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Fighting fire with detection.

Fighting Fire with Detection

Arson and suspected arson are the largest cause of property damage due to fire in the United States, according to the National Fire Protection Association (NFPA). The fire industry authority reported that in 1988 structural fires caused more than $700 billion in property losses. Of that figure, more than $150 billion - one of every four dollars - was caused by arson of an incendiary or suspicious nature. Although the NFPA gave some insight on arson and arsonists, studies could ascertain no pattern or place of origin for security professionals to target.

According to the NFPA, "... in most properties, incendiary and suspicious fires are not concentrated in just a few areas but are scattered among all levels of a building.... The diversity of areas of origin of incendiary and suspicious fires also underlines the importance of complete building coverage for fire protection features, like automatic sprinklers and detection and alarm systems, that mitigate losses when prevention fails."(1)

When security officers fail to stop arsonists from entering a building, a reliable, early-warning fire protection system that provides total building coverage may be the best way to mitigate damage. Specifying fire protection equipment to ensure total building coverage is more complex than specifying a security system.

In addition to analyzing the building thoroughly, specifications for effective fire protection systems require examining the characteristics and behavior of anticipated fires; the building's construction and layout; what a building is used for; the contents of a building; and the capabilities and limitations of fire equipment for early detection, warning, and control. Using a consultant as well the accompanying checklist for detection systems at the end of this article will make the specification process easier and help ensure that your system is effective.

Fire detectors are the first line of defense in a fire. According to the NFPA standard on automatic fire detectors (72E), detectors are "designed to detect the presence of fire and initiate action"(2) and can "initiate emergency action"(3) in conjunction with other fire protection equipment, such as alarm and suppression/control systems. An integrated fire system with automatic fire detectors installed to activate alarm and suppression systems will help speed up evacuation and suppression efforts.

Depending on its fuel, its starting place, and the environment, a fire may be fast or slow burning, flaming or smoldering. A fire can ignite by flames, electrical malfunctioning, vapors, or even extreme heat over an extended period.

Automatic fire detectors are designed to generate specific, early-warning responses to fires. All detectors that are tested and approved by Underwriters Laboratories must meet the same requirements. Anticipating the kinds of fires likely to occur in a building will help determine whether smoke, heat, flame, or gas will be the primary byproduct of the burned substance. Knowing this information will help you select appropriate detectors for your building.

Classifying detectors is based on each detector's strength in sensing the characteristics specific to certain fires. Types of detectors include smoke, flame, gas, heat, air duct, and special combination. The most commonly used automatic fire detectors for commercial, industrial, institutional, and residential buildings are smoke, heat, and air duct detectors. Flame and gas detectors are generally used in environments where explosions can occur.

Smoke detectors. Smoke detectors sense the airborne visible and invisible particles of combustion produced by a fire. Optical or photoelectronic smoke detection and ionization smoke detection are the two main technologies used to sense smoke produced by a fire. (See Exhibits 1 and 2.) The difference between the technologies is in design, affecting the way their sensing chambers respond to smoke particles. Photoelectronic smoke detectors respond best to smoke particles from smoldering fires, whereas ionization smoke detectors are more suitable for smoke produced by flaming fires.

In most cases, ionization detectors should be specified for invisible particles of combustion, less than the size of one micron, produced by most flaming fires. The ability of ionization detectors to detect fires decreases with larger particles of combustion typical of smoldering fires. Ionization detectors are commonly used in places that have contents - such as grease, gas, chemicals, or paper - that would immediately cause a fire if ignited.

The technology used in ionization detectors is chemical as opposed to photo-optic, The detector unit houses a small amount of radioactive material that ionizes the air in the sensing chamber at a predetermined level. The sensing chamber monitors the electrical conductance or current flow in the air between two charged electrodes. When invisible smoke particles enter this ionized area, they attach themselves to the ions, causing the air's current flow to decrease. Once the detector's sensing unit counts less than the preset number of particles per unit volume of air, it initiates an alarm.

There are two types of photoelectronic smoke detectors - light-scattering and light-obscuration. Both incorporate a light source and a photosensitive sensor to detect the change in light levels caused by smoke.

While ionization detectors detect small invisible particles of combustion produced from flaming fires, photoelectronic light-scattering smoke detectors are more responsive to visible particles of combustion, larger than the size of one micron, often produced by smoldering fires. Detection capability decreases with smaller particles of smoke, typical of flames or black smoke.

Photoelectronic light-scattering detectors are ideal for fires involving acrylic clothes, wood, sofas, foam, and other contents that emit large amounts of smoke before flames are visible. These detectors often are recommended for offices and homes, where fires are usually slow-burning and smoldering.

The light source and photosensitive sensor built into photoelectronic light-scattering units are designed so light does not enter the detector's sensing chamber. When smoke enters, the light reflected from the smoke particles scatters and strikes the sensor. The sensor initiates an alarm.

At one time, photoelectronic detectors used a photocell instead of solidstate photodiodes as sensors. Today's optical detectors are more sensitive. They are also less prone to sounding false alarms because of advances in technology, such as photodiodes, faster switching times, custom integrated circuits, infrared LEDs, and pulsing circuits to confirm the presence of smoke before sounding an alarm.

