Fire Alarm Initiating Devices Can Be Lifesavers
A Gamewell pull station is mounted on a telephone pole in a Ridgewood, N.J., neighborhood. Photo via Ben Schumin (Wikimedia).
Heat, Smoke, Gas, Light Are Most Common Means of Early Detection
Automatic fire detectors are designed to provide early detection of fires and protection in normally unattended areas. Automatic detectors are designed to detect common signatures of combustion at an early stage before the fire gets out of hand. Heat, smoke, gas and light (radiation) are the most common means of early detection. Proper detector selection requires consideration of local environmental factors that can produce or mimic these signatures and using detectors that are immune to them yet provide the fastest possible response.
Automatic detectors are classified as spot type or linear type. Spot type detectors, which have the detection element concentrated in a single location, are the most common. Linear types have their detection element(s) spread out over an area. Signals received from the area are processed at a single location by a control unit.
Spot detectors are typically less expensive than linear detectors, but they cover a smaller area so more are required. Spot detectors are also more labor intensive to install since each unit must be installed and connected separately. An example of a spot detector would be a standard smoke detector or heat sensor.
Linear detectors are more expensive than spot detectors, but cover a larger area so fewer units are required. Linear detectors can be less labor intensive to install (depending on the type used). An example of a linear detector would be a beam smoke detector.
Heat Detectors Include Fixed, Rate-of-Rise, Thermistor Models
Heat detectors are the oldest type of automatic fire detector. They are generally mounted on or near the ceiling and are designed to detect the thermal energy from a fire. They are the least expensive and most reliable of all the automatic detectors, but they are also the slowest to respond. Heat detectors can operate under several different operating principles, but all generally respond to a specific physical change to the detection material (melting or expanding).
Fixed temperature heat detectors respond when the temperature at the detector itself reaches a predetermined, specific level. This type of detector is extremely slow to respond due to the time it takes it to reach the temperature of the surrounding air. This delay time is known as thermal lag. Fixed temperature detectors are available that can be used in areas with high ambient temperatures (above 100° F).
Fixed temperature detectors are available in restorable and nonrestorable types. Restorable detectors automatically reset themselves once the heat level has dropped below its operating range. Nonrestorable-type detectors must have their detector element or the entire detector replaced when they have been activated.
Rate-of-rise detectors respond when the temperature of the detector rises more than 15° per minute. A flexible metal diaphragm covers an air chamber in the detector such that a rapid rise in temperature expands the air in the chamber and pushes the diaphragm up against contacts. The detector self-restores after activation. An air vent allows the detector to compensate for slow temperature changes.
This type of detector usually also employs a fixed temperature element as well. A low temperature alloy holds a spring in place. If the temperature of the detector gets high enough to melt the alloy, the spring is released. The spring pushes the diaphragm into the contacts, causing an alarm condition. Once operated in this mode, the detector must be replaced.
Thermistor heat detectors have a sensing element that increases electrical resistance in response to increasing temperature. The detector element is supported by electronic circuitry, which allows them to respond like a fixed temperature or rate-of-rise heat detector (or both). Thermistors heat detectors are commonly found in smoke detectors with a built-in heat sensor.
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