Douglas Krantz - Technical Writer - Describing How It Works

How do I Troubleshoot the NAC Circuit?

By Douglas Krantz | Maintenance

How do I Troubleshoot the NAC Circuit?

How do I Troubleshoot the NAC Circuit?

Greetings Douglas

Thanks for your webpage! I discovered it while troubleshooting an alarm system at my building. As a retired firefighter (31 years) I am fairly familiar with the operation of these systems, but the mechanics are taking a little research and your page on "how do I find this NAC trouble?" is proving to be a great help.

One additional question that came up as I was following up on a NAC 4 trouble alarm. Is it normal to have some horn/ strobes that simply strobe?

There are 20 Horn / Strobes in this system split across 4 NAC circuits. While walking the building during an alarm test, I found that, while all of the strobes were flashing, there were six that the horn was not sounding on. (this is a silent knight SK-5208 panel, system sensor P2WL horn strobes and this system was installed about 11 years ago).

I have yet to isolate which NAC circuit each is on, but wondered if this was normal or do I have some faulty horns? I would assume that the wiring is intact since the strobes work, but is this an accurate assumption? If this is an issue, should I simply replace the horn strobes or is there something I should check first?

Thank You, TG

The Data Sheet for the horn/strobes can be found at:

These data sheets show a lot of details, including the wire-base that the horn/strobes are connected to.

Strobes Only or Horns and Strobes

Even though the horns and strobes are in the same box-on-the-wall, the design coverage areas for the horns and strobes are different. Strobes, for their location criteria, are supposed to be within line-of-sight from almost everywhere in the common areas of the building; horns, for their location criteria, have to produce enough sound, straight line, around corners, and even through some closed doors, to be loud enough to hear.

These are two different methods for the design criteria. In most buildings, there will be a greater number of strobes than horns. This is one possible reason for the lower number of horns than strobes.

Look Similar, but Different

Look closely at the horn/strobes on the data sheet
  • The left two devices pictured are horn/strobes. The strobe is obvious, the horn is behind the holes above the strobe

  • The upper right device is a horn only. No strobe, but notice the holes for the horn

  • The lower right device is a strobe only. No holes for the horn above the strobe

Holes but No Sound

There is synchronization circuitry, but beside the synchronization circuitry connected to both the horn and the strobe, the horn and the strobe are completely different devices.

If there are holes for the horn, but even though the strobe flashes, no sound is coming out, the problems could be caused by:
  • A faulty horn inside the horn/strobe

  • A faulty synchronization module or Notification Appliance Circuit (NAC) output from the Silent Knight control panel

  • On extremely rare occasions, a badly wired circuit in the building, and no one noticed the silent horns until now

First Check It Out

It could be that the system was designed that way. Look for the holes above the strobes. If there are holes above the strobes in the horn/strobes, you probably have a faulty horn/strobe; if you don't have holes, there shouldn't be sound.

Douglas Krantz

Greetings Douglas

Thanks so much! This is very helpful. I feel somewhat unobservant, but all 6 of the devices that were strobing and not sounding are strobe only devices, so they are operating as designed.

I think the next step to finding my intermittent trouble alarm will be to map out which devices are on which NAC circuit, and then remove and inspect all that are on the NAC 4 circuit.

This week could be a good week to do that as the building is empty for the holidays break. I will let you know what I find out.

Thanks again for your help! TG

Before you examine the horns and strobes for trouble, you need to know a little about the circuit you're looking at.

When the panel is sounding the alarm, the panel is applying "Forward Voltage" on the Notification Appliance Circuit (NAC). Inside each and every horn and strobe is a "Blocking Diode" that allows current from the NAC circuit to run through the horn or strobe, while in alarm.

On the other hand, when the panel is not sounding the alarm, it is "Supervising the Wires" of the circuit. The panel has reversed the voltage on the NAC, and the "Blocking Diode" inside the horns and strobes prevent any current from flowing through the horn or strobe, while not in alerm.

Yes, it would be nice if the panel could supervise the horns and strobes to make sure they are fully functional, but the horns and strobes aren't complicated enough to be able to send any information to the panel. When the panel is supervising the wires of the circuit, the horns and strobes are no better than terminal blocks.

Continuity Check

The panel, when supervising, isn't really checking to see that the horns and strobes work; the panel, when supervising, is checking to make sure all the horns and strobes are connected. It checks to make sure they're connected by running a small current, backwards, through all the wires.

It takes the whole string of red wires, the end of line resistor, and the whole string of black wires to make a complete circuit. The terminal blocks on the horns and strobes are also part of the circuit.
  • If a screw terminal on the horn or strobe is loose, the circuit is broken, and the current stops. The panel then shows a trouble

  • If a wire breaks anywhere in the building, the circuit is broken, and the current stops. The panel then shows a trouble

  • If the end of line resistor doesn't make a good connection, the circuit is broken, and the current stops. The panel then shows a trouble

  • If a horn or strobe is unplugged so the wires don't pass on the current, the circuit is broken, and the current stops. The panel then shows a trouble

Not Quite Properly Made Connections

One time, in a church, our company made several trips there for an intermittent trouble on a strobe circuit. It was always fine while we were there, so we couldn't even tell which circuit was in trouble.

Finally, one time while there, the circuit went into trouble for a short while. The best I could do was to determine which circuit had a trouble, and then the trouble cleared.

What I did was to go to all 8 strobes, and tighten down the screws of the circuit. Most of them were very tight when I tried to tighten them, but a few screws could be tightened just a tiny bit. There was no obvious problem, but the troubles on the strobe circuit quit and never returned.

Mismatched Wires

When two different size wires are under a single screw terminal plate, one of them is held very tight, while the other one of them is loose, or at best not as tight.

Another common issue I've found is mismatched sizes of wires under the same screw. At your end of line resistor for the NAC circuit, look closely at the wires under the screw terminals. The end of line resistor wire is very thin, while the wire for the NAC circuit is quite fat. If both the thin and fat wires are under the same screw terminal, the thin wire could be building up tarnish.

For the fire alarm installer, there doesn't seem to be a problem. But after a few years, if one of the wires isn't tightened down hard, tarnish builds up on the copper wire and the screw terminal. The tarnish makes for a poor contact, and as the contact is in the process of going totally bad, the contact becomes intermittent. Sometimes it can take many hours to find and fix the intermittently poor contact.

Usually, installing a short wire to match the thicker wire makes a better contact. One end of the added wire goes under the screw plate, and the other end of the added wire is wire-nutted to the end of line resistor.

It's the Wires

  • If you have a trouble, it's the wires, not the horns or strobes

  • When the alarm isn't sounding, the voltage on the circuit is reversed from the alarm voltage. Don't change the polarity

  • Always call the monitoring company before doing anything to the fire alarm system; make sure they are not going to call the fire department if the alarms accidently sound off. It's embarrassing to meet your former coworkers that way

Douglas Krantz

Greetings Douglas

Well, I went through the system and tightened a few connections that were minimally loose. Nothing real obvious found, but so far, the intermittent problem has not returned. I am hoping this solved it.

I went ahead and purchased your book and have started going through it. You are a wealth of knowledge!

Thanks again for your help and I will let you know if it comes back.

Thank You, TG
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