obstructions greater than 250mm.

(ii) Where the obstruction is less than 250mm, detectors should not be mounted within twice the depth of the obstruction.

(iii) If any lantern light within a protected area is 800mm or more in depth, or is used for ventilation, a fire detector should be sited in the lantern light.

(iv) Detectors should not be mounted within 1m of any air inlet of a forced ventilation system. 

(v) Other than in category L4, L5 and P2, if any vertical shaft penetrates one or more ceilings, a fire detector should be sited at the top of the shaft and on each level within 1.5m of the penetration. E.g. Lift shaft, escalator etc

(vi) Obstructions greater than 10% of the ceiling height, treat as a wall.

(vii) Cellular structure ceilings:

- Down-stands less than 10% of the ceiling height, standard spacings.

- Down-stands greater than 10% of the ceiling height, reduced spacings.

- Detectors located on beams for small cells where the width is less than 4 x the depth.

- Detectors located in the cells where the width of the cell is greater than 4 x depth.

(viii) Structural Beams:

- Provided the length of the beam is no greater than 10.6m for smoke and 7.5 for heat detectors then special spacing are provided.

- If the beams are greater than above, a detector is required in each cell.

(ix) Partitions or storage racks that reach within 300mm of ceiling should be treated as walls.

Control equipment
The control and indicating panel will depend on the size of the protected building and the extent of the automatic protection provided.

Specification of equipment would be based on numbers of zone circuits, sounder circuit, battery standby, remote centre link ancillary control relays, and other individual customer requirements.

Next to the control unit should be a diagrammatic plan showing zone locations.

Siting of control and indicating equipment:

1. In an area of low fire risk.
2. On the ground floor by entrance used by the fire brigade.
3. In an area common to all building users.
4. Where automatic detectors are in use, the control equipment area must be protected.
5. Alarm sounders must be sited next to the control unit.

Standby power supplies
Standby supplies will usually be from secondary batteries with automatic chargers. These batteries must have an expected life of at least 4 years and the code specifically bans the use of automotive type batteries.

When the mains supply fails the standby must be able to operate the alarm load for 30 minutes after a certain minimum duration. The minimum duration varies with the type of system and building occupancy.
For life protection (L) a standby duration of 24 hours is required. A greater duration may be needed if the probability of a mains failure longer than 24 hours is considered significant. For property protection (P) if the mains failure will be recognised within 6 hours, then a 24 hours standby is required, if not, then the required duration is 24 hours longer than the building may remain unoccupied, up to a maximum of 72 hours.

Wiring
All cables used for the critical signal paths, and for the mains supply to the system, should comprise one of the following:

- MICC with an overall polymeric covering, conforming to BS EN 60702-1.

- Cables that conform to BS 7629.

- Cables that conform to BS 7846.

Enhanced fire resisting cables (MICC) with appropriate methods of support and jointing should generally be used in the following:

- In unsprinklered buildings which involves the evacuation of the occupants in four or more phases.

- In unsprinklered buildings greater than 30m in height.

- In unsprinklered premises and sites where a fire could affect cables of critical signal paths in other remote areas where people might remain in occupation during a fire condition e.g. Hospitals or large sites.

MICC does not require further protection. (SWA not suitable for analogue fire systems)

Others should be protected if they are:

- Less than 2 metres above floor level (other than benign environments e.g. Offices, shops).

- Liable to damage or rodent attack

Generally fire alarm cables should be segregated from other cables of other systems.

Where the cable shares a common trunking, it should be compartmented and reserved solely for the fire alarm system.

The recommended colour for the cable is RED.

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