A project engineer would never intentionally design a building sub-system to be obsolete a year or two after completion - unless the whole project was a temporary venture. What factors must an engineer consider to ensure the longevity of a fire detection, alarm or signaling system? And, what is the useful life of a typical fire alarm or signaling system?
The answer to the second question is directly related to the first. For a properly designed system, its life cycle is defined only by possible changes in the required mission or by component failures and replacement availability.1 To varying degrees, engineers can design to reduce component failures and provide for possible changes in a system's intended mission. There are many fire alarm systems still in service today that are 30 or more years old. They might not be as sophisticated as today's systems, but they continue to reliably perform their intended functions.
Like sprinklers, today's fire alarm and signaling systems are often designed for a specific mission. Building codes, fire codes and life safety codes have requirements for the use of fire detection, alarm and signaling systems in certain occupancy or use groups. The requirements in the codes are coordinated with other prevention and protection requirements to achieve some level of life safety and property protection contemplated by the code. (See Figure 1.)
In some cases, code changes are made retroactively and would require either a new installation, if one did not already exist, or a change to an existing system. A good example are the code changes that took place to require sprinkler and alarm retrofits in many places of assembly as a result of the Station Night Club fire in 2003. Changes in occupancy and changes in owners' goals might also trigger changes to a fire detection, alarm or signaling system.
There are several ways that an engineer can help ensure that a system designed to meet today's needs has the infrastructure to be adapted to meet changes in its intended mission without requiring a total replacement. One of the best ways to provide a flexible infrastructure for a fire detection, alarm or signaling system is to ensure that there are an adequate number of circuits, properly distributed and of sufficient wire size, to permit changes and growth.
Most new fire detection and alarm system designs use addressable control units and addressable detectors for all but very small systems. In NFPA 722 terms, an addressable circuit is called a "Signaling Line Circuit" (SLC). Control unit manufacturers design their equipment to allow a single SLC to have as many as 200 addressable devices in various combinations of initiating devices and control devices (relays for example).
Design engineers would not normally specify the number of SLCs required for a project. Instead, they would show the number of initiating devices and specify the number of control points, and then allow the contractor or manufacturer to determine the required number of circuits.
However, to ensure room for changes and future growth, engineers should specify that SLCs not be loaded more than 80% - or some other limit. This could still result in 100 or more devices on a single SLC that passes through multiple fire areas of a building. While technically feasible, that is not good practice. Therefore, the engineer should specify that either separate circuits be used or that isolation modules be used to limit number of devices that might be out of service due to a fault at any one location. Some local codes might also impose similar limits.
Signaling line circuits also have a "growth" advantage in that they are permitted to be "T-tapped" - unless Class A circuits are specified. Class A circuits can remain fully operational even with a single wire break. On the other hand, a Class B circuit would not be operational beyond the open circuit. (See Figures 2 and 3) Class B SLCs, unlike other types of Class B circuits, are permitted to be "T-tapped" as shown in Figure 3 because they monitor the integrity of the wire by communicating with the devices. (Circuit classes will be described in greater detail in a future installment of this series.)
While it is possible to maintain Class A circuit integrity when adding to a system in an existing installation, it is generally less expensive to "T-tap" an SLC. Also, Class A circuits are not necessarily more operationally reliable then Class B circuits. Class B circuits can be more reliable, depending on the number of "T-taps," the wiring type, the type of fault and the number and location of isolation modules.
As with SLCs, Notification Appliance Circuits (NACs) should have a load limit specified by the designers. NACs are more likely to require changes during the life of a system. Therefore, a lower load limit of 70% or so is recommended. Design engineers should also specify that no one NAC serve more than one fire or smoke zone - even if the system is set up for general alarm. This would permit future "zoning" or selective communications options to be implemented by changing or reconfiguring the control equipment, which is easier and less expensive than rewiring a building.
Regardless of the specific load limits for the initial installation, the wire size for each circuit should be based on 100% loading so that it can properly handle future additions. Future changes to circuit length should also be incorporated in the initial installation by up-sizing the wires. For NACs, the wire size can be increased one or two sizes without any downside consequences - the only limit being the cost. However, for SLCs, the smallest wire size that meets the load and distance requirements is best. This is because larger wires have a greater capacitance, which degrades digital signaling and slows communications speeds. In most cases, No. 18 or No. 16 AWG (0.823 mm2 or 1.31 mm2) wires are sufficient for SLCs. The panel manufacturer can provide resistance and capacitance limits.
Many facilities now install Emergency Communications Systems (ECS) to provide notification and information to occupants for more than just fire emergencies.2 The back-bone of an ECS is the use of voice for the audible part of the system. This can have a growth advantage over conventional direct current audible appliances.
Conventional NACs used for horns are direct current circuits and are limited by the available current - typically one to two amps per circuit. NACs used for speakers are limited only by the available wire size and amplifier size. Thus, if properly planned, they have greater potential for future growth. The disadvantage is that separate circuits must be provided for visible signaling appliances. However, since the need for ECS is increasing, providing voice capability even when not required results in a more flexible system. In many installations, the cost can be offset by using the ECS for day-to-day communications and paging needs as permitted by NFPA 72.
Engineers should also address future component failures and replacement availability. The first step is to choose and specify the correct type of equipment for the environmental and situational conditions. In certain situations, mechanical protection, listed for use with the particular detector of notification appliance, might be required.2 The availability of parts over time varies among different manufacturers and for different product lines. Engineers should work with manufacturers and distribution chains that have a track record for maintaining parts availability.
A system that can last forever and meet all future needs does not exist. However, by requiring certain features and upgrading from minimum requirements, engineers can design and specify systems that will be more likely to meet or contribute to future needs.
- "Mission Effectiveness and Failure Rates Drive Inspection, Testing, and Maintenance of Fire Detection, Alarm and Signaling Systems," Fire Protection Engineering, Bethesda, MD 20814, Summer 2002.
- NFPA 72, National Fire Alarm and Signaling Code, National Fire Protection Association, Quincy, MA, 2010.