RECENTLY, A POST APPEARED on one of NFPA’s LinkedIn pages (linkedin.com/company/nfpa) questioning the placement distance of smoke detectors near fluorescent light fixtures. The post resulted in an interesting exchange—commenters offered no concrete answer except to confirm that the 2013 edition of NFPA 72®, National Fire Alarm and Signaling Code, did not contain an answer. So where do we start to look for help?
We should first define the basis for the question. The concern in the post is electrical interference from a fluorescent light fixture. In reality, this kind of interference seems rare. When it does happen, the costs to fix the problem can become significant. But smoke detectors can be affected by more than just the occasional “rogue” electronic ballast generating a harmonic signal.
NFPA 70®, National Electrical Code®, has always required that “communications wires and cables shall be separated by at least two inches (50 millimeters) from conductors of any electric light, power, Class 1, nonpower limited fire alarm, or medium power network-powered broadband communication circuits.” This requirement intends to provide for safety and not to improve performance of any system. Additionally, it only applies to wiring not enclosed in some form of metallic raceway, which would shield the fire alarm wiring from electrical interference. So in fact neither NFPA 72 nor the NEC addresses the issue that arises when strong electromagnetic fields may induce unwanted current and voltage into a cable installed in close proximity. With addressable fire alarm systems, any excessive electromagnetic interference (EMI) hinders the ability of the fire alarm control unit to successfully communicate with the devices connected to it.
We know from experience that too little separation from the 60 Hz noise from an electrical system can affect the transmission of fire alarm system signals to the control unit and may induce false trouble signals at the control unit. A number of installation guideline manuals will advise the installer to segregate the circuits of fire alarm systems from other circuits in order to minimize any likelihood of the other circuits causing a malfunction of the fire alarm system. Additionally, the manufacturer of the fire alarm equipment may require a minimum separation distance between the cables of fire alarm circuits and those of other services.
Where installation manuals require such separation, an installer must give careful attention to the physical location of those fire alarm circuits susceptible to electromagnetic interference. The installation must maintain proper separation from other circuits. Where an installer cannot maintain separation, the circuits should cross each other at a right angle. If you peruse various installation manuals, you will discover that they do not state the actual separation distance, or that it can vary depending on the manufacturer.
Underwriters Laboratories includes transient electrical testing for all listed smoke detectors, which assures a minimum level of rejection of electrical interference. So the answer to the original question may be that it is not necessary to maintain any particular separation between smoke detectors and fluorescent lighting fixtures.
But as every designer, authority having jurisdiction, and installer knows, field conditions often present unexpected problems. NFPA 72 already requires a three-foot (one-meter) separation between various components, though the requirement does not directly address the EMI issue. It seems that using the three-foot distance for the separation of circuits to ensure code-compliant performance of a fire alarm system might offer a reasonable alternative. We should remember that good practice is at least as important as any minimum code requirement.