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. (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".).
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. 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.
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. 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. 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. The other type of detector, known as "intelligent", can be specifically identified by the fire alarm control panel. When an intelligent detector is malfunctioning, by observing the fire alarm control panel, a technician can determine exactly which device to go troubleshoot.
When considering a fire detector, one must determine how the fire alarm system powers its detectors. 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.