Fire alarm, communication and low-energy systems

Commercial and industrial buildings contain far more than power wiring. Fire alarm systems detect fires and warn occupants; communication cabling carries data, voice and building automation signals; and many control and signaling circuits run at low, power-limited voltages, including the Power over Ethernet circuits that now supply lighting, cameras, wireless access points and even desk equipment. These systems have their own code sections, product standards and installation rules, and in many jurisdictions electricians install them alongside or instead of specialty contractors. This lesson covers fire alarm system components and circuits, power supplies and battery calculations, survivability and interconnections with other building systems, verification and testing, and the rules for power-limited circuits, communication cabling and Power over Ethernet.

Fire alarm systems

A fire alarm system has a control unit (panel) that monitors initiating devices, activates notification appliances, and communicates with the fire department or a monitoring station. Its parts:

• Initiating devices: manual pull stations, smoke detectors (photoelectric and ionization), heat detectors (fixed temperature and rate of rise), duct smoke detectors, sprinkler waterflow switches and supervisory devices such as valve tamper switches.
• Notification appliances: horns, bells, speakers, and visual signals (strobes) for occupants who are deaf or hard of hearing; voice evacuation systems give spoken instructions, used in high-rise and large assembly buildings.
• Ancillary functions: releasing magnetic door holders so that fire doors close, recalling elevators, shutting down air handlers or operating smoke control fans, unlocking doors, and signaling sprinkler and suppression systems.

Conventional systems use zones of devices wired on the same circuit, telling the operator only which zone is in alarm. Addressable systems give each device an address on a signaling line circuit (loop), so the panel identifies the exact device, and analog addressable detectors report their sensitivity, allowing drift compensation and maintenance alerts. Almost all new commercial systems are addressable.

In the US, the building and fire codes (IBC and IFC) determine where fire alarm systems are required, NFPA 72, National Fire Alarm and Signaling Code, governs design, installation, testing and maintenance, and NEC Article 760 covers fire alarm wiring, distinguishing power-limited (PLFA) and non-power-limited (NPLFA) circuits and their cable types. Many states license fire alarm installers separately, and NICET certification is widely required for designers and technicians.

Circuits and supervision

Fire alarm circuits are supervised: the panel continuously monitors them for open circuits, ground faults and, on some circuits, short circuits, and reports a trouble signal if a fault occurs, so that a broken wire is discovered immediately rather than when a fire happens. Conventional initiating and notification circuits use an end-of-line resistor at the far end, through which a small supervisory current flows; an open circuit stops the current and causes a trouble signal. Circuit classes describe performance under faults:

• Class B: a single open circuit disables devices beyond the break, though the panel reports trouble.
• Class A: the circuit returns to the panel by a separate path, so devices on both sides of a single open continue to work. Class A wiring is required in many larger or more important buildings, and the outgoing and return conductors must be routed separately so that one incident cannot damage both.
• Class X and other classes in current standards add protection against shorts and ground faults with isolators.

Short-circuit isolators on addressable loops limit the effect of a short to the section between two isolators.

Notification appliance circuits and voltage drop

Notification appliance circuits (NACs) carry substantial current to horns and strobes, and the voltage at the last appliance must stay above the appliance's minimum operating voltage when the battery is near the end of its discharge (about 20.4 V on a 24 V system). Voltage drop calculations are therefore essential, often using the "lumped load" method that conservatively places all the load at the end of the circuit.

Power supplies and batteries

Fire alarm systems need a reliable primary supply and secondary (standby) power from batteries or, in some cases, an emergency generator, so that they work during a power failure. Codes require the batteries to supply the system in its normal standby condition for a specified time, followed by full alarm load for a specified time, typically 24 hours of standby plus 5 minutes of alarm in many cases (with longer alarm times, such as 15 or 30 minutes, for voice systems and some occupancies, and different requirements where a generator is provided). The required capacity is:

where \(F\) is a safety factor for aging and temperature, typically 1.2 to 1.25.

