Douglas Krantz - Technical Writer - Describing How It Works
Get the Book Make It Work - Convetional Fire Alarms
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Why Won't the Horns and Strobes Work?

The horns and strobes are polarity sensitive. Only when sounding the alarm should the positive wire be connected to the plus terminal and the negative wire by connected to the minus terminal. When the alarms shouldn't be sounding, the polarity on the wires need to be reversed.

Why Won't the Horns and Strobes Work?

Why Won't the Horns and Strobes Work?

Greetings Douglas,

Question: I'm troubleshooting a system for a friend. It's an Edwards IO64, with 3 pull stations and some smokes that all seem to work just fine. They all throw the system in alarm. However, the horn/strobes aren't doing anything.

They seem to be in a single loop connecting to the NAC1 and not throwing any errors other than a missing battery. Is there anything you could think of that may be a simple fix?

Thank you, KC

This is a guess, but this is a very common problem, even among the most professional installers. If what I think is happening, this is a polarity issue. There may be some other problem, but checking polarity is the fastest, easiest, and cheapest test.

The voltage polarity for this test needs to be checked at a horn/strobe. I know that this doesn't sound right, but if a voltmeter was originally used to make sure the voltage on the horn/strobes was correct, the voltmeter gave the wrong answer.

Wrong Polarity - Not in Alarm

To start with, when the alarms are not sounding, the fire alarm control panel (all brands, including Edwards) is supervising the building's wiring. It does this by performing a continuity check of the building's NAC (Notification Appliance Circuit) wires.

This sounds simple, but when running the continuity check, the panel has to send voltage out on the wires. However, the horns and strobes can't be sounding off. To solve this, the panel reverses the polarity on the circuit to make sure the horns and strobes can't sound off when they aren't supposed to.

In essence, the polarity on the wires, when the panel isn't in alarm, won't sound the alarms. The polarity on the wires won't match the polarity marks on the horns and strobes.

Right Polarity - Alarms Sounding

When the panel is in alarm and supposed to be sounding the alarm, the panel is sending the correct polarity to the horns and strobes. That's when the voltage on the wires matches the polarity marks on the horns and strobes.

Check Polarity

When the panel is not in alarm, take a voltmeter and go to a horn/strobe. Without disconnecting the horn/strobe, measure the voltage being applied to the terminals. If, when there is no alarm, the voltage of the + screw terminal is positive and the - screw terminal is negative, the polarity is wrong.

+ is Negative and - is Positive

When the panel is not in alarm, connect the wire that is measured as positive onto the screw terminal marked -, and the wire that is measured negative onto the screw terminal marked +.

Test Like You Tested (Check to Make Sure It Works)

After changing the wires, test the system the same way you already tested. Try activating a pull station and see if the strobes flash and the horns sound. Check each one, just to make sure.


The missing battery is a different issue. If you're still having problems with that, send another email.

Douglas Krantz
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