Douglas Krantz - Technical Writer - Describing How It Works

How Do You Troubleshoot a Shorted NAC?

I have a question about something

Good morning.

I was wondering what the easiest way to troubleshoot a short circuit on NAC2 would be.


The easy way to troubleshoot a circuit is found only after many years of guess-and-confirm troubleshooting. Even then, troubleshooting is still guess-and-confirm. In essence, after a while you learn which guesses to skip because those guesses are never confirmed. That's what experience is.


First off, we have to understand what a NAC (Notification Appliance Circuit) is: it's the wiring outside the fire alarm panel that goes to the horns and strobes, (the notification appliances - the devices). The circuit is a pair of wires that connects all the devices in a chain fashion, like a single strand of beads on a necklace.

Guess-and-Confirm --- Divide-and-Conquer

Troubleshooting is a guess-and-confirm process. Guess where you want to first start looking, and then, using your test instruments (ohmmeter, voltmeter, eyes, ears, and common sense), confirm that is where the trouble is leading you. In other words, check to see if the guess you took is the right guess. When the guess is wrong, take another guess and confirm that one. Just make sure to confirm each guess because, in reality, most guesses are going to be wrong.

Troubleshooting is a multistep divide and conquer process. Guess-and-confirm what kind of problem you have, divide the circuit in half and guess-and-confirm what direction to look, divide that in half ad guess-and-confirm which half of the circuit the problem is on, divide that in half again and guess-and-confirm which half of the circuit again, divide and guess-and-confirm. Eventually, the problem will be narrowed down to a device or a wire. This could be an open wire or a shorted wire, or both. This kind of guess-and-confirm process is called Divide-and-Conquer.

Troubleshooting is a confusing process. The wires are hidden in the walls, ceilings, and floors. This wouldn't be so bad, but because all fire alarm systems are custom built, the wires in one building are not going the same places that the wires go to in another building. This is another issue that you are going to have to guess-and-confirm.

Troubleshooting is a dirty process. Problems with the wiring are going to take you above the suspended ceiling, upstairs, downstairs, into offices, apartments, mechanical rooms, across high ceilings using ladders and lifts, through attics and garages, etc.

Troubleshooting uses a "low hanging fruit" process. Before checking the wiring through the walls or high up, go to where it's easy to confirm the guesses. Many times, by checking where it's easy to confirm the problems, the issue can be found and fixed. Only go to the difficult to reach places when all other possibilities have been ruled out. Checking first for the problems at the "low hanging fruit" locations will at least help narrow down the area where you need to look using a ladder. Do a walk-through first, though. Looking for visible problems can often save a lot of time.

Troubleshooting takes patience, but not too much patience. If, using one type of guess-and-confirm method, the problem eludes you, change to another type of guess-and-confirm.

Get used to using the guess-and-confirm process, it's what you'll be using in almost every troubleshooting situation.

Continuity Checking

The panel isn't smart enough to be able to tell if there's any horns or strobes attached to the wires, the panel is only smart enough to test the continuity of the wires. It's the wires, or a shorted or reverse wired device, that's causing the short on NAC2.

Keep in mind that once in a while, more than one short or open wire is causing the trouble. Be on the lookout for confusion. Just take care of one problem at a time, or you'll be chasing "ghost" problems.

The last device on the NAC string is a tiny resistor which is used to complete the circuit without shorting the end wires together. Usually, the resistor is somewhere else in the building (Class B Wiring), but sometimes the resistor is actually in the fire alarm control panel itself (Class A Wiring). The one thing to remember about the end of line resistor, though, is that it is always at the end of the circuit with all the wires of the circuit between the panel and the end of the line resistor.

Once in a while, the end of line resistor is not at the end of the circuit because the circuit was not wired correctly in the first place, or the circuit was incorrectly added to. It does happen, and you'll have to deal with it once you figure it out. Just keep that possibility in the back of your mind.

Start at the Panel

In all of this, I'm assuming that the NAC1 Notification Appliance Circuit is not showing a trouble on the panel, but NAC2 is showing a short.

Because the panel is showing you that there's a problem, without disconnecting any wires, look at what the panel is looking at. Standing there at the panel, check the voltage of the circuit at the screw terminals of the panel. Compare the voltage reading of NAC2 to the voltage read at NAC1.

The voltages you read show you what condition the circuit is in. Yes, you need to know about all three possibilities, Normal, Open, and Short, because you are going to mess around with the wiring as you confirm voltages or resistances.
  • If the voltage is the same on NAC2 as was measured on NAC1, that means that the NAC2 circuit is "Normal". The wiring for the NAC2 circuit is connected properly to all devices, including the end of line resistor.

  • If the voltage is much higher on NAC2 as was measured on NAC1, that means that the NAC2 circuit is "Open". Somewhere on the NAC2 circuit a wire is broken or a connection has come loose. The circuit is not continuous.

  • If the voltage on NAC2 is much lower as was measured on NAC1, that means the NAC2 circuit is shorted. Somewhere the electricity used for continuity checking (supervision current) is taking a short cut around the end of line resistor. This could be a wire-to-wire short somewhere, or it could be that a device is shorted. The short could be a hard short (dead short) and pull the NAC2 voltage to near zero, or the short could be a soft short (partial short) and pull the NAC2 voltage down only part of the way to zero.

Once you have read the voltages at both NAC2 and NAC1, write these voltages down. As you confirm your guesses around the building, you'll need to refer to these voltages.

Using your cell phone, take pictures of the wiring at the panel. As a matter of fact, take pictures of any wiring you take apart anywhere in the building. These pictures are your memory of where the wires were originally connected so that you can reconnect all the wires exactly as they were before disconnecting them.

Disconnect both NAC2 and NAC1 from the panel. Use your ohmmeter to check the resistance of both circuits. Reverse the connections of the ohmmeter and check the resistance of both circuits again. Sometimes the resistance is different when the plus and minus wires are reversed. Write down all four readings so you can use these as you confirm your guesses around the building.

Reconnect the wires for both NAC2 and NAC1 so you can guess-and-confirm your readings as you troubleshoot.

Nothing is Consistent

I don't know how the building is wired because I'm not there confirming. You're the one on site, so you're the one who has to take all the guesses and confirm every one of the guesses.

Douglas Krantz
THANK YOU for your response

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