Survivability and wiring methods

In larger buildings, especially high-rises and buildings with voice evacuation or staged evacuation, circuits that must keep working during a fire, such as risers to notification circuits on floors not yet affected, need survivability: protection so that a fire in one area does not disable the system elsewhere. Methods include two-hour rated fire separations around risers, two-hour circuit integrity cable (cables tested to keep operating in a fire, such as mineral-insulated cable or listed circuit integrity cable systems), and routing outgoing and return Class A paths separately. Fire alarm conductors are identified (often by red cable jackets or red-painted raceways and boxes) and separated from other circuits as the code requires.

Interfaces with other building systems

Fire alarm systems coordinate with many other systems: sprinkler waterflow and valve supervision, kitchen and special-hazard suppression systems, elevator recall (with smoke detectors in elevator lobbies, machine rooms and shafts initiating recall to a designated floor), air handler shutdown by duct detectors, smoke control systems, magnetic door holders, access-controlled doors that must unlock, and generator and fire pump status. Each interface must be designed, installed and tested so that it works in the right sequence. The sequence of operations, often shown in a matrix of inputs and outputs, is one of the most important documents for a fire alarm system, and integrated testing of all interfaces before occupancy is required in many jurisdictions.

Verification and testing

New fire alarm systems are verified before occupancy: every device is tested for correct operation and indication at the panel, every circuit is checked for supervision, battery capacity and charger operation are confirmed, sound levels and strobe coverage are measured, interfaces are tested, and a verification report is issued. In Canada this is done to CAN/ULC-S537 by qualified technicians, and in the US to NFPA 72 acceptance testing requirements. Existing systems are inspected and tested annually, with some items tested more often, to CAN/ULC-S536 or NFPA 72, and records kept. Electricians installing fire alarm wiring should test insulation resistance and continuity before devices are connected, label circuits, and provide as-built drawings for the verification technician.

Power-limited (Class 2) circuits

Many control and signaling circuits are supplied by sources whose output is inherently limited, so that they present little risk of fire or electric shock: thermostats, doorbells, security systems, building automation controls, LED lighting drivers with Class 2 outputs, and low-voltage lighting. The code classifies these as Class 2 circuits (and, in the US, Class 3 for somewhat higher voltages with limited power), typically limited to about 100 VA and to voltages of 30 V AC or 60 V DC in dry locations for Class 2. Because of their low energy, they can be wired with lighter cables and fewer protections than power circuits, but they must be kept separate from power and lighting circuits (in separate raceways or compartments, or separated by barriers or distance), so that a fault cannot energize the Class 2 wiring at line voltage.

In the 2023 NEC, Article 722 covers cables for power-limited circuits, Article 724 Class 1 circuits, and Article 725 Class 2 and Class 3 circuits, with 725.144 addressing bundles of 4-pair cables that transmit power and data. Articles 800 to 840 cover communication circuits, coaxial cable and network-powered broadband.

Communication cabling and Power over Ethernet

Structured cabling for data and voice follows standards such as ANSI/TIA-568 (adopted in Canada as CSA T529) for horizontal cabling from telecommunications rooms to outlets (maximum 90 m permanent link plus patch cords, 100 m channel), backbone cabling, pathways and spaces, labeling and testing. Category 6 and 6A copper cables support 1 and 10 gigabit Ethernet; optical fiber is used for backbones and long runs. Electricians installing communication cabling must respect bend radius and pulling tension limits, avoid crushing cables with tight ties, keep cables away from sources of interference such as fluorescent ballasts and motors, use plenum-rated or appropriately fire-rated cable for the space, firestop penetrations (Lesson 2), and test each link with a certified tester.

Power over Ethernet (PoE) delivers DC power over the same twisted pairs, with standards up to about 90 W at the source (IEEE 802.3bt Type 4) for devices such as LED luminaires, pan-tilt-zoom cameras, wireless access points and displays. Large bundles of PoE cables heat up because each conductor carries current, so the code and cabling standards limit bundle sizes or require cables with higher temperature ratings or larger conductors for high-power PoE, and recommend avoiding tightly packed bundles in enclosed spaces.