Note for Guidance

The Fire Safety Advice Centre

Fire Alarms

INTRODUCTION.

The choice of fire alarm system depends on the type of building structure, the purpose and use of the building, current legislation and the local fire officer’s requirements. In all new buildings the enforcement body is the local building control or in existing buildings the fire authority whose interpretation and approval must be sought before any installation work begins. This is particularly important since none of the legislation gives any detailed information on the type of system required. The addresses of the Chief Fire Officers for the counties throughout Great Britain are given in the Fire Brigades Directory.

The British Standards relating to fire alarm systems are listed at the end of this guidance and should be read in conjunction with the following information. The section on component description and design check notes points out typical items in a fire alarm system. The design check notes indicate the variation in type, size, voltage, etc. possible in fire alarm components, as a reminder and checklist for the designer and specifier. It is not the object of this  note to consider the details of different circuit types or to discuss their relative merits.

This guidance contains brief descriptions of the major components which go to make up a fire alarm system. The design check notes are intended to highlight the variables which can exist and need to be considered whilst designing and compiling a specification for component. parts.

MANUAL CALL POINTS

There are two basic types each containing a breakable front plate of either glass or plastic. The call point for use in open circuit systems contains contacts held open by the pressure of the front plate. Breaking the plate closes the contacts and initiates an alarm. Call points for closed circuit systems operate in the reverse manner, the contacts being held in the closed position and open upon the breaking of the Front plate. Alarm testing facilities are normally provided for open circuit points. Closed circuit points do not necessarily require this facility since the circuits are continuously under test. Flameproof call points are available, also handle operated points for use in areas where broken glass may cause a hazard. In certain instances internal telephone systems may be used.

Specification and Design check notes  

• Open circuit, closed circuit, fault monitored circuit
• Surface, flush mounting
• Weatherproof, internal location
• Spare breakable material
• Contact rating suitable for load under alarm conditions
• Special call points for flameproof or special hazard areas
• Hammer for call points with breakable front plates

ALARM SOUNDERS

Many types of alarm sounders are available and include:

1. Dome bells - operating mechanism contained within the bell.
2. Bells with operating mechanism external to the bell.
3. Electronic solid state sounders with mono or multi tone output normally in the range of 800 - 1000 Hz.
4. Small sirens operating in the range of 1,200 - 1,700 Hz.
5. Sirens ranging widely in size from 0.17kw to 11kW generally operating in the frequency range of 400 - 800 Hz.
6. Horns operating in the range of 300 - 400 Hz and either motor or pneumatic operations.

Figure 1 gives a broad indication of the sound levels of the various alarm sounders. Also indicated are typical sound levels for various industrial and commercial situations.

An alarm noise level of not less than 5  decibels above ambient should be provided in general areas for adequate audibility but in sleeping areas a minimum level in the order of 65 decibels and 75 decibels at a bed head to wake sleeping occupants.

It should be noted that most dome hells are intended for use with flush conduit or wiring systems. For surface wiring or conduit installations surface mounting adaptor boxes are normally required. Generally bells, electronic sounders and small sirens are available for use with 6, 12, 24 and 48V d.c. and 12, 24 and 240V a.c. Sirens are normally operated at mains voltage, single or three-phase depending on the motor rating. Outdoor sirens should be fitted with heaters and thermostats to protect against low temperature conditions.

Public address systems may be used for alarms, and visual signals may be used in special circumstances.

Specification and Design Check notes

• Operating voltage
• a.c. or d.c.
• Installation with surface or flush conduit system
• Internal or weather-proof installations
• Bell gong size
• Colour
• Motor rating of sirens, note some sirens are rated for continuous duty whilst others are rated for limited time periods.
• Television interference suppression

Whisper at three feet = 30dB        Casual conversation at three feet = 65dB       Threshold of pain = 130dB

DETECTOR HEADS 

These can be divided into two main types Heat detectors and Smoke detectors.

HEAT DETECTORS

Heat sensitive point detectors.

Point detectors can again be subdivided to a further two types.

a)   Fixed temperature which will operate when it is exposed to a pre-determined temperature. Normally fixed temperature detectors employ a fusible alloy element which must be replaced after the detector has operated. Different temperature rated elements are available to take account of varying ambient air temperatures.

b)    The second type operates on the rate of temperature rise. The rate of rise temperature detector may also include a fusible element for fixed temperature operation.

Both types are suitable for inclusion in open, closed or line monitored systems.

Linear detectors

These can take the form of a heat sensitive cable which will operate, at a predetermined temperature, as an open circuit device. Melting of the cable insulation provides a short-circuit between conductors. After operation the destroyed length of cable must be replaced. Linear detectors may be used in large areas such as warehouses. Alternative types of linear detector exist including the heat pneumatic operating on the rate of rise principle.

