Clearing the Smoke

April 1, 2007
There's no doubt that recent global events have created a greater awareness of life safety and security issues associated with commercial buildings, resulting in more low-voltage special systems being incorporated into overall design criteria. Today, fire alarm and detection systems are installed almost routinely not only in new construction, but also in retrofit projects associated with the remodeling

There's no doubt that recent global events have created a greater awareness of life safety and security issues associated with commercial buildings, resulting in more low-voltage special systems being incorporated into overall design criteria. Today, fire alarm and detection systems are installed almost routinely — not only in new construction, but also in retrofit projects associated with the remodeling or expansion of existing facilities. Despite the fact that a fire alarm system is more frequently considered an integral part of the design process, some uncertainty still exists on the part of designers, specifiers, and installers regarding various types of fire alarm systems, their applications, and associated peripheral devices.

One of the most common issues relates to the difference between a “zoned” fire alarm system and an “addressable” system. Although there is a major difference between the two types of systems, drawings and specifications often use these words interchangeably, leading to confusion regarding what exactly is expected — and being specified — for a particular project.

Of the two technologies, the zoned system is the earlier type, which is why it's often referred to as a “conventional” system. In a conventional system, the fire alarm control panel provides a minimum of two to four zones, with expansion capabilities for 32 to 36 zones. Individual zones support initiating, or change-of-state devices, such as smoke or heat detectors, manual pull stations, and supervisory switches.

In essence, each zone is a 24VDC circuit from the fire alarm control panel supporting a certain number of initiating devices located in the same general area of a given structure (often an entire level or floor of a building). Although the total number of devices carried on a single zone may differ somewhat, depending on the system's manufacturer, an average of 20 detection devices per zone is the norm.

In the event of an alarm condition originating from a single detection device, the fire alarm control panel (and any remote annunciator unit) will indicate, via lamps or alphanumeric display, that the entire zone associated with that device is in alarm. As such, if a smoke detector on the east side of the second floor in a facility goes into alarm, the fire alarm control panel will indicate that the alarm condition is “somewhere” on the second floor. This same general information regarding the alarm's source or location is also transmitted to the local fire department. The possibility that this type of general communication might lead to a delay in fire fighter's response time was recognized as a potential life safety issue, leading to the development of addressable systems.

In an addressable system, each initiating device, such as a manual pull station, smoke or heat detector, or duct detection unit, has its own unique identification number or “address.” This allows each appliance's individual status to be transmitted to the fire alarm control panel. Upon alarm, the control unit — and any associated remote annunciator — will designate both the type of device in alarm (smoke, heat, pull station), and its exact location (Second Floor East — Room 201), with this same specific information being forwarded to the local fire department.

In addition, control and monitoring functions associated with elevator recall, shutdown of HVAC units, door control, and status of sprinkler system tamper and flow switches, which must be supported by individual, hard-wired zones on a conventional fire alarm control panel, can be accomplished using an addressable control or addressable monitor module, providing the same specific device type and location information to the addressable control unit as transmitted by initiating devices. While the conventional system may have several zones, or circuits, made up of initiating devices or supporting monitor and control functions, the addressable unit usually has only one circuit, supporting the very same devices and functions, with individual annunciation of each appliance type and its specific location. Thus, the fundamental difference between conventional and addressable systems is the nature of the information transmitted upon alarm — very general versus very specific.

When a project requires an addressable fire alarm control system, which is becoming the norm (see Fig. 1), drawings and specifications should avoid using the words “zone” or “zoned,” to prevent any confusion regarding the specific type of system to be provided. Another common source of uncertainty is the nomenclature assigned not only to various types of fire alarm system devices, but also to the various types of circuits that make up a fire alarm system.

A fire alarm system consists of two types of devices: those that send information to the control unit (inputs), and those that receive information from the control unit (outputs).

