Basic Fire Detection & Alarm System Design

Basic Fire Detection & Alarm Design BS5839

This guide provides a basic overview to anyone involved in the design or installation of a fire detection system. It will identify the current legislative requirements as well as clarify the responsibilities placed on the three key roles involved with the provision of a new system, namely the Designer, Installer and Commissioning Engineer, as well as remind the End User or Owner/Occupier what part they play in ensuring that the best possible system is supplied to protect life and property from fire.

It is important that everyone involved is conversant with the current British Standard Codes of Practice BS 5839-1:2013 for general buildings and BS 5839-6:2004 for dwellings including those of multiple occupancy. The Installer should also be conversant with the British Standard relating to general wiring BS 7671.

This guide, which has been prepared by Gent by Honeywell, one of the UK’s largest manufacturers of fire detection systems, is intended to offer practical advice and is not a substitute for any of the standards or legislation referred to.

Legal elements

·         Regulatory Reform Fire Safety Order 2005*

·         Disability Discrimination Act 1995 part III (October 2004)

Regulations

·        Building Regulation Approved Document B

·         Building Regulation Approved Document M

All these laws, Building regulations and Standards in some way affect what is included in the system, however the Owner/Occupier is ultimately responsible for the level of protection provided.

It is recommended that the Owner/Occupier carries out a Fire Risk Assessment to identify the level of protection required i.e. one of the categories detailed within BS 5839-1:2013 (L1,L2,L3,L4,L5,M,P1 or P2).

The full responsibilities of the Owner/Occupier are detailed within the new Regulatory Reform Fire Safety Order* (RRO) that replaced the majority of existing laws within the UK from October 2006.

* Note the RRO, at time of press, effects England and Wales whilst Scotland is covered by the Fire (Scotland) Act and NI is still in abeyance.

Any design should be prepared by a competent individual/organisation, who has consulted all interested parties and created a set of drawings, a specification, a cause & effect or fire plan, a list of Variations and completed a G1 Design certificate, detailed within BS 5839-1:2013.

If designs are undertaken without this research being carried out, the fire detection system is unlikely to comply

The Designers’ responsibilities:

• ·         Agree the level of protection or category with Owner/Occupier
• ·         Justify any Variations and document reasons
• ·         Detail the detection & alarm zones
• ·         Prepare specification and drawings including;
• ·         Siting of manual call points
• ·         Siting of point type heat and smoke detectors
• ·         Siting of beam detectors
• ·         Siting of any other forms of detection
• ·         Specify type of cable for each circuit
• ·         Specify type of system and equipment
• ·         Include detail for on/off site links with other equipment
• ·         Take into account the risk of false alarms – use the Gent sensor application guide at the back of this section
• ·         Allow for correct level of sounders and visual alarms
• ·         Prepare a fire plan or cause and effect chart
• ·         Sign a G1 design certificate

Note BS 5839-1:2002 recommends that a fire detection system is designed by a competent person, who takes responsibility for completing the design and signing off a ‘Design certificate’ G1. This should not be confused with other certificates relating to Installation G2 and Commissioning G3, that are completed by the parties responsible for those parts.

Also if the contract allows, it is suggested that the Designer witness tests the completed system to ensure the original design is still appropriate – the Design certificate can then be completed after any amendments have been included.

Design Stage 1

Talk to the interested parties to decide on the level of protection or category and agree Variations

The importance of pre-design planning cannot be overstated. Many parties are likely to have an interest in what the fire detection is expected to do. Ultimately it is up to the Owner/Occupier, who is responsible by law, to make the final decision on the level of protection provided for a particular building.

In most circumstances the Owner/Occupier will appoint a competent Designer to carry out this work and take liability for the design as a whole.

The nominated Designer is expected to consult the following organizations:

• ·       The User or Facilities Manager
• ·       The Building Control Officer
• ·       The Health and Safety Executive
• ·       The Insurer
• ·       The Local Fire and Rescue Service
• ·       A specialist fire alarm system supplier

Issues to be covered by the designer should include:

• ·       The Fire Risk Assessment demands
• ·       The requirements necessary to comply with the Regulatory Reform (Fire Safety) Order (RRO) 2005, the Disability Discrimination Act (DDA) 1995 and Building Regulations Approved Documents B & M
• ·       prime purpose of the system (Property or life protection or both)
• ·       The level of protection suggested by the interested parties. (Category P1 or P2, M or L1 L2 L3 L4 or L5
• ·       The list of Variations identified by the interested parties

Determine the System Category or Level of Protection

Systems designed for Protection of Property only, fall into two classifications P1 or P2.

