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Life or Property
BS5839 Part 1, classifies systems and divides them into six different types:
L1 Intended for life safety and covering the whole building
L2 Intended for life safety and covering escape routes and other areas of high risk
L3 Protecting only the escape routes
P1 Intended for property protection and covering the entire building
P2 Intended for property protection covering any potentially high risk areas
M Intended for giving the alarm in response to operation of a manual call point and having no means of automatic detection

System Design
Before staring 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.
The information which should be available includes:
A The type of system required ie: L1, L2, L3 etc and where appropriate, parts of the premises to be covered.
B The action to be taken in the event of fire
C Whether other occupants of a multi occupancy building will be affected
D Whether other work is to be done at the same time. If so then consultation with other contractors may be required.
E A Method of calling the Fire Brigade
F Whether the type of occupants or activity in the building will require a greater provision of Manual Call Points than normal
G A likely attendance time of the Fire Brigade
Coverage L1 Systems
In an L1 System, all areas of the building should be covered. It is normal not to cover the following areas:
  Lavatories and Water Closets unless they contain electric hand dryers
  Voids less than 800mm in height

Coverage L2 Systems
Coverage of L2 systems depends on the vulnerability of the likely occupants and the probability of ignition of fires, ie:
Sleeping Areas without supervision

Areas having a high probability of ignition ie: day accommodation, store rooms, kitchens and plant rooms

Where occupants are especially vulnerable due to illness, age or are unfamiliar with the building. It should be noted that L2 systems always include L3 coverage.


Coverage L3 Systems
In L3 systems, coverage should be provided for escape routes and any areas in which the occurrence of a fire would directly threaten escape routes. The following areas should therefore be covered:
Corridors, passages and circulation areas
The top of vertical risers
At each level within 1.5m of an access point to a lift shaft or other vertical riser
In all rooms opening onto an escape route

Coverage P1 Systems
In a P1 system all areas of the building should be covered with the exception of:
Lavatories and Water Closets
Voids less than 800mm in height and such that extensive spread of fire or fire products cannot take place in them prior to detection by detectors outside the void

Coverage P2 Systems

P2 systems give coverage of only part of the building. The areas covered would normally have a high fire risk and unprotected areas should be separated by fire resisting construction.

Both P1 and P2 systems would invariably be connected to the Fire Brigade via a central station. It would be on an extremely irregular basis that you would ever be requested to fit a P type of system.

Manual only Systems

An A M@ system provides for Manual only alarm and systems intended for multi occupancy buildings are given the suffix A X @.

The Survey
If you are surveying a building we would suggest you start at the top of the building and work down. Two basic reasons for this are as follows:
A If you are as athletic as the writer of this guide then you can ride the elevator to the top floor and the rest of the survey is downhill!
B If you start at the top of the building then you can be sure to check the top of each stairwell. This will need a detector (except a Manual only System) even on an L3 System. As you work your way down the building you can then check how often detectors are required vertically down the stairwell. On Type L Systems, BS5839 states that this shall be at vertical intervals not exceeding 10.5 metres which normally works out to be every third floor. On Type P Systems, there should be detectors on every main landing.

The Design
If you are designing onto a set of drawings then we would recommend that you adopt the following procedure.
1 Identify and plan out where all break glass points are required.
2 Note where all the Sounders are required. Indicate where the Control Panel will be. You now have an > M=System.
3 Consider fitting door holders/closers onto doors which might otherwise get propped open. Doors leading onto stairwells should not be fitted with Door Holders. There was an old GLC regulation which requires a Smoke Detector to be fitted within 2m either side of the door or pair of doors fitted with a Door Holder/s - Closer/s and many authorities ie: West Sussex still require this.
4 Mark down where all detectors are required in escape routes, top of stairs, landings, ceilings at vertical intervals not exceeding 10.5m, top of vertical risers, within 1.5m of access to lift shafts and within rooms opening onto escape routes.
5 You should now effectively have the design of an L3 System and can now go on to add detectors to bring the system up to L2 or L1 as required.
6 Indicate the number of Zones that will be required.

Siting of Manual Call Points

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 [4] 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 sited [5] 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.

Siting of Sounders

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

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.

Siting 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 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. On the rear of all Photain Smoke Detector Data Sheets, a diagram showing the relevant coverage they provide is shown.

In corridors the number of detectors required depends on the corridor width. When installing Smoke Detectors the following data can be used:

Corridor Width (m)

Allowable Radius of Cover (m)

Maximum Spacing between Detectors (m)































5.0 or more



The maximum height that smoke detectors should be installed at is as follows:

Point Smoke Detectors 10.5m

Optical Beam Smoke Detectors 25m

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. Optical Beam Smoke Detectors should be mounted as follows: (m = Metres)

The minimum height above floor level = 2.7m
Maximum height above floor level = 25m
Minimum Optical Beam length = 10m
Maximum Optical Beam length = 100m
Minimum distance of Optical Beam
From a flat ceiling or apex
= 0.3m
Maximum horizontal distance between Optical
Beams measured at right angles to a Beam
= 14m
The Maximum horizontal distance between
Optical Beam and an adjacent wall or partition
= 7m

Heat Detectors

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.

On the rear of all Photain Heat Detector Data Sheets, a diagram showing the relevant coverage they provide is shown.

There is also information regarding detector coverage in corridors using Heat Detectors. As Heat Detectors are very seldom used in corridors then please consult British Standard BS5839 Part 1, should you require this information.

The maximum height that Heat Detectors should be installed at are as follows:

Grade 1 Heat Detector 9m

Grade 2 Heat Detector 7.5m

Grade 3 Heat Detector 6m

High Temperature Heat Detectors 6m

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