Specification and Design Check notes

• Open, closed, fault monitored circuits
• Temperature setting for fixed temperature fusible elements
• Spare fusible elements
• Surface or flush mountings
• Temperature setting for fusible elements in the rate of rise detection., if included
• Mounting height
• Spacing to manufacturers’ recommendations
• Rate of rise detectors located in positions where abnormal increase in temperature is likely, e.g. space heating equipment, industrial processes

SMOKE DETECTORS

There are three basic types operating by ionisation, light scattering and light obscuring.

Ionisation

These generally contain two chambers. One is used as a reference to compensate for changes in ambient temperature, humidity or pressure. The second contains a radioactive source, usually alpha particle, which ionises the air passing through the chamber where a current flows between two electrodes. When even invisible smoke enters the chamber the current flow decreases. This drop is used to initiate an alarm.

Light obscuring

In the obscuring type the smoke interferes with a light beam between a light source and photo cell, the variation in photo cell output being used to initiate an alarm. This type of detector is normally used to protect large areas with the source and photo cell positioned some distance apart.

Light scattering

The light scattering detector operates on the Tyndall effect, a photo cell and light source are separated from each other by a darkened chamber such that the light source does not fall on the photo cell. The passage of smoke into the chamber causes the light from the source to be scattered and fall on the photo cell, the cell output being used to initiate an alarm.

The light scattering and light obscuring detectors both, detect visible smoke. The ionisation detector and light scattering detector are normally each a single unit suitable for BESA conduit box mounting. In some models the smoke detector head is attached to the main body by a bayonet fixing for easy removal for maintenance or replacement. It should be noted that some detectors are suitable for two-wire circuits whereas others require three or four wire connections. Smoke detectors require a continuous power supply. Under quiescent conditions they draw a current of some 100 micro amps, and under alarm conditions, some 45 milliamps. This needs to be borne in mind when sizing the power supply. Smoke detectors generally operate on 24 d.c. Refer to British Standard Codes of Practice and manufacturers literature for information regarding the positioning of smoke detectors. Detectors are not suitable for positioning in kitchens, near fireplaces or areas with excessive’ exhaust fumes, or within 2m of air supply ducts or diffusers.

Specification and Design Check notes

• Open or closed circuit
• Fault monitored circuit
• System voltage
• Surface or flush mounting
• Detector operated indicator
• Two or three-wire system
• Quiescent current demand
• Smoke detector location
• Spare detector heads

CONTROL OF INDICATING PANELS 

Two basic types of fire alarm system exist:

1. Manual systems
2. Automatic systems which may be completely automatic or include manually operated devices.

Panels for automatic systems are fully specified in British Standards. This requires that circuits are monitored continuously and that both audible and visual indication is provided for fault and fire alarm conditions.

Further requirements include that alarm sounders may only be silenced manually, after which the control panel must provide audible and visual signals until the system is reset. Silencing of alarm sounders must not prevent the alarm being raised in other zones.

It should be noted that the standards require all fault/alarm indicator lamps to be in duplicate or a single lamp with audible signal of lamp failure.

Control and indicating panels may include facilities for operation of ancillary services such as fixed fire extinguishing, door closing etc.

Specification and Design Check notes

• Number of zones required
• Surface or flush mounting
• Maximum alarm load per alarm zone
• Automatic system, compliance with British Standard
• Manual system compliance with British Standard
• Maximum current per detector zone
• Maximum detectors per zone
• Open, closed or fault monitored system
• Single or two stage alarms
• Provision for connection to remote manned centre
• Provision for conduit and wiring compatible with building conduit and wiring system
• Provision for operation of ancillary services

POWER UNITS

British Standards  refer to power supplies for both automatic and manual systems. It recommends that a fire alarm system should operate at a voltage not exceeding 50V between conductors, or 30Va.c, or d.c. to earth, although it does not rule out the possibility of using voltages not exceeding 250V.

A fire alarm system must have its own exclusive power supply. The fully charged battery must be capable of maintaining the system in full operation for 48 hours and afterwards operate the alarm for 30 minutes.

For manual system in sma1l buildings alternative power systems may he accepted by the appropriate authority and British Standards, three possible alternatives exist, 

1. An a.c. mains power supply unit and standby primary battery, each independently able to meet the maximum alarm load with provision for automatic changeover to and from the standby battery as necessary. Fault waning must be provided for failure of the power supply unit. The primary battery capacity must be able to supply standing load for 72 hours and maximum alarm load for 30 minutes.
2. Direct from a.c. mains supply
3. Where no mains supply exists, a primary battery with one held in reserve. The battery capacity must be such as to supply the standing load for 90 days plus maximum alarm load for 30 minutes.

