Douglas Krantz - Technical Writer - Describing How It Works

Why do Some of the Horns and Strobes Fail?

By Douglas Krantz | Maintenance

Why do Some of the Horns and Strobes Fail?

Why do Some of the Horns and Strobes Fail?

Greetings Douglas,

Before I got hired onto this job, some previous employees installed some horn/strobe alarms inside of a building.

Yesterday I was made aware that some of the alarms only have horns while the strobe doesn't work, and on another floor the strobe works but the horn doesn't. Would you happen to have any idea where I go about starting to solve this issue? I'm pretty new to setting these up.

Thank You, CN

You indicate that over time, previous building employees have replaced some horns and strobes. The replacement over time may have caused multiple problems.

Voltage Polarity Issues

If the employees used their voltmeters to determine the voltage polarity on the wiring, and put the wire measured positive on the plus terminal, the horn or strobe won't work.

On a Notification Appliance Circuit (NAC), the voltage on the wires change polarity, depending on whether the control panel is sounding the alarm, or just standing-by:
  1. When the fire alarm system is sounding alarm, the wire measured positive goes on the plus terminal, while the wire measured negative goes on the minus terminal. This allows current to flow through the horn or strobe, sounding the alarm.

  2. When the fire alarm system is standing by so it isn't sounding the alarm, the wire measured positive goes on the negative terminal, while the wire measured negative goes on the plus terminal. This is backward wiring, but this backward wiring prevents current from flowing through the horn or strobe, keeping the horn or strobe silent.

Almost all of the time, the horn or strobe is replaced when the alarm system is standing by. It's counterintuitive, but the second way of wiring is required, or the horn or strobe won't work.

Wiring the horn or strobe backward is a common problem, even among some trained fire alarm personnel. You may have to check each horn, strobe, or horn/strobe for the proper voltage polarity.

Incompatibility Issues

No one can use just any old horn or strobe to replace a horn or strobe; the replacement has to be compatible with the fire alarm system itself.

In other words, all horns, strobes, and horn/strobes have to be replaced with the same manufacturer's original model, or else the official replacement model. The problem is with synchronization. All strobes must flash together, and all horns have to sound off together.

If the strobes don't flash together, people with epilepsy may have seizures; if the horns don't sound off together, the sound becomes confusing.

All manufacturers do the synchronization differently. If another brand was used as a replacement than was designed for at the fire alarm control panel, the replacement won't work correctly, and may not work at all.

If someone replaced a horn or strobe that was not compatible, that can be a real problem. You may have to check all of the horns, strobes, and horn/strobes to make sure they are the original manufacturer's compatible horn, strobe or horn/strobe.

To make sure about compatibility, call the fire alarm system manufacturer's technical support team.

Wiring Issues

Many of the fire alarm systems use a single pair of wires for both the horns and strobes; many of the fire alarm systems use one pair of wires for the horns, and one pair of wires for the strobes.

If someone else wired the horns and strobes, and there are problems, check the wiring; if you wired the horns and strobes, and there are problems, recheck the wiring, anyway. Everyone, including you and me, makes wiring mistakes.


With a NAC circuit, there are two types of overload: power supply overload and circuit overload.

Power Supply Overload

If the power supply is overloaded, the NAC Circuit will be turned off. The horns and strobes may not come on when there's a fire alarm, or the horns and strobes may turn off withing seconds of an alarm. Usually, the control panel will show some sort of trouble.

The overload can be caused by someone having added extra horns or strobes on the circuit.

More commonly, however, the overload may be caused by the settings on individual horns or strobes being set at a higher level than the circuit was designed for. This could be settings like the circuit is designed for 15 Cd (Candela) strobes, but they are set to 115 Cd. Or maybe, the circuit was designed for low audio level on the horns, but they are set to high levels.

Circuit Overload

The wires of a NAC circuit have resistance. It's only during an alarm that this is an issue. Because of the resistance, if there's greater current, there's more power being lost in the wires; if there's less current, there isn't as much power lost in the wires.

The horns and strobes all use current. If the horns and strobes use more current than the circuit was designed for, there isn't enough power to reach the horns and strobes at the end of the circuit.

This is a real problem when horns and strobes have been added to the circuit. The horns and strobes use more current. Making it worse, the wire has been lengthened, increasing the amount of power the wires sap from the circuit.

In a county courthouse, I've found that because of several remodels to the building, the wires were more than doubled the length that a NAC circuit was designed for, and several extra horns and strobes had been added. While troubleshooting that one, I had to come early in on a Saturday morning just to sound the alarms.

The again, the overloaded circuit may be just that the replacement horns and strobes were set higher than the circuit was designed for. Even though the higher settings on the horns and strobes aren't overloading the power supply, the higher settings may still overload the circuit.

Fixing an overloaded circuit may take some rewiring.

Settings on the Horns and Strobes

The horns, and especially the strobes, often have adjustments. These adjustments make the horns louder or quieter and the strobes brighter or less bright.

A result of these adjustments are that a louder horn draws more current than one that is quieter, and a brighter strobe draws more current than one that isn't as bright. Incorrect settings can affect both the power supply and the circuit capacity.

If you can, make sure the loudness and brightness settings on the devices are set to the original levels.

Fix the Problems One at a Time

Each time a horn or strobe was replaced, the system should have been checked. It's far easier to find a problem right away, rather than letting the problems accumulate over long periods of time.

To fix a system with many problems, start out with a walk-through examination. Visually make sure all horns and strobes are compatible with the panel. If you have to, replace the ones that are not compatible.

Next, pick a horn or strobe that doesn't work near the panel, and fix the problem with that one. Test it before going on. When that one works, go on to the next one.

Remember. When you fix anything, test it to make sure it works. Make noise. The previous employees didn't test the system by making noise each time they changed something. If they had, there wouldn't be all these troubles with the system.

Yes, I know. The repeated noise from testing as you are fixing the system will annoy people. Do what you can to reduce the annoyance. Remind people, though, if the fire alarm system is going to be fixed, it's gotta be tested by making noise as you fix the system.

CYA Paperwork

CYA paperwork is documentation that you did your due diligence in testing and correcting problems.

Before actually troubleshooting, test the system. Sound the alarms, go around and listen to each horn, and watch each strobe. Make sure each one does it job.

If any horn or strobe fails to work, write it down. When you correct a problem with the wiring, write down what you found, and what you did to fix it.

When you get through fixing everything, go around and make sure everything works. The making sure everything works includes what was fixed and what worked before. Keep that paperwork on file.

Remember, no one reads the paperwork until something goes horribly wrong. Then the CYA Paperwork had better be there.

Douglas Krantz
Life Safety
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