Smoldering Stage

Smoke detectors have become increasingly important as finishes and furnishings become more flame resistant and therefore more likely to smolder for a long time without flame at temperatures too low to trigger sprinklers. NFPA 72, Household Fire Warning Equipment, and NFPA 101, Chapter 22 regulate the use of residential smoke detectors. The goal should be to provide sufficient time to evacuate residents and to take countermeasures.

The particles in smoke at the smoldering stage are large enough to be visible to the eye. Photoelectric smoke detectors use a beam of light projected to a photo-sensor. When the beam is broken by smoke, the alarm goes off. Dust, dirt, or heavy fumes can obscure both the photocell and the lamp, which along with aging of the lamp results in false alarms. Photoelectric smoke detectors require continuous maintenance and periodic recalibration. They are used for smoldering fires and smoky fires from plastics and chemicals.

Projected beam photoelectric smoke detectors (Fig. 46-2) can cover even greater distances. They use a beam transmitter and beam receiver mounted on the walls on opposite sides of the space somewhat below the ceiling. They are used in spaces with high ceilings, such as atriums, churches, malls, and auditoriums, where spot-type detectors are difficult to reach for maintenance. Projected beam detectors can be physically shielded for use in very dirty, corrosive, humid, hot, or cold areas. The range from the transmitter to the receiver is from 9 to

92 meters (30-300 ft). Units are spaced 9 to 18 meters (30-60 ft) apart. Projected beam photoelectric smoke detectors are expensive. They must have an unobstructed view, which may be a problem with exposed ductwork or pendant lighting fixtures.

Scattered light photoelectric smoke detectors are also called photoelectronic or Tyndall-effect detectors. A beam of pulsed light-emitting diode (LED) light is directed at a photocell. If the light is scattered by particles, it strikes an alarm cell. Scattered light detectors are not sensitive to normal dust, dirt, or light source depreciation and do not require continual maintenance. They are used for commercial and high-quality residential construction.

Laser beam photoelectric devices are scattered light type detectors that use a very high-sensitivity laser diode source. They are able to differentiate between smoke and dust particles, but work best in clean environments.

Air sampling detection systems sample air throughout a space by using piping with holes at sampling points. A fan powers them, and the piping is zoned to indicate the area of the problem.

A basic residential system places a listed smoke detector outside and adjacent to each sleeping area, in each sleeping room, and at the head of every stair, with at least one on every level including the basement. Combined smoke and heat detectors are recommended in the boiler room, kitchen, garage, and attic. An alarm in any detector should set off an alarm in all audible and visible units.

Codes specify which occupancies require smoke detectors but don't always give specific locations, so the

Figure 46-2 Pair of projected beam smoke detectors.

interior designer must then determine the best placement. Smoke detectors are subject to false alarms from moisture and particles in the air. The greater the sensitivity of the detector, the more false alarms. Choosing the appropriate type and avoiding placement where conditions cause problems will limit false alarms. If you must locate a smoke alarm in a poor location, use more than one type of detector, specify extra maintenance, and provide for verification of alarms.

Kitchens, laundries, boiler rooms, shower rooms, and other spaces with high humidity and steam create problems for smoke detectors. Repair shops and laboratories with open flames used in their work and garages and engine test facilities with exhaust gases affect sensors. Smoking rooms and areas near designated smoking areas can be a problem, as can areas with heavy accumulations of dust and dirt. High volumes of air movement near loading docks, exit doors, and discharging ducts and registers are also problems.

Avoid putting smoke detectors where normal cooking processes will activate the alarm in kitchens. Units are usually placed 15 to 30 cm (6-12 in.) from the ceiling when mounted on a wall. If the alarm is too close to the intersection of the wall and ceiling or too near a doorway, the air currents may carry smoke and heat past the unit. If you are unsure of proper placement, check with the manufacturer or with code officials.

Most jurisdictions require installation and hard wiring of smoke detectors in residential occupancies and hotel or motel units. Interconnected detectors tied into the building electrical system and with a battery backup are required in many new homes and homes with new additions or alterations. Other homes are required to have at least battery-operated units. Residences are usually required to have smoke detectors outside each sleeping area and on all habitable floors. Townhouses have even stricter requirements.

Smoke detectors in apartment houses, dormitories, hotels, motels, and rooming houses are governed by NFPA 101 and NFPA 72. Alarm systems are designed to provide early warning and orderly egress at times when the building occupants may be asleep. Audible and visual alarms are positioned so that all sleeping persons, including those with sight or hearing impairments, will be wakened. Be aware that living rooms may be regularly used as sleeping areas. There should be an alarm light over the door of each apartment or suite to indicate the alarm location, especially if the central panel only shows a zone location. In high-rise residential buildings, an emergency voice alarm communication system should be provided.

Smoke detectors should be located in the corridors of multiple dwelling buildings, and in service spaces and utility and storage rooms. Battery powered detectors are not permitted in multiple dwellings. All fire alarm circuits should have standby power. All alarms must be identifiable by addressing or annunciation, which indicate the location of the alarm. Annunciator panels that have a map and lights can be located at a system control panel in the building management office or at the lobby desk of a hotel or dormitory. Lobby annunciators are helpful to firefighters.

In apartments, false alarms are common from kitchen smoke and excessive dust. Some apartment building alarm systems give only a local alarm for evacuation of the apartment. A separate central heat detector system sounds a remote alarm. This reduces the number of false alarms but increases the risk of a fire growing before activation of the fire-suppression system or before firefighting crews are dispatched.

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