Douglas Krantz - Technical Writer - Describing How It Works
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I have a question about something

M K Asked:

Why reverse polarity for fire alarm horns?

Douglas Krantz Answered:

A fire horn is turned like a light; it turns on when power is applied and turns off when power is removed. But then again, to constantly check continuity of the wires (supervise the wires), the Fire Alarm Control Panel (FACP) always has power applied to the circuit.

This is a conflict. Only when there is an alarm are the horns supposed to make noise, but just to check continuity of the wires, the panel has to apply power all the time.

To prevent the fire horns from making noise while the wires are being checked for continuity, the voltage for the continuity check (supervision) is reversed. The horns, then, have a diode inside them to block the electrical current, so the horns stay silent.

It's an automatic turn on/turn off. When there's an alarm and the panel wants to turn on the horns, the panel changes the Notification Appliance Circuit (NAC) voltage to forward polarity. Once the voltage is forward, the diode in the fire horn allows electrical current to flow and the horn turns on, making noise.

That's why the voltage reverses when the horns are turned on.


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It's just one more strobe, what's the problem? -- Sometimes, after a building has been established for years, the fire marshal or the building owners want to add another horn or strobe. Or maybe, with remodeling, more horns and strobes are just added to an existing Notification Appliance Circuit (NAC). Really, it is just one more strobe and the circuit works... Read More

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See how Class A Wiring works


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