In general, manual pull stations, detection devices, and supervisory switches are input devices, while audible and visual signaling devices are considered output devices. Unfortunately, several different terms are often applied interchangeably not only to each type of device, but also to the circuits supporting those devices. NFPA 72 identifies three separate types of fire alarm system signaling paths: initiating device circuits (IDC), signaling line circuits (SLC), and notification appliance circuits (NAC).

IDCs use an end-of-line resistor to monitor current flow and support conventional, change-of-state devices. SLCs are essentially data circuits, supporting addressable devices. NACs use an end-of-line resistor to monitor polarity and are made up of audible and visible notification devices. Although NFPA 72 delineates the performance characteristics and applications for each type of circuit, there is still some confusion regarding the differences between them.

Specifiers often define an IDC as any circuit supporting initiating devices, whether conventional or addressable. Specifications supporting this definition usually identify an SLC as being either the transmission path in a network system between the fire alarm network control station and local fire alarm control panels or nodes that it supports, or the connection between a fire alarm control panel and its associated remote annunciator.

This is a common misconception that has been specifically addressed by the 2007 edition of NFPA 72. In it, an SLC is defined first by stating that it is a circuit capable of supporting any mixture of addressable devices. In basic terms, initiating devices associated with a conventional fire alarm system are supported by IDCs, while signals from addressable initiating devices are transmitted via an SLC to their associated addressable control unit.

Another common problem seen in fire alarm system specifications involves multiple terms applied interchangeably to notification appliance circuits. A prime example of this is the use of “indicating devices,” or “indicating device circuits,” as a label for fire alarm system audiovisual devices and circuits. The obvious difficulty in this example is the confusion caused when NAC circuits or devices are defined by terms that closely resemble the language and abbreviations normally associated with conventional detection devices, or IDCs. The best solution for a specifier is to identify fire alarm system audible and visible devices — and the circuits that support them — as NACs. This reduces the potential confusion caused by the use of terms similar to those applied to IDCs.

An additional area of uncertainty involves the various classes and styles of fire alarm system circuits defined by NFPA 72. Class and style descriptions are assigned to circuits based upon their ability to perform during abnormal circumstances, as depicted in the tables provided for each type of circuit in NFPA 72.

The greatest obstacle in understanding these categories seems to be the difference between Class A and Class B fire alarm system circuits. The basic distinction between the two types is their ability to continue operating under conditions that compromise the integrity of the circuit itself.

In a Class B configuration (the more common of the two), outgoing conductors originate at the fire alarm control panel and support peripheral fire alarm devices. In the event of any circuit being compromised, such as a ground or wire-to-wire short, all of the devices downstream from the trouble condition are unable to report to the control unit, and are, in essence “lost.”

Class A wiring ensures a much greater degree of survivability, as it requires both outgoing and return conductors. As in Class B, circuits not only originate at the fire alarm control panel to support peripheral devices, but also return to the control panel after the last device on the circuit. Using this “loop” configuration, when a circuit is damaged, devices upstream from the trouble are still able to communicate with the control unit, as is the case with Class B. However, devices downstream are not lost; they're able to transmit their status to the fire alarm panel on the returning, redundant set of conductors.

The survival of a Class A circuit is further enhanced by NFPA 72 requirements that call for each set of conductors to be installed in dedicated conduits that are physically separated from one another. When conductors and conduit are routed horizontally, the recommended minimum spacing is 4 feet. If conduit and conductors are routed vertically, a spacing of at least 1 foot is the recommendation.

As the inclusion of a fire alarm and detection system as an important part of design methodology continues, it will be essential for specifiers, designers, and installers to have a more thorough understanding of the life safety features of each system. As technology advances, fire alarm systems will have a larger role in overall design, being interfaced with other building systems to provide a greater degree of protection (Fig. 2). Clearing up some of the basic misconceptions surrounding fire alarm systems, their associated devices, and wiring methods is the first step in creating a greater awareness of these systems and their capabilities.

Shaver holds a NICET Level IV certification in Fire Protection Engineering Technology — Fire Alarm Systems and serves as an engineering associate, CET IV, for Stanley Consultants in Muscatine, Iowa.

About the Author

John Michael Shaver

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!