Smoke Detector Selection and Use
features
Most fires, no matter how large they may become, begin as a small fire. The sooner a fire is detected, the better the chances are for the survival of the persons in danger.
What can be done to reduce serious injuries and unnecessary loss of life? To answer this question, let’s take a look at fires in the home. A New York City Fire Department survey indicates that 80 percent of fire deaths occur in the home. Seventy percent of these fatalities occur between 11 p.m. and 7 a.m., when most people are asleep.
Most deaths in the home result from the effects of smoke and toxic gases, and not as a result of being exposed to the flames. All the toxic products responsible for fire deaths are usually not known because detailed pathological examinations of fire victims are usually not conducted. Pathological examinations conducted have shown that carbon monoxide is the overwhelming killer. Carbon monoxide is produced by incomplete combustion and it is present in large quantities at most fires.
Respiratory stimulant
Carbon dioxide is another toxic product of almost all fires. Although not nearly as toxic as carbon monoxide, carbon dioxide is a respiratory stimulant. A 8 percent concent ration of carbon dioxide will increase the respiratory rate by 100 percent. This will, in turn, increase the intake of toxic gases at a fire, making lower concentrations of toxic gases fatal in shorter periods of time. This is known as the synergistic effect of toxic gases.
When you are asleep, your metabolic rate is at its lowest. Your respiration and, therefore, your inhalation of toxicgases will also be at its lowest rate. When you awaken into a panic situation, your respiration rate is greatly accelerated. This, in combination with the synergistic effect of the toxic gases, could rapidly result in death.
When human lives are at risk, time becomes important. Time is needed to alert the occupants, and time is needed for them to reach an area of refuge. Throughout this time period, it is essential that an escape route be passable.
Smoke detectors, properly placed and maintained in your home, will sound an alarm before the fire has time to develop toxic gases in concentrations that are fatal. This alarm gives the occupants time to escape.
Fire detectors for home use are of two general types: smoke detectors and heat, detectors. Until recent years, only heat detectors were available for home installation. These included fixed temperature, rate of rise and rate compensation detectors. They ranged from the simple and inexpensive freon-activated or windup types, to the costly rate of rise detector systems wired into the home. This type of detector, which depended upon the heat of the fire to trigger them, generally did not offer an early enough warning to sleeping occupants to provide the protection needed to save lives during a fire. Usually by the time heat buildup was high enough to trigger the alarm, the generation of toxic gases was so great that it affected the ability of the occupants to escape from the dwelling. A detector which would give an earlier alarm of fire was needed.
This type of early detection was available in the product of combustion detector, commonly called a smoke detector. These detectors were in industrial use since World War II, but their cost was beyond the means of the average householder. However, due to technological advances in the electronic field, these units became available to the average household in the late ’60s. In 1972, only about 50,000 were produced. In 1974, production rose to 2.5 million units. In 1976 8 million were produced and there are now over 100 brands from which to choose.
Ionization operation
To assist in making an intelligent selection of a smoke detector for your home, and to advise the general public as to the type they should install in their homes, let us take a look at how these smoke detectors operate.
There are two common types of smoke detectors: the ionization type and the photoelectric type.
Ionization detectors use a radioactive source (usually americium 241) to transform the air inside them into a conductor of electrical current. A small electrical current is constantly being passed through this ionized air. When the products of combustion, whether visible or invisible, enter the detector, they impede the flow of this current. The reduced current flow is used to trigger the alarm. This type usually responds quicker than photolectric detectors to a flaming fire that produces little or no smoke but slower to a smoldering fire. However, tests have indicated that they will sound an alarm for a smoldering fire in adequate time for escape before the toxic products incapacitate the occupants.
A question has been raised as to the hazard of the radioactive source used in these detectors. The United States Nuclear Regulatory Commission and the Consumers Union have conducted independent studies, and have confirmed that there is no radiation danger from these detectors. However, caution should be used in disposing of any of these detectors to prevent their falling into the hands of children, who may disassemble them and ingest the radioactive source.
These ionization detectors come with different power sources. They may be battery operated or powered by house current. Those that are powered by house current may be plugged into a receptacle or wired permanently into the electrical system.
Photoelectric operation
Photoelectric spot-type detectors have a light source, either incandescent or light emitting diode (LED), that directs a light beam into a chamber. The chamber contains a light-sensitive photo cell (usually cadmium sulfide cell) which is normally tucked out of the way of the light’s direct beam. When smoke enters the chamber, the particles scatter the light beam. The photoelectric cell now “sees” the light and triggers the alarm.
The photoelectric detector will usually respond to smoldering fires faster than the ionization type, but it is slower in responding to flaming fires. However, as in the case of the ionization detectors, tests have demonstrated that they will sound an alarm in adequate time.