The maximum demand load for equipment not complying with the British Standard is defined as the load resulting from the simultaneous operation of 5% of zone circuits with a minimum of two, and for any additional load imposed on a battery when the a.c. mains supply to the system is disconnected,

British Standards requires that battery and charging equipment should incorporate automatic control features to maintain the battery within limits specified by the manufacturer and take into account any quiescent load imposed on the system. The charger should be capable of charging the battery from a discharged state together with supplying the quiescent load and fault signals. Within 24 hours the condition should be such that the battery would provide the standing load for 72 hours (24 hours if connected to a remote manned centre) and supply the load resulting from alarms originating in two separate zones for one hour. If the system provides emergency evacuation signals it should be capable of supplying the full evacuation alarm load for 10 minutes.

Where a system is not connected to a remote manned centre and the building is unoccupied for more than 72 hours the British Standard requires that either the battery capacity should be increased to account for the total unoccupied period, or inspection at intervals of less than 72 hours should be adopted.

Specification and Design Check notes

• System voltage
• Automatic systems compliance with British Standard
• Manual systems compliance with British Standard
• Battery charger output
• Battery capacity
• Indication of battery and/or mains supply failure
• Mains supply servicing F/A system only
• Secondary battery exclusive to fire protection system
• Where system voltage exceeds extra-low voltage compliance

DIVERSION RELAYS AND SUPERVISORY BUZZERS

No British Standard exists specifically for this item and indicating panels. A diversion relay permits the audible signal until the system is reset.

Specification and Design Check notes  

• System voltage
• a.c. or d.c
• Open or closed circuit system  
• Surface or flush mounting  
• Current rating of alarm system

SELF CONTAINED FIRE ALARM UNITS

These units normally incorporate break glass contact, fire alarm sounder, battery and charger. The units are suitable for small hotels, shops, guest houses etc. A single sounder can be expected to give a 3 hour alarm. This reduces to one hour when three additional 6 bells are installed. Self contained units incorporating smoke detectors are also available, They normally include visual and audible indicators of the circuit and power supply faults in accordance with the British Standard.

 Specification and Design Check notes

• Open, closed or fault monitored circuit
• Smoke detectors to be incorporated
• Operating voltage
• Number of additional bells
• Resistance of connecting cables
• Spare glasses for contact
• Mains supply connection for unit
• Spare cartridge fuses  

WIRING AND INSTALLATION

Recommendations as to suitable types of wiring and cables are included in the British Standard together with minimum conductor sizes, It also indicates suitable cable types for monitored or non monitored circuits according to the type of installation. The Institution of Electrical Engineers Regulations for the Electrical Equipment of Buildings,

Regulations refers to the necessary segregation of fire alarm circuit wiring. Provision should be made for end line devices to be fitted where necessary on line fault monitored

Specification and Design check notes  

• Conductor rating for alarm load and volt drop requirements
• Compliance with the British Standard and IEE Regulations for Segregation of Services  
• Cable type and installations suitable for monitored or non monitored systems  
• Earthing in accordance with the IEE Regulations  
• Cables suitable for ambient air temperature  
• Mains voltage power supply to control equipment in accordance with the Regulations  
• For surface laid insulated and sheathed cables protection provided where mechanical damage or attack by rodents or where cables are installed less than 2.5 metres from floor  
• MICC cables provided with PVC sheaths in corrosive or damp situations  

CONNECTION TO FIRE AND RESCUE SERVICE

British Standards discuss the various methods available. The alternative methods are:  

1. Direct monitored line  
2. Shared multiplex line to a commercially operated central fire alarm depot and from there by direct line to the fire brigade  
3. VF remote control system  
4. A 999 auto dialer non-monitored, not necessarily reliable and not approved for insurance purposes.
5. British Standards include the requirements for control and indicating panels for remote manned centres other than those operated by the local authority fire brigade.

STATUTORY REGULATIONS

In addition to government legislation, byelaws and local policy documents exist, created by local authorities at both district and county level, demanding the installation of fire alarm systems. These vary from area to area and advice must be sought from the appropriate local authority on any regulations in force.   

BRITISH STANDARDS RELEVANT TO FIRE ALARMS 

This handout provides a general overview and basic information on this topic. It may not apply to everyone, consequently to find out if this guide applies to you and to get more information on this subject, study all the relevant legislation, guides and British/European Standards. Also you should seek advice from an expert on the subject or your local Fire Safety Officer or Fire Safety Professional.

Merseyside Fire Liaison Panel. Permission is granted to print and photocopy this material for nonprofit educational uses.

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This page was last checked and modified on Saturday, 22 March, 2008 22:41