Fire alarm equipment
Smoke Detectors
As the name implies, these devices are designed to identify a fire while in its smoldering or early flame stages. Smoke detectors operate on either an ionization or photoelectric principle, with each type having advantages in different applications.
Photoelectric Detectors:
Photoelectric smoke detectors use light and how it is reflected to detect smoke. Normally light is projected into a smoke sensing chamber inside the detector assembly. The light hits a black background of the chamber and is absorbed. When enough smoke enters the chamber it reflects the light on to a sensor inside the chamber. This causes the sensor to indicate an alarm. Photoelectric detectors are suitable for most applications giving the fastest response to slow burning fires – the most common start to fire events. Use of photoelectric detectors is highly recommended to provide coverage for escape routes due to their superior ability to detect optically dense smoke that would easily obstruct the use of escape routes.
Ionization Detectors:
Ionization detectors were the first type of detector to be commercially developed and are also a popular choice. 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. Even when invisible smoke enters the chamber, it disrupts the flow of current and generates an alarm.
Ionization detectors have superior response to fast burning fires but an inferior response to slow smoldering fires, which are typical with modern construction materials. Ionization detectors are also less acceptable from an environmental point of view due to the radioactive material that they contain. There is increasing restriction on the transportation and disposal of ionization detectors so it is recommended that alternative types are used where possible.
Considerations in Selecting Smoke Detectors
The characteristics of an ionization detector make it more suitable for detection of fast flaming fires that are characterized by combustion particles in the 0.01 to 0.3 micron size range. Ionization smoke detectors are sensitive to the presence of ions, which are electrically charged particles produced by the chemical reactions that take place during combustion.
Photoelectric smoke detectors are better suited to detect slow smoldering fires that are characterized by particulates in the 0.3 to 10.0 micron size range. Photoelectric detectors react to visible particles of smoke.
Each type of detector can detect both types of fires, but their respective response times will vary, depending on the type of fire.
Because the protected buildings normally contain a variety of combustibles, it is often very difficult to predict what size particulate matter will be produced by a developing fire. The fact that different ignition sources can have different effects on a given combustible further complicates the selection. A lighted cigarette, for example, will usually produce a slow smoldering fire if it is dropped on a sofa or bed. However, if the cigarette happens to fall upon a newspaper on top of a sofa or bed, the resulting fire may be characterized more by flames than by smoldering smoke.
The innumerable combustion profiles possible with various fire loads and possible ignition sources make it difficult to select the type of detector best suited for a particular application.
For more information, see NFPA 72-1996, paragraphs A-5-3.6.1.2, and tables A-5- 3.6.1.1,A-5-3.6.1.2(a), and A-5-3.6.1.2(b).
Self Contained Smoke Detectors/Alarms
Some smoke detectors are self contained with a sensor to sense the smoke and a very loud electronic horn to wake people up. These are commonly used in apartment suites and houses that are technically referred to as smoke alarms. Smoke alarms have a builtin audible alarm device in addition to a smoke sensor, and are intended to warn only the occupants in the room or suite in which they are located. Smoke detectors on the other hand are connected to the building fire alarm system and are designed to initiate an alarm signal to warn the occupants of the entire building.
Self contained smoke detector/alarm can run off of a 9-volt battery or 120-volt house current. Some models run off of house current and change to battery backup if the power fails. NFPA 72 requirements dictate that alarm notification appliances (including smoke detectors with built in sounders) produce the 3-pulse temporal pattern fire alarm evacuation signal described in ANSI S3.41. (Audible Emergency Evacuation Signals)
Smoke Detectors have Limitations
Smoke detectors offer the earliest possible warning of fire. Nevertheless, smoke detectors do have limitations.
They may not provide early warning of a fire developing on another level of a building. A first floor detector, for example, may not detect a second floor fire. For this reason, detectors should be located on every level of a building.
In addition, detectors may not sense a fire developing on the other side of a closed door. In areas where doors are usually closed, detectors should be located on both sides of the door.
As already indicated, detectors have sensing limitations. Ionization detectors are better at detecting fast, flaming fires than slow, and smoldering fires. Photoelectric smoke detectors sense smoldering fires better than flaming fires. Because fires develop in different ways, and are often unpredictable in their growth, neither type of detector is always best. In addition, a given detector may not always provide significant advance warning of fires when fire protection practices are inadequate, or when caused by violent explosions, escaping gas, improper storage of flammable liquids such as cleaning solvents, etc.
Heat Detectors
Heat detectors warn of fire when the temperature in the area around the smoke detector reaches a certain level. The static response temperature of a heat detector should be a minimum of 29°C above the maximum ambient temperature likely to be experienced for long periods of time and 4°C above the maximum temperature likely to be experienced
for short periods of time.
