Residential fire accounts for the vast majority of fire deaths. One of the most powerful tools at the disposal of each and every homeowner is the automatic smoke alarm.
“With home fire deaths still accounting for 2,580 fire deaths or 80 percent of all civilian deaths, fire safety initiatives targeted at the home remain the key to any reductions in the overall fire death toll,” writes Michael Karter Jr. of the National Fire and Protection Association’s (NFPA) Fire Analysis and Research Division in Fire Loss in the United States During 2006.
According to Karter, there are five areas homeowners need to address that can reduce the likelihood of death or injury in their home. They are 1) public education; 2) smoke detector use and maintenance, and the development of escape plans; 3) use of residential sprinkler systems; 4) more fire safe products; and 5) address fire safety needs for special groups of people.
This month, we’ll take a look at automatic smoke detection and how fire equipment companies are working to better protect every person’s castle.
Home Smoke Alarms Dysfunctional
According to a report titled Home Smoke Alarms and Other Fire Detection and Alarm Equipment, published by Public/Private Fire Safety Council, an overwhelming number of homes lack proper automatic fire protection.
“An estimated 20 percent of U.S. homes have smoke alarms present but none that are working. Nearly all of this 20 percent involves dead or missing batteries, as opposed to problems with AC power,” says the report. “Nearly half of the households with nonoperational smoke alarms that gave a reason cited nuisance alarms or continuous alarming as the reason for disabling the smoke alarm. They represent roughly 21 million housing units and an estimated 30 million or more smoke alarms.”
Smoke detectors are one way to curb home fire deaths and injuries. This is because smoke detectors are made to more exacting specifications than the common smoke alarm. In addition, smoke detectors are traditionally powered by an alarm system that uses a central power supply equipped with a rechargeable battery for back-up power.
Use The Right Detector For Task
One of the first steps in providing better fire protection for homes is to use the right smoke detector for the job. In general, photoelectric smoke detectors are better suited to home applications. This is because the sensor in this type of detector is better able to detect the type of smoke usually encountered in homes.
Ionization smoke detectors are better suited to detect small, invisible smoke particulates where photoelectric models are better at detecting larger, visible smoke particulates. While the former involves the presence of flash fires that burn hotter with less visible smoke, the latter involves a slower-burning, low-energy fire that is typified by the presence of larger, more visible smoke.
The smoke chamber in a typical photoelectric smoke detector contains an emitter that transmits light as well as a light-sensitive sensor that receives it. In most conventional detectors light is generated by a light emitting diode (LED) and received using a light sensitive photo diode (LSPD).
Typically, the LED and LSPD are separated by an opaque object or the chamber is offset at an angle. In either case, light from the LED is prevented from reaching the LSPD under normal circumstances. But when smoke is introduced into the air chamber, light from the LED is refracted, striking the LSPD.
This causes the LSPD to generate current, which is monitored by a current comparator circuit within the detector. When the amount of current generated by the LSPD reaches a predefined threshold, the smoke detector will issue an alarm.
2- Vs. 4-Wire Smoke Detectors
There are two basic types of smoke detectors on the market: two- and four-wire. There are two primary differences between them that fire technicians should know. The first is how each detector derives its operating power and the second pertains to how they trigger an alarm.
Two-wire smoke detectors use two conductors for power and signal initiation. When a two-wire device detects the presence of smoke in the surrounding atmosphere, it effectively draws many times more current than it normally does in standby mode. It is this sudden increase in current that cues the fire alarm panel a smoke detector is in alarm.
Four-wire smoke detectors typically use two conductors for power and two for signaling. When an alarm occurs, current may increase on the power circuit, but it’s the act of closing a Form A relay inside the detector that cues the fire alarm panel the detector is in alarm.
Physically, there are two formats found in the construction of the smoke detectors we use in the field. The first type of smoke detector consists of two pieces — a smoke head and a base. The second type combines the base and head into one integral unit.
The two-piece method is often preferred to the single unit for two reasons. The first is because of the many ways a single smoke detector head can be used with a variety of bases. Secondly, because a separate base allows fire technicians to rough-in a new fire alarm system without installing the smoke head.
For example, a single smoke detector head can be used with any number of bases in both two- and four-wire configurations. Bases are now available that contain an additional Form C relay. Another type contains a separate sounder unit.
Installing Spot-Type Detectors
The installation of smoke detectors is not that complicated, but a thorough knowledge of the various codes that regulate their use is necessary. First, per code, always consult the manufacturer’s instructions before you commence installation.
Some of the most important aspects of installation work involve: 1) environmental conditions; 2) mounting in regard to ceiling height; and 3) the purpose of the detector. There are two basic types of smoke detectors on the market: spot and line. In the remainder of this article we will discuss spot-type detectors.
The first order of business is the environmental conditions you need to avoid when installing spot-type smoke detectors. According to Section 18.104.22.168 of NFPA 72, 2007 Edition, you must avoid the following:
Temperatures below 32° F
Temperatures above 100° F
Humidity levels above 93 percent
Air velocities greater than 300 feet per minute
When mounting a smoke detector on the ceiling, Section 22.214.171.124.1 of NFPA 72, 2007, says to maintain at least 4 inches between the detector and the sidewall. Also, when mounting a spot-type detector on a sidewall, maintain a distance of 4 to 12 inches from the ceiling.
Section 126.96.36.199.3.1 allows for a general spacing of 30 feet under normal conditions. But always consult NFPA 72, chapter 5, as well as the manufacturer’s instructions before you arbitrarily use this number.