Smoke Detector

From CT.wiki

Smoke detectors are designed to alert the occupants of a building to a fire, allowing the occupants to exit before the fire grows to dangerous proportions. They are required in ALL residential buildings.

Types There are two different types of smoke detectors used in residential areas, Ionization and Photoelectric. Each unit has different strong points.

Ionization detectors rely on a small piece of radioactive material to ionize the air entering the sensing chamber. When smoke enters the chamber, it changes the conductance of the ionized air, and the change triggers the alarm. Ionization detectors are more sensitive to the smaller smoke particles emitted by flaming fires, and less sensitive to the larger particles emitted by smoldering fires. Most smoke detectors sold in home centers and retail outlets are ionization units. They are cheaper than photoelectric units.

Photoelectric detectors use a small light beam shining in the detection area. When smoke enters the chamber, it scatters the light beam, and a photo sensor picks up the scattered light and triggers the alarm. These types of detectors are more sensitive to the larger smoke particles emitted by smoldering fires, and less sensitive to the smaller smoke particles emitted by flaming fires. Photoelectric detectors are more expensive than ionization units, but because they are more sensitive to smoldering fires, they are a better fit for residential use. Photoelectric detectors also frequently have additional features such as a "clean me" function, to indicate when the detector needs to be cleaned, and more extensive self-diagnostic capabilities.

There are several additional options for smoke detectors. Some are available with built in sounders, so all detectors will sound if one is tripped. You can also get detectors with built in Heat detectors. Another option is to have the supervisory relay built into the detector. There are even dual ionization/photoelectric versions. Depending on the manufacturer, these options may be available in different combinations.

There are other types of smoke detectors, but they are specialty units normally used in industrial settings, and thus beyond the scope of this description.

Installation: Smoke detector requirements vary from jurisdiction to jurisdiction, but they are all based on NFPA 72. At minimum, there should be one detector on each floor of a dwelling.

In new construction, there should be a detector in every bedroom, plus a detector on the outside of the bedroom within 10 ft of the door. This is in addition to the detector on every floor. The detectors should also be connected in such a way that if one unit is tripped, all areas of the dwelling should be notified. In most cases, this means that the detectors are wired so that all the other detectors will sound when one is triggered. All detectors are required to draw power from the mains (120 VAC) and a form of battery backup. Because of false alarms, detectors should not be installed near bathrooms or other sources of moisture. They should also not be installed in kitchens or garages for the same reason. If detection capabilities are needed in these areas, heat detectors should be installed instead.

There are commonly two types of installations used in new construction. In the most popular, 120 VAC ionization units are installed. A single circuit powers all the detectors, and the power wire also serves to trigger the alarm in the other units when one is tripped. Because the electrician can install this type of setup during construction, it is very popular with large developers. The smoke detectors must be installed and tested prior to the house being occupied, and since burglar alarms are frequently installed after the house is occupied, this presents a scheduling problem for smoke detectors installed by a security firm. Also, because fewer people are involved, and the units themselves are cheaper, this type of installation is less expensive (and thus more popular) to the developers.

In the second type of installation, 12 VDC smoke detectors are installed, connected to a security system by a special type of cable called "firewire". Firewire is designed to resist fire, and thus will continue to operate in a fire longer than normal cables. Firewire is designated by the code FPL or FPLR, and is always colored red. In this type of installation, the mains power is connected to the security panel, where it is converted to 12 VDC and sent to the smokes. Rechargeable batteries inside the panel provide backup power. This type of installation usually uses photoelectric detectors, and can be monitored by a security company, resulting in timely dispatch of the fire department in an emergency.

Detectors attached to security systems come in 2 and 4-wire configurations. The 4-wire units have two wires for signaling, and two wires for power. The 2-wire units combine signaling and power on the same pair. 4-wire units are cheaper than 2-wire units, but they must have a supervisory relay that 2-wire units do not need. The supervisory relay is placed at the end of a run of 4-wire smokes, and connected so that if power is lost to the smoke detectors, a trouble indication will be sent to the panel. 4-wire units can be mixed and matched among manufacturers, but 2-wire units must be explicitly listed for use with the panel. Generally, if you install one model of 2-wire smoke detectors, any replacements or additional detectors should be the same model.

As mentioned earlier, there is frequently a scheduling problem in tract houses. In this case, the security company can either run another set of smoke detectors in parallel with the existing units, or they can use a special unit to connect the alarm system to the existing units. When building a custom house, the building schedule is more flexible, and the security system and smokes can be installed prior to occupancy.

Notes:

• Contrary to the opinion among many inspectors, it is not required for all detectors to sound when one is triggered. The rule is that all areas within the dwelling must be notified of the alarm. A single siren can fulfill this requirement if sufficiently loud, and there are specific sound levels that are required to be reached in every bedroom.
• Regardless of the statement above, it's usually easier to install smokes with integrated sounders than it is to argue with the inspector.
• It NEVER pays to argue with the inspector.
• Detectors should be replaced every ten years, and should be tested AT LEAST twice a year.
• NFPA 72
• Heat detectors