Types of Fire Alarm Systems
All Fire Alarm Systems essentially operate on the same principle. If a detector detects smoke or 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 signaling equipment which would alert the fire brigade via a central station.
Our Fire Alarm Systems are broken down into four categories, Conventional, Addressable, Analogue Addressable and Wireless systems.
Conventional Fire Alarm System
Conventional Fire Alarm System, 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 reason for having Zones is to give a rough idea as to where a fire has occurred. Knowing where a fire has started is indicated by a Zones at the Control Panel. The Control Panel is wired to a minimum of two sounder circuits which could contain bells, electronic sounders or other audible devices. Each circuit has an end of line device which is used for monitoring purposes.
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. Sounders are wired in a minimum of two sounder circuits exactly as a Conventional System. Loop Isolation Modules are available for fitting on to the detection loop/loops such that the loop is sectioned in order to ensure that if a fault occurs this will cause the loss of a minimal part of the system.
Analogue Addressable Fire Alarm Systems
Analogue Addressable Fire Alarm Systems are often known as Intelligent Fire Alarm Systems. With an Analogue System each detector effectively incorporates its own computer which evaluates the environment around it, and communicates to the Control Panel whether there is a fire, fault or the detector head needs cleaning. Essentially 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. With the Analogue Addressable System, up to 127 input devices i.e.: Smoke Detectors, Call Points, Heat Detectors, Contact Monitors and other interface devices may be wired to each detection loop. In addition to the 127 Input Devices, up to 32 Output Devices such as Loop Sounders, Relay Modules and Sounder Modules may be connected. Analogue Systems are available in 2, 4 and 8 loop versions which means large premises can be monitored from one single panel. Isolator units should be connected between sections of detectors as described for Addressable Systems.
Wireless Fire Alarm System
Wireless fire alarm systems are an effective alternative to traditional wired fire alarm systems for all applications. They utilise secure, licence-free radio communications to interconnect the sensors and devices (smoke detectors, call-points, etc.) with the controllers. It provides many unique benefits and is a full analogue addressable fire detection system without the need for cables.
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.
The equipment should normally be sited in an area of low fire risk and on the ground floor by the entrance 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.. 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 detectors, Heat sensitive point detectors Point detectors can again be subdivided to a further two
- 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.
These can take the form of a heat sensitive cable which will operate, at a predetermined temperature, as an open circuit device.
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.
Carbon Monoxide Detector
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.
Multi Sensor Detector
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.
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 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.
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.
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.
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.
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.
We can advise on all types of fire alarm system, thei installation and location, we can design new systems, upgrade existing and maintain those already installed.
We design and install the emergency lighting systems that guide you to safety and we can supply and fit ancillary systems such as smoke venting, air sampling, automatic door releases and fire suppression systems.
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