Douglas Krantz - Technical Writer - Describing How It Works

Why do I have a NAC Short on the Panel?

By Douglas Krantz | Maintenance

Why do I have a NAC Short on the Panel?

Why do I have a NAC Short on the Panel?

Greetings Douglas,

I removed a GX90 Gentex fire horn from system to clean the plate. After reattaching it, I'm getting a NAC short on the panel.

I have checked a second unit in the apartment and all wiring looks the same. When checking the wires themselves, I have 12VDC on one disconnected pair, and no voltage on the other pair, which continues on to another apartment.

I measured polarity, and installed it as shown in GX90 instructions - Neg to Neg and Pos to Pos.

Any help would be greatly appreciated.

Thank You, DC

The Gentex GX90 is a minihorn commonly used in condominiums and apartments to alert the residents when there's a fire. The wires inside the wall are a conventional Notification Appliance Circuit (NAC),

Wire Supervision Voltage

The wires on the NAC are always being checked for continuity by the panel, using a small electrical current. When there isn't a fire alarm, and you measure the voltage on the wires, you are detecting voltage of the supervision current.

The minihorns have blocking diodes inside that prevent the supervision current from passing through the minihorn. With some minihorns, this small current passing through the minihorns would be enough to make a quiet noise. When there isn't a fire, the diodes, by blocking the supervision current, prevent the quiet noise.

Alarm Voltage

Later, when the panel detects a fire, the panel reverses the voltage on the NAC circuit, and feeds lots of power from its internal power supply to all the minihorns. Because the voltage is now correct to pass through the blocking diodes to the minihorns, the minihorns wake up the residents and alerts them to evacuate the building.

Fire Panel Display

The supervision current is important. If a wire comes loose from a connection or a wire breaks, the supervision current is stopped, the panel thinks something is wrong, and displays that there's a trouble with the system.

If a minihorn is wired backward, the minihorn will be using current because the blocking diode is passing current through to the minihorn itself. The panel sees that extra supervision current is being used on the NAC than normal, and interprets the extra current as being a short circuit.

If it detects a loose or broken wire on the NAC circuit, the panel only displays a trouble. If it detects a short on the NAC circuit, not only does the panel display that there's a short, but the panel will protect itself by not connecting the short circuit to its internal power supply.

Shorts on the NAC circuit are really bad because they can prevent anyone from hearing that there's a fire.

How is the Short Fixed?

Use your voltmeter again, but remember that you are making the measurements during normal conditions. In this case, the wire you measure as Negative should be connected to the Plus screw terminal, and the wire you measure Positive should be connected to the Minus screw terminal.

Yes, you read it correctly. If the alarms aren't sounding throughout the building, the minihorns should be wired backwards. That way, when the alarms do sound, the voltage on the NAC circuit, provided by the panel, will become forward.

Should It be Tested?

Now is the annoying part. Yes, because you have worked on the fire alarm system, you will have to annoy everyone.

The fire alarm system is a life-safety system. If anything is wrong with the fire alarm system, people can be injured or worse. In order to have confidence that the fire alarm system will detect fires and alarm all the residents, you have to test the system.

Testing involves:
  • Making sure the monitoring company doesn't take the action to dispatch the fire department while the testing is taking place

  • Activating the fire alarm system, usually by triggering a pull station somewhere

  • Going around and into all the apartments, listening to make sure all the minihorns work (you have changed wiring, and the wires go to other apartments, so they all have to be checked)

  • Writing down the date, your name, why the work was done in the first place, what was done with the original minihorn, how the test was performed, and what the results were of the test

CYA Paperwork

The writing part is CYA Paperwork. No one reads the CYA Paperwork, unless. If anything else bad happens to the system, and someone finds out that you personally have done any work on the fire alarm system, the CYA Paperwork had better be in the files of the building, or you personally can be held liable. One state fire marshal says "If it aint on paper, it didn't happen." Paperwork is important.

Licensed Fire Alarm Company

Another issue, though, is that many states or cities require licensed fire alarm companies to perform any work or any testing on any fire alarm system. The licensed fire alarm company is used to dealing with wiring and wiring issues, performing tests, and CYA Paperwork.

To protect yourself from liability, it might be a good idea to spend the money and hire a licensed fire alarm company to test the system.

Douglas Krantz
Life Safety
This website uses cookies. See Privacy for details.
Fire Alarm Q&A Articles

No Charge - Unsubscribe Anytime

Make It Work Series of Books by Douglas Krantz
Free - Click and Download
Make It Work Series of Books by Douglas Krantz
Make It Work Series of Books by Douglas Krantz