Heat detectors are highly reliable and have good resistance to operation from nonhostile sources. They are also very easy and inexpensive to maintain. On the down side, heat detectors do not notice smoke. They do not function until room temperatures have reached a substantial temperature, at which point the fire is well underway and damage is growing exponentially. Subsequently, thermal detectors are usually not permitted in life safety applications. They are also not recommended in locations where there is a desire to identify a fire before substantial flames occur, such as spaces where high value thermal sensitive contents are housed. However, a heat detector could be valuable additional protection in areas such as kitchens and attics, where smoke detectors are not recommended. They are not recommended for the use in bedrooms or sleeping areas. There are several types of heat detectors including:
Fixed-temperature heat detectors:
Fixed temperature heat detectors operate when the sensing mechanism reaches its specific temperature threshold. Usually there is a fusible metal element which melts and causes a short on the initiating circuit.
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The most common units are fixed temperature devices that operate when the room reaches a predetermined temperature (usually in the 135°-165°F/57°-74°C). 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. A typical set temperature might be 57.2ºCentigrade. These detectors are non-restoring type (because it is destroyed when activated) and have to be replaced, if another setting is required.
When a fixed temperature device operates, the temperature of the surrounding air will always be higher than the operating temperature of the device itself. This difference between the operating temperature of the device and the actual air temperature is commonly spoken of as thermal lag, and is proportional to the rate at which the temperature is rising.
Rate-of-rise (ROR) heat detectors:
The second most common type of thermal sensor is the rate-of-rise detector, which identifies an abnormally fast temperature climb over a short time period. Rate of rise detectors also have a fixed temperature backstop to ensure that even very slow increases in temperature will eventually raise an alarm, if the increase continues for a sufficiently long period. Rate of rise detectors are not usually used for suppression systems because they operate on a 12 to 15ºF temperature rise per minute. This makes them too sensitive to sudden environmental changes causing false alarms and unexpected discharges.
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The rate of rise type is the most sensitive type of heat detector, particularly when used in areas where the ambient temperature can reach low levels and therefore create a large difference between the ambient temperature and the trigger temperature of a fixed temperature detector. In order to avoid false alarms, the rate of rise detectors should not be used in areas subject to frequent temperature swings, such as in kitchens, boiler rooms and warehouses with large doors to open air. In most of these detectors, when the rate of rise element alone has been activated, the detector is self-restoring.
Both rate of rise and fixed temperature heat detectors are “spot type” detectors, which mean that they are periodically spaced along a ceiling or high on a wall and are suitable for inclusion in open, closed or line monitored systems.
Rate Compensating Type:
Rate compensating heat detectors operate when the surrounding air temperature reaches a specific temperature threshold. As a result the thermal lag associated with fixed temperature detectors is eliminated. Usually there is a hermetically sealed tube with two (2) sensing elements, an outer metal tube and an internal pair of bi-metallic struts which are connected to both ends of the tube. During a slow rise in temperature, the struts and the outer shell expand at the same time until the unit reaches its specific temperature value, and operates. As the temperature rises quickly, the outer shell expands faster than the struts, pulling them closer together, allowing the contacts to close sooner. This compensates for the thermal lag time.
Fixed Temperature Line Type Detector:
The fourth detector type is the fixed temperature line type detector, which consists of two cables and an insulated sheathing that is designed to breakdown when exposed to heat. 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. The advantage of line type over spot detection is that thermal sensing density can be increased at lower cost.
Considerations in Selecting Heat Detectors
Each type of heat detector has its advantages, and one cannot say that one type of heat detector should always be used instead of another. If you were to place a rate-of-rise (ROR) heat detector above a large, closed oven, then every time the door is opened a false alarm could be generated due to the sudden heat transient. In this circumstance the fixed threshold detector would probably be best. If a room is protected with a fixed heat detector filled with highly combustible materials, then a fast flaming fire could exceed the alarm threshold due to thermal lag. In this case the ROR heat detector may be preferred.
A general comparison of smoke v/s heat detectors is as follows:
1) A smoke detector transmits a signal to the control unit when the concentration of airborne combustion products reaches a predetermined level. A heat detector transmits a similar signal when the temperature reaches a predetermined level or when there is an abnormal rate of temperature rise.
2) The key advantage of smoke detectors is their ability to identify a fire while it is still in its incipient. As such, they provide added opportunity for emergency personnel to respond and control the developing fire before severe damage occurs. Smoke detectors give the earliest warning of fire, typically responding to a fire 1/10th of the size as that required to operate a heat detector.
3) Heat detectors are not prone to false alarms although it is rather insensitive to smoldering fires of low temperature. Heat detectors are, therefore, preferred for the environments where the ambient conditions might cause false alarms.
4) Heat detectors must be mounted closer together than smoke detectors, so while the mounting bases are compatible for all types, care should be taken to ensure that the spacing between detectors is appropriate for the detector type fitted. With analogue systems it is possible for the photo thermal detector to act as a thermally enhanced smoke detector during certain times and as a pure heat detector at other times. If this mode of operation is envisioned, then spacing must be those appropriate for heat detectors.