Photoelectronic light-obscuration smoke detectors (also called projected-beam detectors) also use photo-optic technology. (See Exhibit 3.) The projected-beam detectors, however, are responsive to both visible and invisible particles of combustion. They are also unaffected by color. The only real drawback is their higher cost per unit than photoelectronic light-scattering and ionization detectors.

Beam detectors create a long beam of light across an open area. They have monitoring capacities of 300 feet or more in open spaces. As such, they are ideal in large, unobstructed areas like atriums, lobbies, and warehouses.

Typical applications recommend that two beam detectors be placed on opposing walls. One detector projects a light beam onto a photodiode or other photosensitive sensor used to read the beam's light level. When smoke particles enter the created light beam, the smoke obscures it, causing less light to reach the photosensitive device. The reduced light alters the receiving device's output and initiates an alarm.

Heat detectors. Smoke-filled environments can still use automatic fire detectors to guard against arson. Heat detectors are specifically designed for places where smoke, exhaust, or flames are present every day. If these environments were equipped with smoke and flame detectors, false alarms would be frequent and costly. Places where heat detectors are installed include restaurant kitchens, garages, airplane hangars, and smoky manufacturing plants. For total building coverage, NFPA recommends installing heat detectors in closets and other enclosed areas.

Two types of heat detectors are used to sense abnormally high temperatures. Fixed-temperature heat detectors have a built-in, thermal-sensitive switch that activates the detector if the heat rises to the preprogrammed temperature.

Rate-of-rise heat detectors react when the temperature rises quicker than the predetermined rate of rise. A thermistor device in the detector changes its resistance when exposed to heat that rises faster than the preset level. The detector's external electronics then respond to the resistance change by initiating an alarm. In addition to early-warning fire detection, rate-of-rise detectors help protect contents that need to be stored at a constant temperature.

Air duct detectors. Air duct detectors are designed to prevent panic, injury, and property damaged caused by smoke and gases that spread through a building's air conditioning and ventilating system. When duct detectors sense smoke, they initiate action with other fire safety equipment to close automatic dampers, shut off fans, operate smoke doors, and activate other life-safety and property-protection procedures.

Although air duct detectors operate with photoelectronic or ionization detection technology, they usually are calibrated differently than open-area detectors in order to operate properly in the high air flow of an HVAC system. Duct detectors typically are installed directly in air ducts or are mounted outside of ducts, using sampling tubes to monitor the air inside.

Often specifications call for installing combination detectors or several detectors in a building. This is especially true in specifications for total building coverage.

Total building coverage, as defined by NFPA standard 72E on automatic fire detectors, "... shall include all rooms, halls, storage areas, basements, attics, lofts, spaces above suspended ceilings and other subdivisions and accessible spaces, and inside all closets, elevator shafts, enclosed stairways, dumbwaiter shafts, and chutes. Inaccessible areas that contain combustion material shall be made accessible and protected by detector(s).... Detectors shall also be required underneath open loading docks or platforms and their covers, and for accessible under floor spaces or buildings without basements." The NFPA concedes to certain exceptions under specific conditions and as permitted by the local jurisdiction.

Combination detectors and mixed detectors are equally effective; however, installing several different detectors to meet your building's needs is generally more cost-effective than purchasing a lot of combination detectors.

A qualified consultant is usually an excellent source of information on cost-effective, compatible equipment for various codes and applications. Specialists are also helpful in selecting appropriate automatic fire detection systems. Once you've selected appropriate fire detectors, a plan for integration must be in order to help provide total building protection against arson and other fire disasters.

In the event of arson, automatic fire detectors also can act as auxiliary security officers. In most commercial, industrial, and institutional businesses, detectors usually are wired to a central control panel and monitored by security or maintenance personnel. When the detector senses a fire, it signals to the control panel which zone of the building is burning. This signal helps fire fighters quickly locate the fire and help evacuate the building. However, in the case of arson, an early-warning fire detection system can help officers capture the arsonist.

Although automatic fire detection systems may hinder the escape of arsonists, they are no substitute for security personnel. Personnel are the foremost preventive factor against arson.

To help ensure that your building's security and fire protection systems are effective, review all codes - local, state, national, and NFPA. Plan your system around your objectives, not around those of your local city hall. Local codes often will only keep the fire from spreading to neighboring buildings. The codes usually don't have your building's security needs in mind. And they generally do not offer provisions on how to protect your building from arson. Planning a complete, security-integrated fire detection system based on your own building's needs is essential for maximum protection of your property as well as the lives of your employees.

About the Author ... Kathleen Sowder is vice president of marketing for Electro Signal Lab Inc. in Hingham, MA. (1) John J. Hall Jr., "US Arson Trends and Patterns." (Quincy: National Fire Protection Association, 1988.) An arson update can be obtained this October by contacting NFPA, One Batterymarch Park, Quincy, MA 02269-6101.

Incendiary fire is a fire showing physical or legal evidence that it was deliberately set by a person capable of understanding the act. Suspicious fires are fires showing evidence that they were deliberately set (for example, multiple points of ignition), yet there is not enough evidence to be conclusive in the judgment of the investigating officers. (2) National Fire Codes, Automatic Fire Detectors, NFPA Standard 72E, 2-1. (3) NFPA Standard 72E, 1-2.2
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Title Annotation:includes detector specification checklist
Author:Sowder, Kathleen
Publication:Security Management
Date:Oct 1, 1990
Previous Article:Checking in with fire protection.
Next Article:Igniting interest in fire safety.

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