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Fire detection devices can be either Manual or Electronic.                                                         

Manual Call Points                                                                                                              Go to top of page

Manual call points are used to give a manual indication of a fire. These are normally protected by means of a glass/plastic cover that needs to be broken in order to trigger the alarm.

These must be clearly recognisable and should all be identical in type when installed on the same system. The person operating this device must be made aware of the success of the operation by an audible and sometimes a visual alarm. Certain types of manual call points are reset able and these must not have a delay of more than 3 seconds before triggering an alarm. On conventional systems manual call points within an area connected to a specific zone can be included on that zone, but it must be remembered that people will not always trigger the call point closest to the fire. For this reason manual call points are often all on one zone reserved for this purpose.

 Location of manual calls points.

Manual call points should be located on all exit routes, in particular on floor landings or stairways and at exits to the open air. They should be located so that no person in the premises should need to travel more than 30m to trigger a manual call point. If it is difficult to travel at normal speed in certain areas than this should be taken into account and the manual call points should be closer than 30m. In areas where potentially hazardous conditions exist then these should also be mounted closer than 30m

 

There are various types of fire detectors, the main types are; 

                - Smoke Detectors

                - Heat Detectors 

                - Flame Detectors.

It is essential that the correct type of detector be installed in the correct environment.

 

There are several types of smoke detectors. The Main types are Optical Smoke Detectors, ionization Smoke Detectors, Aspirating Smoke Detectors and Optical Beam Detectors.

 

Optical Smoke Detectors detect visible smoke particles in a dark chamber. A light source is projected into the chamber out of view of a light sensitive receiver. When smoke enters the chamber, the visible smoke particles reflect this light onto the light sensitive receiver and trigger the smoke detector. Optical smoke detectors are more efficient at detecting denser smoke produced by smoldering materials, like those found in homes or general offices where furniture is found. Optical detectors are far better at detecting polyurethane foam than ionization detectors, which can be insensitive to larger smoke particles. Optical smoke detectors are more prone to false alarms in dusty areas. Due to environmental storage concerns optical detectors are becoming more and more popular. Lately this technology has improved at a rapid rate.

Ionization Smoke Detectors.                                                                                     Go to top of page

Ionization Detectors detect small invisible smoke particles when these enter the chamber. These smoke particles reduce the normal ionisation current flow, created by a small radioactive source, causing the detector to trigger. lonisation Detectors are more efficient at detecting fast burning flaming fires and are less likely to false alarm in areas with tobacco smoke. Diesel exhaust fumes, certain chemicals (like ether), high humidity and wind are more likely to cause false alarms with lonisation Detectors. While lonisation Detectors give off less radiation than the human body, sufficient quantities when stored together, can create harmful amounts of radiation. For this reason European authorities are phasing this type of detector out.

Aspirating Smoke Detectors.                                                                                 Go to top of page

This type of smoke detector "breathes" the air at intake points before pumping this air through a filter and then into an extremely sensitive chambers which is far more sensitive than the standard smoke detectors chamber. The air is then exposed to an intense light source and analysed for a smoke to air ratio. This information is converted into an electronic signal and the signal is then compared to algorithms. This type of detection is expensive but highly accurate and less susceptible to false alarms. One of these types of detectors can have many sampling points reducing the cost of equipment dramatically. With this type of system it is essential that piping from each sampling point to the detector be installed exactly according to the manufacturers specifications.

Optical Beam Smoke Detectors.                                                                             Go to top of page

Optical Beam Smoke Detectors use an active beam to detect visible smoke between the transmitter and the receiver. Certain modules can also detect the turbulence from a fire by detecting the refraction of the beam. If any part of the beam is obscured by smoke an alarm will be triggered. Optical Beam Smoke Detectors are ideal for protecting areas such as churches and certain factory warehouses as the detection area can be up to a 100m (or more depending on the make and model), directly between the transmitter and the receiver. This type of smoke detector can be mounted up to 25m high, far higher than any other smoke detector. This type of smoke detector can go into a state of false alarm due to birds or bats obstructing the beam, or due to bright light sources like sunlight or flash bulbs.

Heat Detection.                                                                                                                     Go to top of page

There are two types of Heat Detectors: Fixed Temperature Heat Detectors & Rate of Rise Heat Detectors. These can also be combined into one detector to provide more sensitivity using logic with an "or gate", or less sensitivity using logic with an "and gate". Both of these types of heat detection are also available in two configurations. The "Point" type configuration determines the heat at a specific place, while the "Line" type heat detector determines heat at any place along the detector length. "line" type detectors either compare heat in one area compared to the rest of the length or by comparing the heat at any point of the detector against a pre-programmed temperature or rate of rise.

Fixed Temperature Heat Detectors.                                                                        Go to top of page

Fixed Temperature Heat Detectors are designed to trigger when a specific pre-determined temperature is reached, often 60C or 80C. Fixed Temperature Heat Detectors are normally once off devices and have to be replaced after triggering. These normally have a substance that melts at the specified temperature separating the contacts. When this substance melts the contacts close and an alarm is triggered. Fixed Temperature Heat detectors are more reliable than rate of rise heat detectors and combinations of the two are normally required to have an "or gate".  Fixed temperature detectors can false alarm where ambient temperatures have not been adequately taken into account, either due to direct sunlight or heating elements being present.

Rate of Rise heat detectors.                                                                                     Go to top of page

Rate of Rise heat detectors are designed to trigger when the ambient temperature rises faster than a specific rate. This often operates using two types of metal with different expansion rates. If the temperature increases slowly, as it would on a warm day, the expansion of the one metal will slowly catch up with that of the second metal, which would not create an alarm. If the temperature were to increase rapidly the first metal would expand faster than the second metal, resulting in contact between the two metals to create an alarm. Rate of Rise Heat Detectors are less likely to respond to slow burning fires, and may not be used in escape routes.

These detectors can false alarm where low temperatures are increased to normal room temperature by closing outside loading bay doors, etc in a cold climate. Heat detectors are normally used in areas where there may be smoke present, like generator rooms, etc and are unlikely to trigger until flames reach up to the ceiling. They should be avoided in areas where small fires can create unacceptable losses like computer rooms.

Flame Detectors.                                                                                                                 Go to top of page

Flame detectors detect ultra violet and/or infra red radiation.

Ultra-violet flame detectors detect ultra-violet radiation emitted from flames and normally restrict this to specific bandwidth that are not common in solar radiation due to the ozone layer. Flame detectors are often used for outside fire detection and are often the best method of detection for fires where smouldering is unlikely such as liquid fuel or gas fires. Normally, if these are used indoors the areas are also covered with smoke or heat detectors. These must be installed within line of sight of the protected areas.