Beam Detectors
Beam detectors provide a cost effective method of covering wide open plan areas such as galleries and atria, however care should be taken that activities in the space do not obstruct the beam, and that the building structure is such that the beam does not ‘move’ or false operation may result. This detector consists of two components, a light transmitter and a receiver, that are mounted at some distance up to 300 ft (100 m) apart.
As smoke migrates between the two components, the transmitted light beam becomes obstructed and the receiver is no longer able to see the full beam intensity. This is interpreted as a smoke condition, and the alarm activation signal is transmitted to the fire alarm panel.
If optical beam detectors are mounted within 2 feet (600 mm) of the ceiling level, they should be positioned such that no point in a protected space is more than 25 ft (7.6 m) from the nearest part of the optical beam. Should the beam detector be mounted more than 600 mm below ceiling level, then spacing should be altered to 12.5% of the height of the beam detector above the highest likely seat of any fire.
Other than the part of the beam within 500 mm of the beam’s transmitter or receiver, if any other section of a beam which runs closer than 500 mm to any wall partition or other obstruction to the flow of hot gasses, that section of the beam should be discounted from providing protection.
Where optical beam detectors are mounted in the apex of pitched roofs then the same enhanced spacing can be applied as for point smoke detectors.
The area covered by a single optical beam detector should not exceed that of a single detection zone.
Aspirating Systems (VESDA)
Air aspirating detectors are extremely sensitive and are typically the fastest responding automatic detection method. This type of system aspirates the smoke from various locations into a tube where the smoke is analyzed electro-optically by a line of sight transmitter-receiver set. This device consists of two main components: a control unit that houses the detection chamber, an aspiration fan and operation circuitry; and a network of sampling tubes or pipes. Along the pipes are a series of ports that are designed to permit air to enter the tubes and be transported to the detector. Under normal conditions, the detector constantly draws an air sample into the detection chamber, via the pipe network. The sample is analyzed for the existence of smoke, and then returned to atmosphere. If smoke becomes present in the sample, it is detected and an alarm signal is transmitted to the main fire alarm control panel.
Aspirating systems should be specified where protection is required in areas such as cold stores or areas where a very fast response to fire is needed, and while each sense point can be considered a smoke detector, special training is needed to design such systems as they are normally required to cover special risks. Many high technology organizations, such as telephone companies, have standardized on aspiration systems. In cultural properties they are used for areas such as collections storage vaults and highly valuable rooms. These are also frequently used in aesthetically sensitive applications since components are often easier to conceal, when compared to other detection methods.
Flame Detectors
The Optical detector is an electronic device containing electro-optical sensors that are sensitive to electromagnetic radiation in the UV, VIS, IR spectral bands. The Optical detector “sees” the fire by detecting the electromagnetic radiation emitted by the combustion products. They are line of sight devices that operate on either an infrared, ultraviolet or combination principle. As radiant energy in the approximate 4,000 to 7,700 angstroms range occurs, as indicative of a flaming condition, their sensing equipment recognizes the fire signature and sends a signal to the fire alarm panel. The advantage of flame detection is that it is extremely reliable in a hostile environment.
They are usually used in high value energy and transportation applications where other detectors would be subject to spurious activation. Common uses include locomotive and aircraft maintenance facilities, refineries and fuel loading platforms, and mines. A disadvantage is that they can be very expensive and labor intensive to maintain. Flame detectors must be looking directly at the fire source, unlike thermal and smoke detectors which can identify migrating fire signatures. Their use in cultural properties is extremely limited.
DETECTORS SUB-CLASSIFICATION
To further break down the detector groupings, there are two sub-groups known as “Spot type” and “Line type” initiating devices. The NFPA definitions of Spot and Line type are as follows:
NFPA Preferred Definition of a Line type device
A device in which detection is continuous along a path. Typical examples are rate-of-rise pneumatic tubing detectors, projected beam smoke detectors, and heat sensitive cable.
NFPA Preferred Definition of a Spot type device
A device in which the detecting element is concentrated at a particular location. Typical examples are bimetallic detectors, fusible alloy detectors, certain pneumatic rate-of-rise detectors, certain smoke detectors, and thermoelectric detectors.
A Spot type detector will provide coverage for a limited area, or small spot, while the line sensing type can protect or monitor very large areas, such as large atriums. Spot type detectors have a maximum theoretical rated coverage of 900 sq. ft (30 ft x 30 ft) in large open rooms. If placed in a narrow hallway, the maximum allowed rated coverage might be increased.
The Line type sensors are typically of the “projected beam” or heat sensitive cable variety. In the average residential home, all detectors will most likely be of the Spot type. The maximum theoretical rated coverage area for the projected beam detector can be as large 20,000 sq. ft. Note: This maximum coverage area for Spot type and Line type detectors is only a general statement, and should not be used in every circumstance.
