Fire Alarm System: Introduction and Importance of Fire Alarm System



Fire alarm systems have become increasingly sophisticated and functionally more capable and reliable in recent years. They are designed to fulfil two general requirements: protection of property and assets and protection of life. As a result of state and local codes, the life-safety aspect of fire protection has become a major factor in the last two decades.

What is Fire Alarm System? Why do we use Fire Alarm System?

Fire Alarm System block diagram

All Fire Alarm Systems essentially operate on the same principle. If a detector detects smoke or heat, or someone operates a break glass unit, then alarm sounders operate to warn others in the building that there may be a fire and to evacuate. For the system protecting property, it is additionally likely that the Fire Alarm will incorporate remote signalling equipment which would alert the fire brigade via a central station.

A fire alarm system comprises a central Control and Indicating Equipment (CIE) with various types of detectors, manual call points (MCP), interface units and sounders connected to it.

Wired Fire Alarm Systems can be broken down into three categories: Conventional, Addressable and Analogue Addressable.

Addressable Fire Alarm System

The detection principle of an Addressable System is similar to a Conventional System except that the Control Panel can determine exactly which detector or call point has initiated the alarm.

The detection circuit is wired as a loop and up to 99 devices may be connected to each loop. The detectors are essentially Conventional Detectors, with an address built in. The address in each detector, is set by dial switches and the Control Panel is programmed to display the information required when that particular detector is operated. Additional Field Devices are available which may be wired to the loop for detection only ie: it is possible to detect a normally open contact closing such as sprinkler flow switch, or a normally closed contact opening. Sounders are wired in a minimum of two sounder circuits exactly as a Conventional System.

Analogue Addressable Fire Alarm System

Analogue Fire Alarm Systems are often known as Intelligent Fire Alarm Systems. There are several different types of Analogue Systems available which are determined by the type of protocol which they use. It is left up to the Control Unit to decide whether there is a fire, fault, pre alarm or whatever. Essentially however, Analogue Systems are far more complex and incorporate far more facilities than Conventional or Addressable Systems. Their primary purpose is to help prevent the occurrence of false alarms.

It should be noted that with Addressable and Analogue Addressable Fire Alarm Systems the method of wiring to a detector base is different to the method of wiring to a Conventional Base.

Conventional Fire Alarm System

In a Conventional Fire Alarm System, a number of call points or a number of call points and detectors are wired to the Fire Alarm Control Panel in Zones. A Zone is a circuit and typically one would wire a circuit per floor or fire compartment.

The Fire Alarm Control Panel would have a number of Zone Lamps. The reason for having Zones is to give a rough idea as to where a fire has occurred. The accuracy of knowing where a fire has started is controlled by the number of Zones a Control Panel has, and consequently, the number of circuits that have been wired within the building. The Control Panel would then be wired to a minimum of two sounder circuits which could contain bells, electronic sounders or other audible devices. Sounder Circuits and Detection Zones are wired in a star configuration. Each circuit would have an end of line device which is used for monitoring purposes.

The conventional system is more suited to smaller installations and usually does not contain the processing power or sophistication of the addressable system and hence is usually used in the smaller less complex installation.

Consultation for designing Fire Alarm System

It is always advisable to consult the Local Fire Prevention Officer at an early stage, regarding the legislation covering a particular premises. The Fire Officer will interpret the Fire Precautions Act or any other act covering a particular premises and advise on the particular type of Fire Alarm System that may be required.

It must be remembered that the Fire Prevention Officer is concerned with LIFE, his concern for property is secondary.

We would advise that you only consult with a Fire Prevention Officer with the client's consent.

When designing a Fire Alarm System, it is important to consult with all other interested parties, for example:

  1. The Local Fire Authority
  2. The System Installer
  3. The Health and Safety Officer
  4. Any Consultant or Architect
  5. The Insurance Company

During early discussions it is important to establish the purpose of the Fire Alarm System, ie:

  • To enhance the safety of the occupants
  • To minimise damage to the property

Whilst Insurance Companies give good discounts to clients who fit sprinkler systems the fitting of complex Fire Detection Systems seldom lead to a reduction in premiums sufficient to encourage a client to fit a Fire Alarm System for property protection. The vast bulk of Fire Alarm Systems fitted are normally for the protection of Life.

Fire Alarm System Design

Before starting the design you will need to ensure that certain information is available. This may be given in the specification or it may have to be obtained by consultation. As well as the purchaser, there may be a requirement to consult with other interested parties. The most important of which will probably be the Fire Prevention Officer of the local Fire Brigade.