The objective of a Category P1 is to provide the earliest possible warning of a fire to minimise the time between ignition and the arrival of the fire fighters.

P1 is designed to protect the whole building whilst P2 is installed in defined parts of the building only, which may have an extraordinary high risk or hazard.

Life protection on the other hand will often depend on the number of people accessing a particular building and depending on the variations, the systems can range from simple Type M to L1 categories, these being detailed in the diagrams on this page.

These diagrams show a typical building with a number of escape routes, side rooms and open plan areas used for escape.

A Category M system requires manual call points on all exits as well as corridors where persons are not expected to walk more than 45m (see Design Stage 3) to operate one.

Category L5, designed for buildings that have a particular risk identified which warrants some special attention. For example if there is an area of high risk which is considered worthy of having some automatic detection but a manual system is also needed, then this will be termed as L5/M.

Category L4 provides detection within the escape routes only, whereas L3 not only covers these areas but all rooms leading onto the escape route. The reasoning behind this is to alert people of the danger prior to the corridor becoming “Smoke logged” so people can escape safely.

L2 is a further enhancement of protection with all the areas covered by an L3 category as well as all high risk areas such as boiler rooms etc.

L1 provides protection throughout the building, and also where Property Protection is the prime reason for the system (this allows for a choice between the P1 or P2 categories). ML5L4L3L2L1.

Design Stage 2 

Detection and Alarm Zones

Generally a building is broken down into smaller compartments to enable the fire fighters to locate the fire as quickly as possible.

Even if the system is addressable it is still considered beneficial to have a separate ‘at a glance’ indication of the location of the fire.

These compartments of a building are called detection zones, which need to comply with the following criteria.

Detection Zones

• ·     A detection zone should cover no more than 1 storey, unless total floor area is less than 300m2. Voids in the same fire compartment should be included in the same floor zone. The maximum floor area of a zone should not be greater than 2,000m2, except for some large open plan areas incorporating manual call points only, which can be extended to 10,000m2
• ·     The maximum search distance for the fire fighters to see the seat of the fire within a zone should not exceed 60m assuming the route taken is the worst possible option
• ·     Vertical structures such as stairwells, liftwells etc should be considered as separate zones
• ·     A manual call point within a staircase should be connected to the zone associated with that floor and ideally be mounted on the accommodation side of the corridor exit. Automatic sensors on the stairwell remain as part of the stairwell detection zone

Alarm Zones

An alarm zone is clearly defined within the standard but generally is an area of the building coinciding with the fire compartment boundaries. There must be a clear break between these alarm zones to ensure alert and evacuation messages are not overheard from adjacent areas.

The only other criteria is that an alarm zone may consist of a number of detection zones but not vice versa.

Alarm zones are only required when phased or staged evacuation is required. It is therefore important that care should be taken to ensure only one message is heard at any one time particularly where two alarm zones are attached.
BS5839-1 contains some recommendations for alarm zones:

• - The boundaries of all alarm zones should comprise fire-resisting construction
• - Signal overlap between alarm zones should not cause confusion
• - The same alarm and alert signals should be used throughout a building
• - A detection zone must not contain multiple alarm zones, alarm and detection zone boundaries should coincide. An alarm zone may contain multiple detection zones

Design Stage 3 

Siting of Manual Call Points

All manual call points, whatever the system, should comply to BS EN54-11 single action Type A version only and should be located as follows:

• ·  All storey exits and all exits to open air irrespective of whether they are designated fire exits
• ·  Nobody should travel more than 45 metres to reach one, except if the exit routes are undefined in which case the direct line distance should not exceed 30 metres
• ·  The above distances to be reduced to 25 and 16 metres respectively, if there are persons with limited mobility or there is a likelihood of rapid fire development
• ·  In all areas with potential high fire risk such as kitchens etc
• ·  Where phased evacuation is planned, call points will need to be sited on all exits from a particular zone
• ·  1.4 metres + or – 200mm above the floor
• ·  Call points fitted with protective hinged covers for whatever reason should be listed as a Variation

Note: In order to comply with the requirements of Building Regulations Approved Document M, which requires electrical switches including manual call points to be mounted at between 1M + or – 200mm on wheel chair access routes, these should be listed as a Variation on the certificate as BS requires MCP’s to be mounted at 1.4M + or – 200mm.

