The component of a fire alarm system that people are most familiar with is the detector. Although most consumers are familiar with the devices in their homes called smoke detectors, many would be surprised to find out that there are many different types of detectors on the market. These types of detectors vary in what they detect, how they detect it, how they are managed by the fire alarm system itself and how they are powered. Further, there are dozens of manufacturers of detectors and they vary widely in price, depending on their features and functions. As such, anyone looking for a detector for their fire alarm system is faced with a plethora of choices. (NOTE: Since all detectors begin the process of placing a fire alarm system into alarm mode, they are one of what may be several devices on the system known as “Initiating Devices”.)
The first decision to make when selecting a detector for a system is to determine what is to be detected. Fire detectors either detect smoke or heat. (You guessed it – the former are called “smoke detectors”, the latter are called “heat detectors”.) Smoke detectors will go into alarm when a sufficient amount of smoke enters the chamber of the smoke alarm. Alternatively, heat detectors have sensors that detect heat and/or an increase in temperature of a certain amount. For example, a heat detector may be designed to go into alarm if one or both of the following conditions are met: 1) the ambient temperature at the detector rises to 130 degrees Fahrenheit or 2) the temperature at the detector rises 15 degrees Fahrenheit or more during a one minute period. The former type is known as a “fixed temperature” heat detector, while the latter is known as a “rate of rise” heat detector. (There are also combination fixed temperature/rate of rise detectors on the market.) Since there are several factors that could cause a fire, but not cause a rise in temperature at the location of the heat detector, including the distance from the fire to the heat detector and normal variations in temperature in the building, heat detectors are not as sensitive to fire as are smoke detectors and are, therefore, not considered reliable enough to be deemed “life safety” devices. As such, they are considered best suited for property protection applications. They also may be used in locations where there is normally a lot of dust or smoke in the air, which would cause false alarms to be triggered by smoke detectors.
Once the decision of what to detect is made, the decision of how to detect it is next. In the case of heat detectors, there is generally only one method used to detect heat. A heat detector will have two resistors which are sensitive to temperature (called “thermistors”) on an electrical circuit in its chamber. When the temperature rises to a high enough point (or, in the case of rate of rise heat detectors, rapidly enough during a specified time period) to deteriorate one of the resistors, the circuit is broken and the heat detector goes into alarm. Smoke detectors sense smoke in their chambers in one of two ways. Photoelectric smoke detectors have both a light source and a light receptor in their chambers. In their normal state, the light from the light source does not make contact with the light receptor. When smoke enters the chamber, however, some of the rays from the light source scatter, thereby contacting the light receptor and putting the detector into alarm. Ionization smoke detectors have charged air in their chambers. The charged air serves as a conductor for electrical current from one part of the chamber to another. When smoke enters the chamber, it reduces the charge in the air, breaking the circuit for the current, putting the detector into alarm.
The third differentiating factor when considering a fire detector for a fire alarm system is the method used by the fire alarm control panel to manage the detector. One type of detector, known as “conventional”, is not differentiated from other devices on the same electrical circuit. That is, the fire alarm control panel can detect that there is a malfunctioning device on a particular circuit, but a technician can not discern which specific device on that circuit is working improperly. The other type of detector, known as “intelligent”, can be specifically identified by the fire alarm control panel. As such, when an intelligent detector is malfunctioning, by observing the fire alarm control panel, a technician can determine exactly which device to go troubleshoot. Conventional and intelligent devices can not be substituted for one another. The benefits of having an intelligent fire alarm system are evident in maintenance, troubleshooting and alarm situations. The drawback is, of course, cost (relative to a conventional system).
Finally, when considering a fire detector, one must determine how the fire alarm system powers its detectors. Some fire alarm systems power their detectors directly from the fire alarm control panel. Such detectors will have two wires, which carry both power and signals. Other fire alarm systems require their detectors to obtain their power from elsewhere. Such detectors will have four wires; two of which connect to the fire alarm control panel and carry signals back and forth from the detectors to the panel; the other two of which provide the conduit for power.
If a fire alarm system owner considers all of these factors and how well they fit his particular application and system, he can be assured of making the right choice when selecting a detector.