Sitting of Manual Call Points in a Fire alarm System

A Break Glass Call Point is a device which enables personnel to raise the alarm by breaking the frangible element on the facia. They should be mounted 1.4m from the floor and sited where they can be easily seen.

Manual Call Points should be sited on the floor landings of stairways and at exits to open air. It should be noted that many Fire Officers prefer Call Points to be fitted on the floor side of an access door to a staircase so the floor of origin is indicated at the Control Panel. Where necessary, extra points should be sited5 so that the greatest travel distance from any point in the building to the nearest call point does not exceed 30m. A greater number of Call Points may be needed in high risk areas or if the occupants are likely to be slow in movement.

Sitting of Sounders in a Fire Alarm System

An Alarm Sounder may be a bell or electronic sounder and it must be audible throughout the building.

A minimum sound level of either 65db(A) or 5db(A) above any background noise likely to persist for longer than 30 seconds, which ever is the greater, should be produced by the sounders at any point in the building. It is unlikely that more than 65DB will be available if the sound has to carry through more than one door. If the alarm system is used in premises such as hotels, boarding houses etc where the alarm is intended to wake sleeping persons then the sound level at the bedhead should be at least 75db(A) with all doors closed. We would strongly recommend that you allow one sounder per bedroom.

A few bells sprinkled down the corridor in hotel will not produce 75db(A) at all the bedheads. It is important to note that the above audibility levels must be produced with all doors shut, after the works on site have been completed. If a Fire Officer even expects that there is a lack of audible sounders, then he is sure to check each area with a db metre and prove it. It can be costly and very inconvenient to have to return to site and fit additional sounders.

A minimum of two sounder circuits should be wired and a larger number of quieter sounders are preferable to a small number of very loud sounders. At least one sounder should be installed in each fire compartment and all sounders throughout an installation must produce a similar sound, ie: you cannot mix an electronic sounder and bells.

Choice of Detectors for Fire Alarm System

Smoke Detectors will generally detect a fire far sooner than heat detectors. It is therefore preferable to fit Smoke Detectors unless there is any possibility of false or unwanted alarms. It is not advisable for example to fit a Smoke Detector in a kitchen as anybody burning toast would cause an unwanted alarm. Heat Detectors should be fitted in boiler rooms, generator rooms, garages and dusty areas. The products of combustion produced by a boiler, a leaky exhaust on a generator or exhaust fumes from a vehicle could all cause a smoke detector to operate and produce an unwanted alarm. Fixed Temperature Heat Detectors should be installed in areas where one would normally expect a sudden rise in temperature for instance kitchens and boiler rooms.

Rate of Rise Heat Detectors should be installed where Smoke Detectors would be unsuitable but one would not expect a sudden rise in temperature for instance, garages, car parks, dusty workshops etc.

There are two basic types of Point Smoke Detectors:

  1. Ionisation chamber Smoke Detectors which are very sensitive to smoke with small particles ie: fresh cellulosic smoke and the source of almost invisible smoke one gets with burning paper and spirit. They are relatively insensitive to smoke with large particles for example, smoke produced by burning plastics or stale smoke.
  2. Optical Smoke Detectors are sensitive to optically dense smoke ie: smoke with large particles and they are relatively insensitive to optically thin smoke.

Some countries ie: Italy, Japan, Qatar only use Optical Smoke Detectors and within parts of the Middle and Far East, only Ionisation Detectors are used. Within the UK systems can comprise of a mixture of the two. The demise of most people is caused by thick dense choking smoke which is normally a greater problem than getting burnt. For this reason Optical Detectors are normally used on escape routes such as corridors and stairwells. Ionisation Smoke Detectors are normally fitted within office and other general areas.

Sitting of Fire Alarm 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 the detectors should normally be sited. Smoke Detectors Smoke Detectors should be sited so that the sensing element is not less than 25mm, nor more than 600mm below the ceiling or roof. If a protected space has a pitched or northern light roof, then Smoke Detectors should be installed in each apex. The maximum horizontal distance between any point in the area being protected and the nearest detector should be as follows:

Under flat horizontal ceilings and corridors more than 5m wide, then the maximum distance for Point Type Smoke Detectors should not exceed 7.5m. The maximum area of coverage of a Point Smoke Detector is 100 square metres.

If detectors are to be fitted in the apex of a pitched or north light roof then a row of detectors should be sited within the apex. One row of detectors should be sited at the highest point a minimum distance of 0.5m from the vertical wall. Add to the maximum horizontal distance 1% for each degree of the slope up to a maximum of 25%.

For instance a point type detector at the apex of a 20 degree slope would work out as follows: 20% of 7.5m = 1.5m.

Therefore the maximum distance between detectors = 7.5 + 1.5 = 9m. The maximum area of coverage may also be increased proportionally.