The photoelectric detector comes wit h the same selection of power sources as the ionization type. Battery powered, photoelectric detectors are relatively new on the market, and use an LED as the light source. This use of the LED reduces the power requirements and makes battery powered detectors feasible.
Under expected residential fire conditions, it appears that there is little difference in the lifesaving potential between ionization detectors and photoelectric detectors. Although some response difference is noticed, depending on the type of combustion (flaming or smoldering), the difference is minimal when compared on the basis of escape time.
It is recommended that for the best coverage of the average household, a mix of ionization and photoelectric detectors be used with a combination of power sources (battery and household current).
A smoke detector in all major areas and rooms of the house will afford the highest level of protection.
For minimum protection, there should be a detector in the following locations:
One in the hallway adjacent to the sleeping area.
Where there is more than one sleeping area, one adjacent to each sleeping area.
One detector on every floor level without a sleeping area (including the basement).
It is also suggested that a smoke detector be installed in the bedroom of anyone who smokes in his bedroom.
A smoke detector should not be placed in a kitchen as it can cause nuisance alarms. If protection for the kitchen is desired, a heat detector is more suitable.
The following are a few precautions to be taken when placing detectors to ensure maximum protection.
Follow the detector manufacturer’s recommendations. If a wall installation is desired, be sure that the detector you obtain is approved for wall installation.
Dead air space warning
Avoid dead air spaces. The most desirable location is in the center of t he ceiling. When installing either a ceiling or wall detector, do not install the detector within 6 inches of the corners of the room. Do not install ceiling detectors within 6 inches of the walls. Wall detectors should be installed more than 6 inches, but not more than 12 inches below the ceiling.
When placing a ceiling detector near a doorway, do not place the detector closer to the wall above the doorway than the distance from the ceiling to the top of the door.
Do not place a detector within 2 feet of an air supply register or between the furnace air returns and the bedroom.
Do not place detectors on ceilings that are directly below an uninsulated attic.
Wall detectors should be mounted only on interior walls.
Battery operated detectors should not be installed where the temperature may go below 32°F because batteries then become unreliable.
Buy approved units
When buying a detector, make sure that it has been approved by Underwriters Laboratories, Inc., or Factory Mutual.
Be sure that any batteries are easily obtainable, easily replaced and reasonably priced.
An indicator to show that the batteries have been removed is a desirable feature.
A battery powered unit should have a low battery warning signal that will sound for a minimum of seven days— preferably longer. Always keep spare batteries on hand. In the winter, as the house cools off at night, a battery approaching its low level will start giving its low level warning. Replacing the battery will stop the warning and keep the unit in service so that protection is not lost.
If an ionization detector is selected, a dual chamber unit is desirable. The second chamber acts as a reference chamber and compensates for changes in humidyty, temperature, and atmospheric pressure. This dual chamber unit is more stable and has less of a tendency to transmit a false alarm.
The alarm should be rated at 85 decibels, or better.
LED has longer life
If a photoelectric detector is selected, an LED model is desirable due to the longer life of the LED lamp. It is also desirable that the unit have an indicator lamp (LED) that shows if the unit is working.
If a unit is installed where the alarm would not be easily heard, select a unit that is approved for multiple hookups. This will require wiring to interconnect the units.
If a household current unit is used, be sure to install it on a circuit that cannot be switched off. If a plug-in model is used, install a retaining clip to prevent accidental removal.
If a household current unit is selected, it is recommended that one be obtained that will automatically switch to battery operation if a power failure occurs.
It is important to remember that detectors require maintenance. Ionization detectors become more sensitive with the accumulation of dust and grease, while photoelectric detectors become less sensitive under the same conditions. Detectors should he cleaned at least once a year to remove dust and grease; more often if the environment requires it. Follow manufacturer’s directions for cleaning.
Have an escape plan
Keep in mind that detectors are only a part of the answer to the fire problem in the home. Even though a smoke detector has sounded its alarm, prompt and proper action must be taken by each and every member of the household to ensure safe escape.
Every member of the household should learn, and practice using, an escape route from the house, including the use of alternate routes if the principal one is impassable. Plans should be made to provide assistance to those members of the household who cannot escape by themselves.
Bedroom doors should be kept closed during sleeping hours. In the event of a fire, they should be checked for heat before opening and, if necessary to open, should be opened slowly to check for conditions in the rest of the house. Close doors behind you during your escape, but do not lock them.
Stay calm, stay low and, if necessary, crawl to the exit. Do not return to the building once outside.
Have a prearranged meeting place outside the home. This will facilitate a check to ensure that everyone has safely escaped.
Conduct exit drills with all members of the household participating at least every six months.
