April 17, 2017

All Fire Alarm NAC Wiring is Supervised, Right?

Supervision of a fire alarm notification appliance circuit (NAC) means that if an open or a short exists on a circuit, the fire alarm control unit will indicate a trouble condition until the malfunction is corrected. Right? Well, that depends.

If a NAC consists of DC operated appliances (horns, bells, strobes and their combinations) then the above statement is true. Before or after the circuit has become activated, if a short circuit occurs on a DC NAC, the fire alarm control unit will isolate that circuit and shut it down preserving the integrity of the rest of the system. However, if the NAC is a voice circuit, that’s a different story.

UL 864, listing the manufacture of fire alarm control units, requires that a voice NAC which has a short circuit will be identified by the control unit and in turn will prevent the NAC from operating. However, UL does not address a voice circuit that develops a short circuit after it has been activated. Therefore, a short circuit will prevent voice evacuation/relocation messages on the affected circuit from being received by the people in or adjacent to the area of the fire. Furthermore, a short circuit in one area of the system’s NAC wiring could even compromise the entire voice evacuation of a building.

NFPA 72, the National Fire Alarm and Signaling Code, states in “An open, ground or short-circuit fault on the installation conductors of one alarm notification appliance circuit shall not affect the operation of any other alarm notification circuit.” Although one might think that the intention here is before or after activation, it does not specifically say that and more importantly, UL does not test voice circuits for compliance after activation.

Consider the following. A flaming fire starts in an electrical room or closet, in which the NAC wiring has been installed for the appliances on a floor of a high-rise. It’s a fast evolving fire. The alarm sounds, and the fire floor, the floor above the fire floor and the floor below are evacuated automatically as programmed by the fire command center. The fire service arrives and after investigation they find that occupants on additional floors need to be relocated to other areas. The fire has advanced into adjacent areas of the fire’s origin and has destroyed one or more NAC circuits, and as a result the conductors of the NAC on that floor have been shorted together. The circuit and the amplifier to which the circuit is connected has been compromised or put out of service due to the short. If the amplifier happens to be of the bulk variety where one amplifier provides the audio for the entire building, the audio evacuation portion of the system for the entire building has now been compromised if not rendered inoperative. Unlikely? Although this scenario may not occur within the first few minutes of the alarm, at some point during the evacuation/relocation process, it is quite possible.

UL should require supervision after activation in the UL864 standard and should test for this during the listing process. Until this occurs, there are some steps a system designer can take to mitigate the dangers. Consider the following:

  • First, consider how the audio circuits feeding a floor or area are placed. Are they all included in one vertical or horizontal riser subjecting them all to total destruction, or are they separated to provide some level of survivability?


  • Are at least two audio circuits being provided per floor or area?
    • Is each circuit connected to a separate power amplifier?
    • Do the circuits alternate between speakers down the corridor?
      • In an apartment building, maybe a section of corridor speakers might be on one circuit and the living unit speakers on the other.
    • Are the speakers installed on either side of the corridor and the circuits installed on either side as well or at least with differing paths?
    • Consider rated enclosures for the wiring or the use of circuit integrity (CI) cable.

Whatever phase of this business you’re in…design, code enforcement, approval, installation, maintenance….we all know that the failure of even one NAC has the possibility of becoming catastrophic. We all need to think about this until our national testing laboratories and our codes and standards people recognize this as a problem that needs their attention.

For comments and questions, contact the author:

Bob Hill,
Robert Hill & Associates, LLC
110 Haverhill Road, Suite 376
Amesbury, MA 01913