Where the passage of Smoke or Hot Gases from a position to a detector is likely to be disturbed by a ceiling obstruction such as a beam having a depth greater than 150mm but less than 10% of the height of the ceiling, then the horizontal distance should be decreased by twice the depth of the obstruction.

For instance for a Point Type Smoke Detector obstructed by a 200mm depth beam then the maximum distance between detectors = 0.2m x 2 = 0.4m.

7.5m - 0.4m = 7.1m.

Where a ceiling obstruction, such as a beam is greater than 10% of the height of the ceiling then the area either side of the obstruction should be considered as separate rooms. Ceiling beams less then 150mm in depth can be ignored. Optical beam smoke detectors are useful for covering large unobstructed roof areas such as those found in most warehouses. They can be quite cost effective as one smoke beam can provide the coverage of many individual point detectors. A smoke beam typically comprises of a Projector, a Receiver, a Remote Manual Reset Unit and a Local Power Supply with battery standby.

Heat Detectors in Fire Alarm System

Heat Detectors should be sited so that the heat sensitive element is not less than 25mm, nor more than 150mm below the ceiling or the roof. The maximum horizontal distance between any point in the area being protected and the nearest detector should be as follows:

Under flat horizontal ceilings and corridors more than 5m wide then the maximum distance between any heat detector and any wall or partition should be 5.3m. The maximum area of coverage per heat detector is 50 square metres.

Fire Alarm System Control Equipment

The Fire Alarm Control Equipment should normally be sited in an area as follows:

Preferably in an area of low fire risk and on the ground floor by the entrance used by the Fire Brigade and preferably viewable from outside of the building. It should be located in an area common to all building users and where automatic detection is in use, the Control Panel should be in a protected area. An alarm sounder should be sited next to the Control Unit, but not too near the telephone position. A suitable zone chart of the building should normally be installed adjacent to the Control Panel.

Fire Alarm System Power Supplies

Two power supplies are required ie: mains and battery and these are normally built into the Fire Alarm Control Panel. Standby batteries must allow the system to operate without mains for 24 hours longer than the building is likely to be unoccupied and then support the sounders for an additional half hour. If the mains supply is supported by an emergency generator then six hours standby plus half an hour alarm load is sufficient. All modern Fire Alarm Systems are 24 volts.

On the medium and larger sized Fire Alarm Systems, the standby batteries will often not fit within the Control Panel. Where standby batteries are contained within a separate housing, then this housing must be as close as possible to the main Fire Alarm Control Panel. If the power supply or battery housing is located more than 10 metres from the main Fire Alarm Control Panel then serious volt drop problems can arise. Standby batteries are invariably of the sealed lead acid variety. Use of nickel Cadmium Batteries is not cost effective and automotive batteries must not be fitted.

Fire Compartments in Fire Alarm System

Buildings are normally split into fire compartments with each compartment so constructed as to prevent the spread of fire from one compartment to another. Each floor and each stairwell within a building is normally a separate fire compartment. Within a small factory, the factory unit will normally be separated from the offices by "firewalls" to prevent the spread of smoke and fire from one to the other. The factory and offices will therefore be in separate fire compartments. A zone should normally only cover a single fire compartment.

Zoning in Fire Alarm System

If the total floor area (ie: the total of the floor areas of each floor of the building) is not greater than 300 square metres then the building need only be one zone, no matter how many floors it has.

In general, if the total floor area is greater than 300 square metres, then each floor should be a separate zone (or set of zones, if the floor is big enough).

There are two exceptions:

  1. If the building is sub divided into fire compartments, then any compartment communicating with other compartments only at the lowest level of the building can be treated as if it were a separate building ie: if a floor area is not greater than 300 square metres then it can all be one zone, irrespective of the number of storeys.
  2. Where stairwells or similar structures extend beyond one floor, but are in one fire compartment, the stairwell should be a separate zone.

There are two restrictions on the maximum size of a zone, irrespective of the size of the building

  1. Its total Floor area should not exceed 2000 square metres
  2. The search distance should not exceed 30 metres. This means that a searcher entering the zone by the normal route should not have to travel more than 30 metres after entering the zone in order to see a fire big enough to operate a detector, even if the fire is only visible from the extreme end of his search path. Remote indicators show an alarm in a closed area and their fitting can enable larger areas to comply to the search distance requirements.

There are two restrictions on the configuration of a zone, irrespective of its size

  1. If the zone covers more than one fire compartment, then the zone boundaries should follow compartment boundaries
  2. If the building is spilt into several occupancies, then each occupancy should lie within a separate zone (or set of zones), no zone should be split between two occupancies
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