Detecting the Difference Between Smoke Alarms and Detectors
The technical side of the fire alarm business often turns on interchangeable terms, such as fire sensors, fire alarms and fire detectors.
However there are times when fire terms cannot be interchanged without altering the technological issues or specific code considerations. A good example of this is automatic smoke alarms vs. smoke detectors. In Section 3.3.179, 2002 Edition, it says a smoke alarm is “A single or multiple station alarm responsive to smoke.”
In Section 3.3.43, it says a detector is “A device suitable for connection to a circuit that has a sensor that responds to a physical stimulus such as heat or smoke.” This month, we will explore some of the more notable details concerning both methods of fire protection and how they differ in the real world.
Smoke Alarms Are Self-Contained
When we examine 3.3.179, it’s obvious there are two types of smoke alarms we must consider: single station and multiple station.
Section 3.3.175, 2002, defines a single-station smoke alarm as, “A detector comprising an assembly that incorporates a sensor, control components, and an alarm notification appliance in one unit operated from a power source either located in the unit or obtained at the point of installation.”
In other words, a smoke alarm is a self-contained unit that detects and notifies within a single unit. They can be powered by 120VAC or by a 9V battery, and in some cases both.
Multiple-station alarm devices share the same characteristics as a single-station alarm with the exception that instead of operating solo, these devices can be interconnected so when one unit goes off, all units go off.
Detectors Use a Supervised Circuit
When we examine the definition of a smoke detector, as contained in Section 3.3.181, 2002 Edition, we find another code reference: Section 3.3.43, Detector. Here it says, “A device suitable for connection to a circuit that has a sensor that responds to a physical stimulus such as heat or smoke.”
The key to defining the difference between the two types of smoke detection units is actually contained in Section 8-1.4.4 of NFPA 72, 2007 Edition. Here it says, “Smoke detectors shall be connected to central controls for power, signal processing, and activation of notification appliances.”
Within the same section it specifies the number of smoke detectors that can be placed on a single initiating circuit. It also defines the number of multiple-station smoke alarms that can be placed on a single circuit, or tandem line, as it’s often called. “Smoke alarms shall not be interconnected in numbers that exceed the manufacturer’s recommendations, but in no case shall more than 18 initiating devices be interconnected (of which 12 can be smoke alarms) where the interconnecting means is not supervised, or more than 64 initiating devices be interconnected (of which 42 can be smoke alarms) where the interconnecting means is supervised.”
The last portion of the above code refers to smoke detectors that utilize supervision to maintain the integrity of the initiating circuit. Here, up to 42 smoke detector units can be installed on a supervised initiating circuit that allows for up to 64 devices. In a smoke alarm circuit, however, only 12 smoke alarms can be used in a nonsupervised initiating circuit that allows for up to 18 devices. In this case, the initiating circuit interconnects each single-station unit so that when one goes off they all go off.
Smoke detectors derive their operating power from a centralized source, usually a fire alarm control panel. Although these devices usually possess a means of signal processing, it is the fire alarm control panel that provides notification appliances with the signal they require to sound off. In most cases, this notification appliance is separate from the smoke detector, but not always. In the case of smoke alarms, a notification appliance must be provided internal to each smoke alarm device. However, in the former, the means of notification appliance activation is usually external to the smoke detectors.
Practical Side of Alarms, Detectors
Smoke detectors have the advantage of an initiating circuit that is constantly supervised by a centralized alarm control panel. Multiple-station smoke alarms usually do not.
Although installing a multiple-station smoke alarm system usually involves a smaller initial monetary investment on the part of the homeowner, there is more of an opportunity for failure when compared to a detector-based system. According to NFPA 72, in smoke alarm networks where the integrity of the interconnecting wires is monitored, reliability is considered to be 88 percent, whereas in a similar network of smoke alarms where these conductors are not supervised, reliability is considered to be 85 percent (Section 8-3.3, NFPA 72, 1999 Edition).
Smoke detector networks that utilize supervision at all levels, including two independent sources of power, are considered to be 95-percent reliable (Section 8-3.1, NFPA 72, 1999 Edition). By far, smoke detector networks are far superior to smoke alarms when they meet all the criteria listed in Section 8-3.1, 1999:
- Utilizes a control panel
- Has at least two independent sources of operating power
- Monitors all initiating and notification circuits for integrity
- Transmits alarm signals to a constantly attended, remote monitoring location
- Is tested regularly by the homeowner and at least every three years by a qualified service technician
No discussion of smoke alarms and detectors would be complete without talking about wireless systems. Here, in Section 8-3.2, NFPA claims wireless smoke-detection networks that meet all but No. 4 of the above points are considered 90-percent reliable when used in homes.
For more information on smoke alarms and smoke detectors, refer to NFPA 72 and NFPA 101, Life Safety Code, available by calling (800) 344-3555 or visiting www.nfpa.org.
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