Fire Alarm Systems
The choice of fire alarm system depends on the building structure, the purpose and use of the building and current legislation. In new or altered buildings the enforcement body is the local building control. All existing buildings except domestic premises are subject to The Regulatory Reform (Fire Safety) Order 2005 and the Responsible Person, as defined in the order, has to conduct a fire risk assessment. Whilst the main aim of your protection strategy should be focused on prevention, automated early warning detection offers essential protection. Each second saved means more time to safely evacuate the building and will also minimise potential damage and disruption.
Types of Fire Alarm
All Fire Alarm Systems essentially operate on the same principle. If a detector detects smoke,heat or someone operates a break glass unit (manual break point), then alarm sounders operate to warn others in the building that there may be a fire and to evacuate. It may also incorporate remote signalling equipment, which would alert the fire brigade via a central station.
Before starting, the designer 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.
Control of Indicating Panels
Control and indicating equipment performs three principal functions, namely:
- Automatic monitoring and control of circuits external to the equipment, such as fire detection and fire alarm device circuits and supply of power to these circuits
- Indication of fire signals, fault signals and their location
- Manual control to facilitate actions such as testing, disablement of devices, triggering of fire signals, silencing of audible fire warnings and resetting the system after a fire signal. Panels 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. The equipment should normally be sited in an area of low fire risk and on the ground floor by the entrance used by the Fire and Rescue Service 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.
These can be divided into four main types Heat detectors, Smoke detectors, Carbon Monoxide detectors and Multi sensors detectors.Heat sensitive point detectors can again be subdivided to a further two types.
- 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.
- 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 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. Points To Consider are as follows:
There are three basic types operating by ionization, light scattering and light obscuring.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 ionizes the air passing through the chamber where a current flows between two electrodes. When any of the products of combustion enters the chamber the current flow decreases. This drop is used to initiate an alarm.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 detection can be used to protect large areas with the source and photo cell positioned some distance apart.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 ionization 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.CO fire detectors are electronic detectors used to indicate the outbreak of fire by sensing the level of carbon monoxide in the air. Carbon monoxide, usually known by its chemical formula CO, is a poisonous gas produced by combustion. They are not the same as CO detectors used for home safety which are used to protect residents against carbon monoxide produced by incomplete combustion in appliances such as gas fires or boilers.CO fire detectors use the same type of sensor but are more sensitive and respond more quickly.CO detectors have an electrochemical cell, which senses carbon monoxide, but not smoke or any other combustion products. The cells do not require much power, so the detectors can be made electrically compatible with ordinary smoke and heat detectors. As fire detectors they are effective but only for certain types of fire. Deep-seated, smouldering fires produce carbon monoxide, which can be detected some distance from the seat of the fire. For this type of fire a CO fire detector will probably operate before a smoke detector. Smoke detectors, however, will almost always give a better response to a fire that has produced a rising plume of smoke. CO fire detectors will give a poor response to flaming fires. Because CO fire detectors work on different principles from smoke detectors, their false alarm behaviour will be different. For example, they will not be affected by steam, dust, or by most cooking fumes. However, because of their high sensitivity, they may false alarm from harmless transient levels of CO produced by gas heaters starting up, or from vehicle exhaust fumes entering through a window. These events would not affect an optical smoke detector.As always, the detector must be selected for the application, to achieve the best balance between fire detection capability and false alarms. There are some known disadvantages of CO fire detectors. One is that the electrochemical cells at the heart of the detectors have a limited life typically seven years and that they are not failsafe. The detector might be “dead” with out this fact being apparent. For this reason a means of checking the CO cell has been incorporated. Another clear disadvantage is the poor response to many types of fire, especially life threatening flaming fires.This detector combines inputs from optical and heat sensors and processes them using a sophisticated algorithm. When polled by the control panel it returns an analogue count which is determined by combined responses from both optical and heat sensors. They are designed to be sensitive to a wide range of fires and may be used in place of an ionisation detector in many instances. Operating principles: Signals from the optical smoke chamber and temperature sensor are independent, and represent the smoke level and air temperature respectively in the vicinity of the detector; the detector’s micro controller processes both signals. The temperature signal processing extracts only rate of rise information for combination with the smoke signal. The detector will not respond to slow increases in temperature but a large sudden change can cause an alarm without presence of smoke, if sustained for 20 seconds. The processing algorithms in the multi-sensor incorporate drift compensation. Points to consider are as follows:
- 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 manufacturer’s recommendations
- Rate of rise detectors located in positions where abnormal increase in temperature is likely, e.g. space heating equipment, industrial processes
- 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
Manual Call Points
A Break Glass Call Point is a device which enables a person to raise the alarm by breaking the frangible element on the fascia. 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 Call Points should be fitted on the floor side of an access door to a staircase so the floor of origin is indicated at the Control Panel. Extra points should be sited, where necessary, 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. Flameproof call points are available, also handle operated points for use in areas where broken glass may cause a hazard.
- 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
Many types of alarm sounders are available and include:
- Dome bells – operating mechanism contained within the bell.
- Bells with operating mechanism external to the bell.
- Electronic solid state sounders with mono or multi tone output normally in the range of 800 – 1000 Hz.
- Small sirens operating in the range of 1,200 – 1,700 Hz.
- Sirens ranging widely in size from 0.17kw to 11kW generally operating in the frequency range of 400 – 800 Hz.
- Horns operating in the range of 300 – 400 Hz and either motor or pneumatic operations.
The following figures 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. 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.
- Operating voltage
- Installation with surface or flush conduit system
- Internal or weather-proof installations
- Bell gong size
- Motor rating of sirens, note some sirens are rated for continuous duty whilst others are rated for limited time periods.
- Television interference suppression
Two power supplies are required i.e. 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.
- System voltage
- Battery charger output
- Battery capacity
- Indication of battery and/or mains supply failure
- Secondary battery exclusive to fire protection system
- Where system voltage exceeds extra-low voltage compliance
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.
- 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, for line fault monitoring.
- 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
Connection to Fire and rescue
British Standards discuss the various methods available.
The methods are:
- Direct monitored line
- Shared multiplex line to a commercially operated central fire alarm depot and from there by direct line to the fire brigade
- VF remote control system
- A 999 auto dialer non-monitored, not necessarily reliable and not approved for insurance purposes.
- British Standards include the requirements for control and indicating panels for remote manned centre’s other than those operated by the local authority fire brigade.
Inspection and Servicing
This information is provided for the general guidance of fire detection and fire alarm system users. As it is a summary, it omits much of the information included in BS5839 part 1. It is therefore not intended to be a replacement for the detailed recommendations included within British Standard. Routine testing by the user: It is vital for a regular test to be undertaken to ensure that there has not been a major failure of the entire fire detection and fire alarm system that may otherwise go unnoticed.
- Weekly tests:
- Test a manual call point during working hours to cheek that the control panel and alarm sounders operate satisfactorily
- Each week, a different manual call point should be tested
- Voice alarm systems should be tested weekly in accordance with BS5839 Part 8. If the system is connected to an Alarm Receiving Centre (ARC) for calling the fire brigade, it is very important that the ARC is notified before testing commences and when it is complete.
- Monthly tests:
- Any automatically started generator used for the fire detection and fire alarm system should be tested
- Any vented batteries used as a standby power supply for the fire detection and fire alarm system inspected