Design Stage 4 

Selection and Siting of Sensors

For further advice please refer to clauses 21 & 22 of BS 5839-1:2002.

The objective is to select the correct sensor for the appropriate application, to provide the earliest warning of fire without the risk of a false alarm.

It is therefore worth trying to visualise the type of fire that is likely to occur in a particular room or area and also to familiarise oneself with the application and the risks that could give rise to a false alarm.

It should also be remembered that a Vigilon system can incorporate a whole range of different sensors using S-Quad multi-sensors. These can be set up for different applications and can be switched from ‘state to state’ should particular risks be present for short periods of time. This is achieved by selecting the ‘enable/disable’ software within the standard panel software. At the end of this section is a full application guide for all sensors including the latest S-Quad multi-sensor with a range of selectable ‘states’ for common applications and risks.

Heat sensors complying to BS EN54-5

Heat sensor has a number of pre-programmed ‘states’ that comply with the requirements of the European standard.

Each state has its preferred use as described in the Sensor Application Guide and incorporates two types of heat sensing element. One can be described as fixed temperature whilst the other is known as a rate of rise element. These elements have a broad range of application specific operating states that will respond quickly in the event of fire without risking a false alarm. See the Sensor Application Guide at the back of this section for specific advice on which state is recommended for a particular application. For example;

Under all normal circumstances point type fire detectors should be mounted on the ceiling - this ensures that the height restrictions are met together with the following table.

	Ceiling Heights (m) General Limits Rapid Attendance*
Heat detectors - class A1	9	13.5
Heat detectors - other classes	7.5	12
Point type smoke detectors	10.5	15
Optical beam smoke detectors	25	40
* Rapid attendance values can be used in type P systems providing fire brigade response time is less than 5 minutes

Smoke sensors complying to BS EN54-7

Traditionally, ‘point’ type smoke sensors have fallen into two main categories, optical or ionisation.

Due to new European Directives for the storage and transport of radioactive sources, ionisation sensors are becoming less favourable and are being replaced by multi-sensors utilising single or dual optical chambers which are also combined with heat and/or carbon monoxide sensing elements.

This creates a whole range of sensors suitable for detecting different types of fires and yet ignore signals that previously have led to false alarms such as white dust or steam particles.

The table below shows the various ‘states’ of these smoke sensor options. This should be read in conjunction with the Sensor Application Guide to ensure the correct sensor is used for a particular location.

Design Stage 5 

Choice and Siting of Alarm Sounders and Visual Alarms

Sounders and strobes are generally provided for systems designed to protect life. However, on the rare occasion when only the property is being protected it is still essential to mount a sounder adjacent to the fire control panel as well as immediately outside the main entrance for the fire fighters.

Before deciding on the number and location of sounders/visual alarms, it is important to establish the ‘Fire Plan’ or cause and effect.

If the building is not going to have a ‘one out – all out’ arrangement, the evacuation procedures must be established. Once this is known, you can then establish the alarm zone areas where different alarm messages may be given, for example an alert or an evacuation tone.

Audible alarm levels within buildings are generally accepted as 65dB(A) throughout. However, the new Standard does accept that in certain locations this can be as low as 60dB(A). This allows some degree of flexibility, although in general the majority of a site must achieve 65dB(A) or greater to be compliant.

The drawing below illustrates the areas where 60dB(A) is permitted:

For areas with high ambient background noise levels, the Standard recommends a sound level of 5dB(A) above the norm although the maximum sound levels should not exceed 120dB(A) for health & safety reasons. Finally it is essential that at least one sounder is placed within each fire compartment and the sounder choice should be common throughout the building. Bells and electronic sounders should not be mixed within the same building although the Gent S-Cubed and S-Quad both offer bell and electronic sounders allowing a system upgrade or switch over from a bell tone to an electronic tone when required.

It is maintained that to rouse sleeping persons you need to achieve a minimum of 75dB(A) at the bedhead.

Sound attenuation is affected by numerous physical structures within a room, including the people, door, furniture and materials used for floor, walls etc.

General internal doors will attenuate at least 20dB(A), whilst heavier fire doors may well attenuate by up to 30dB(A). To ensure 75dB(A) is achieved within a bedroom it is accepted that the sounder is mounted within the room rather than the corridor outside. Use of sensor sounders ensures an even spread of sound throughout the building without the need for separate louder sounders. Visual alarms are generally considered as supplementary rather than the only means of providing an alarm, and are used in areas where the dB(A) level exceeds 90dB(A) or where persons within the area have impaired hearing. The exception could be where sound of any description is undesirable, for example operating theatres, TV studios and places of entertainment where a discreet staff alarm system is the best option to avoid panic.

