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Ask the Technician

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer





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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Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.