Douglas Krantz - Technical Writer - Describing How It Works

Can I Locate a NAC Fault with a Continuity Tester?

By Douglas Krantz | Maintenance

Can I Locate a NAC Fault with a Continuity Tester?

Can I Locate a NAC Fault with a Continuity Tester?

Greetings Douglas,

I have a Fire Lite alarm system for a 3-family home. I am getting NAC Fault and System Trouble message on the panel.

Can I use a continuity tester from one heat sensor to another heat sensor on another floor to determine where the open circuit is? I don't want to fry the alarm panel by doing something risky.

Thank You, MC

  • A NAC Fault is a System Trouble
  • A Battery Fault is a System Trouble
  • A Zone Fault is a System Trouble

Any time there is a fault, or other trouble on the fire alarm system, the System Trouble will show up. The NAC Fault that you see is the panel being more specific about the source of the fault.

NAC stands for Notification Appliance Circuit. It's the Circuit that carries power to the horns and strobes (Appliances) that are used to Notify the occupants of the building about the fire. It's the power that the horns and strobes use to make noise and flash.

The horns and strobes are not able to show the panel that they are having problems, so, to make sure the horns and strobes are connected, the best the panel can do is perform a continuity test of the wires (supervise the wires).

The heat sensors are input devices to the fire alarm system and as such, are on a different circuit.

Before Troubleshooting

Before doing anything to the fire alarm system, fix it so the fire department doesn't come.

Yes, I know. The NAC circuit is an output circuit, and shouldn't accidently sound the alarm and call the monitoring company, which then dispatches the fire department. Don't trust the fire alarm system. You know that there's a problem with the system (Trouble Fault), you don't know that the only trouble is a NAC Fault.

When doing anything with the fire alarm system, the first step is to let the residences know about the troubleshooting, and that you'll be entering to work on the fire alarm system. Let them know that the alarms might be sounding off while you're working on the system. Then they'll be prepared for any alarms and not call the fire department.

The next step is to call your fire alarm monitoring company. Give them your account number and password. Then let them know that you are testing the system so they don't take the action of dispatching the fire department.

When you're through, call them again. Find out if they received any signals from the fire alarm system, and make sure they know the testing is over.


Start by reading the control panel. Check out the lights showing on the panel, including the green power-on light, any red alarm lights, any yellow trouble lights and yellow trouble lights.

The wires inside the walls and the ceilings are hard to get at. Because they are hard to get at, they don't often break or come loose from anything else. Do the easy stuff first; do the stuff that's easily examined.

The fire horns and strobes in all the apartments are accessible. Check every one of them to make sure they haven't been removed. Even if the horns and strobes look like they're still there check the wiring and connections behind the fire horns and strobes, there might be a problem. If any wires have come loose, reconnect the wires.

Probable Cause of the NAC Fault

The fire alarm panel is probably detecting a loss of continuity on the NAC circuit. The loss of continuity could be on the wires inside the walls, but the loss of continuity has a better chance of being on a connection to a horn or strobe.

Most of the time, the horns and strobes are wired in a daisy-chain. The chain starts out at the panel, goes to one notification appliance, then another appliance, and another. The last horn or strobe also has a resistor, called an end of line resistor, or EOL.

When a wire breaks or comes loose from a connection, the current stops - Continuity is broken - There is a NAC fault

If one of the connections for a horn or strobe has come loose, that will break the continuity. The loose connection could have just happened, by itself; the loose connection could have been caused by someone wanting to make sure their horn or strobe won't annoy them again.

Regardless of why the connection has come loose from a connection, the notification appliance at that location won't work; it won't warn anyone about a fire. Because the notification appliances are daisy-chained together, there's a good chance that one or more of the horns or strobes in the rest of the building won't work either.

I've reconnected fire horns in apartments that the resident there disconnected their horn, and then replaced the disconnected horn back in the wall to make it look like it was normal.

If you're looking for a disconnected horn or strobe, you will have to remove it from the wall and check to make sure the wires are still attached.

It's Supposed to be Wired Backward

When there isn't a fire alarm, in other words when the fire alarm system is normal, the fire alarm panel is checking continuity. To check continuity, the panel is backward polarizing the voltage on the Notification Appliance Circuit (NAC).

If you are at a horn or strobe, and use your voltmeter to check polarity of the wires, make sure you land the wires backward; the wire you measure as Positive goes on the Minus terminal, and the wire you measure as Negative goes on the Plus terminal.

That way, when there's an alarm, the panel can reverse the polarity and the Positive power automatically goes correctly to the Plus and Minus terminals on the horns and strobes.

Test the System

Yes. Test the system. See if it actually works. The only way to determine that it truly works, especially after there was trouble with it is to set it off as if there was a fire. Then go into all three units. Listen to every horn and make sure every strobe flashes.

Perform the test once the system is fixed. Even if only one wire is reconnected, performing the test for your confidence that the system still works.

Remember that if a horn or strobe is wired incorrectly, the panel may show that there are no troubles. But if it is wired incorrectly some or all of the horns or strobes might not work to warn anyone about a fire.

Before testing, though, take the same precautions about the fire department; make sure they don't come when testing the system.

CYA Paperwork

Now we get into the nitty-gritty of fire alarm systems. Everything that you did to fix the system, and everything you did to test the system, has to be shown on paper. As one state fire marshal said, "If it aint on paper, it didn't happen."

This CYA paperwork has to be written down at the time the work was done, and kept on file.

One word of caution. Some states or localities require that the work performed on a fire alarm system is to be accomplished by a licensed fire alarm company. Check this out before performing the work.

Douglas Krantz
Life Safety
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