Visual alarms are also included as a requirement of the Disability Discrimination Act and Approved Document M of the Building Regulations and should be included in all sleeping accommodation where people with a hearing disability may be present. 

Alarm Sounders and Visual Alarms complying to BS EN54-23.

Remote indicators should be used in areas where the detector mounting position is such that the detector is not easily viewed, for example in ceiling voids. Remote indicators can also be used to dramatically reduce search distances where detectors are mounted inside rooms, such as in hotels, thus simplifying system zoning and reducing the time taken to locate the source of an alarm.

Design Stage 6 

Control equipment and power supplies

The Control panel itself should comply to EN54-2 and any power supply used should comply to EN54-4. Today the majority of Gent fire control panels incorporate their own battery and charger and as long as the guidelines for loading these systems are complied with, the battery should be sufficient to maintain the system for a period of 24 hours with half an hour alarm load thereafter.

It is however recommended that a battery load calculation is carried out to verify the standby period provided by the capacity of the battery supplied.

Repeater panels are available for most systems and are required where the fire brigade may enter a building from more than one entrance, where security staff are located away from the main panel or where operational staff need the system information in more than one location, for example in hospital wards.

All control panels including most repeaters, require two power supplies. The back up supply is built into the panel and is provided by sealed lead acid batteries, but a secure mains supply is required for the primary power source. Fuses/isolation switches should be clearly marked to ensure that the fire alarm system is not inadvertently powered down.

Irrespective of the size or type of system the control panel should be sited with the following points in mind;

• ·        In an area of relatively low fire risk
• ·        On the ground floor entrance which the fire fighters will use
• ·        In buildings of multiple occupancy, the panel should be sited within a communal area or if this does not exist, a location which is accessible at all times
• ·        Where ambient light levels, ensure visibility at all times
• ·        Fire zonal indication should be clearly displayed by LEDs or an illuminated mimic diagram – it is not acceptable to simply accept the information from an LCD or VDU display

If there are several entrances to the building, consideration should be given to the provision of repeat indicators.

Standby time for life safety systems is normally 24 hrs. For property protection this may need to be increased to up to 72hrs where the building is unoccupied over weekends.

Conventional panels and most repeater panels generally have batteries, which are sized to provide a defined level of standby autonomy based on a fully loaded system. For analogue systems, batteries are typically custom sized to suit the required configuration, because the amount and type of connected equipment can vary considerably.

BS EN 54-2:1997+A1:2006 - Control and Indicating Equipment

BS EN 54-3:2001 - Fire alarm devices: sounders

BS EN 54-4:1998 - Power supply equipment

BS EN 54-5:2001 - Heat detectors: Point detectors

BS EN 54-7:2001 - Smoke detectors: point detectors using scattered light, transmitted light or ionization

BS EN 54-10:2002 - Flame detectors: point detectors

BS EN 54-11:2001 - Manual call points (MCP / MPS)

BS EN 54-12:2002 - Smoke detectors: line detectors using an optical light beam

BS EN 54-13:2005 - Compatibility assessment of system components

BS EN 54-16:2008 - Voice alarm control and indicating equipment

BS EN 54-17:2005 - Short-circuit isolators (SCI)

BS EN 54-18:2005 - Input/output devices (CM/MM/IM)

BS EN 54-20:2006 - Aspirating smoke detectors

BS EN 54-21:2006 - Alarm transmission and fault warning routing equipment

BS EN 54-23:2010 - Fire alarm devices. Visual alarm devices

BS EN 54-24:2008 - Components of voice alarm systems: Loudspeakers

BS EN 54-25:2008 - Components using radio links

BS EN 54-31:2014 - Multi-sensor fire detectors – Point detectors using a combination of smoke, carbon monoxide and optionally heat sensors.

Design Stage 7
BS5839-1:2002 introduced more onerous requirements for the types of cables used in fire detection and alarm systems. Fireproof cables should now be used for all parts of the system and enhanced fire resistance cables should be used where there is a requirement to ensure cable integrity over a longer period of time. For example when connecting to alarm sounders or where the connection between sub-panels provides any part of the alarm signal path.

Fire alarm cables should be segregated from the cables of other systems; they should be clearly marked, preferably coloured red and should be routed through parts of the building that provide minimum risk. This latter point is particularly relevant where the use of the building is being changed - for example if a fuel store is being moved.