Learn How To Choose The Correct Fire Detector
The detector is the part of the fire alarm system that homeowners are the most familiar with and although most consumers are familiar with smoke detectors, many would be surprised to find out that there are lots of different types of detection units on the market.
They vary in what and how they detect, how they are managed by the fire alarm system itself and how they are powered. There are many manufacturers of smoke detectors and they vary widely in price, depending very much on their features and functions. Anyone looking for a detection unit for their fire alarm system is faced with a multitude of choices.
Smoke Detectors Or Heat Detectors?
The first decision that you have to make when selecting a detector for your system is to decide what is to be detected. Fire detectors can either detect smoke or heat. (yes – the former are called ‘smoke detectors’, the latter are called ‘heat detectors’.) Smoke detectors start alarming out when enough smoke enters the chamber of the smoke alarm.
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 criteria are met:
1) the ambient temperature around the detector rises to 130 degrees Fahrenheit
2) the temperature at the detector rises 15 degrees Fahrenheit or more during a one minute period.
The first type is known as a ‘fixed temperature’ heat detector, while the second type is known as a ‘rate of rise’ heat detector. There are also combination fixed temperature/rate of rise detectors available on the market.
There are several factors that can start a fire, but not cause a rise in temperature in the vicinity of the heat detector. That includes the distance from the fire to the heat detector and usual variations in temperature in the building. Heat detectors are not as sensitive to fire as smoke detectors and therefore not considered reliable enough to be thought of as ‘life safety’ devices. Really they are considered best suited for property protection applications. They also may be used in sites where there is normally a lot of dust or smoke in the air, which would cause false alarms to be triggered by smoke detectors.
How To Detect
Once the decision of what to detect has been made, the decision of how to detect is next. In the case of heat detectors, there is usually only one method used for detection. A heat detector will have two resistors called ‘thermistors’ which are sensitive to temperature 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.
Firex 4870 Ionisation Smoke Detector - Battery Backup
- Firex 4870 Ionisation Smoke Detector
- Complies with BS5446 Pt 1 2000
- Complete with flush mount base
- Battery backup approx 4 years (dependant on usage)
- Can be linked in with - up to 36 Firex Detectors
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