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The System’s Heartbeat: Fire Alarm Control Units

Fire alarm.

The heart of any fire alarm system, the component that brings it all together, is the fire alarm control unit (FACU).

The first step is to choose whether a conventional or addressable unit is needed based on the size and complexity of the building. Of course, the owner, who will normally press for a low-cost system, may also drive the choice of system.

As price will undoubtedly come up in your conversation with the owner, it is important that you move discussions from the “installed cost” to the “operational cost.” You’ll have to drive this conversation; the owner doesn’t know what they don’t know about the life safety afforded by a properly designed and installed fire alarm system.

As mentioned in a previous article, you should always ask the owner what he or she wants to have left after the fire. This question often will take the spotlight off the price discussion and instead shine it on the life safety needs of the owner.

Also remember the owner looks to you as the expert. Don’t let him or her down. Ensure that you understand the typical systems operation and the basic code and design tenets of a properly designed and installed fire alarm system. The applicable codes will include the International Building Code—or whatever local building code the jurisdiction has adopted—NFPA 72, National Fire Alarm and Signaling Code and NFPA 70, National Electrical Code.

For a system without great complexity, such as a simple three-story apartment building, you may choose a simple conventional three-zone system for the FACU. In use for decades, conventional fire alarm control units have proven very reliable and easy to wire, though with these systems you cannot T-Tap the circuits. Many buildings still have conventional fire alarm systems in service. At the time of installation, these systems met the price focus of owners.

 The circuits used in a conventional system include Initiating Device Circuits (IDCs) for detectors, pull stations, and waterflow switches, and notification appliance circuits (NACs) for horns, strobes and other audible and visible notification appliances. NFPA 72 defines IDCs as “A circuit to which automatic or manual initiating devices are connected where the signal received does not identify the individual device operated.” NFPA 72 defines NACs simply as “A circuit or path directly connected to a notification appliance(s).”

NFPA 72 also provides requirements for the installation of these circuits and for defining what you must do whether you plan to install them in as Class B or Class A circuits.

The most common type of FACU in use today, even for small, simple systems, is the addressable FACU. The flexibility afforded to you, as a contractor, for both design and operation makes the addressable FACU an easy choice. And, from your point of view, it will likely prove more efficient to train your technicians on one addressable type of FACU from a single manufacturer.

With addressable systems a signaling line circuit (SLC) is used to interconnect your initiating devices—and with some manufacturers their notification appliances, as well. NFPA 72 defines SLCs as “A circuit path between any combination of addressable appliances or devices, circuit interfaces, control units, or transmitters over which multiple system input signals or output signals or both are carried.” You can also install SLCs in a Class A or B fashion. However, NFPA 72 provides for an additional wiring method allowed for SLCs. This method provides more reliability than a Class A circuit. NFPA 72 designates such a circuit as Class X and provides specific installation requirements for this circuit.

The addressable FACU uses a miniature computer to operate the fire alarm system. You and your technicians will need additional and frequent training to maintain the knowledge and skill to install these FACUs correctly the first time. Additionally, these FACUs require programming to connect all devices and appliances, as well as to define their operation.

The same SLC can connect to a control module that controls a fan or magnetic door hold-open, in addition to all the devices and appliances. Most manufacturers will allow hundreds of devices or appliances connected to an SLC.

However, be careful not to put all your eggs in one basket, or rather, in one SLC. You must exercise care not to limit the number of circuits simply to save costs. You should carefully plan the system, keeping a high degree of reliability in mind. Using an addressable FACU has many advantages. Be careful not to abuse their capabilities, especially in larger or more complex buildings.

Remember NFPA 72, National Fire Alarm and Signaling Code, is not a design guide. So, you must hire competent individuals who understand fire and fire protection. A competent designer must understand more than the specifications and operations of a particular manufacturer’s equipment. It takes someone who understands the installation of the equipment, the requirements of the codes, and the application of detection and controls to provide the owner with a life safety system they expect and deserve. Understanding the role an FACU plays in an effective fire alarm system is an important step in meeting these expectations.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker and expert in the life safety field, is a principal member and past chair of NFPA 72, Chapter 24. He is a vice president with Jensen Hughes at the Warwick, R.I., office and can be...

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