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Fire Alarm -- Description

Wires for a fire alarm horn, connected to the screw terminals
Wires from a Notification Appliance Circuit (NAC) are connected plus to +and minus to -. The trouble is that when measuring the voltage polarity on the wires during non-alarm times, the measured polarity is wrong for the horn or strobe.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Why is the Voltage on the Notification Appliance Circuit (NAC) so Weird?

By Douglas Krantz

Notification Appliance Circuit -- Two Conditions

Normally, when the Notification Appliance Circuit (NAC) isn't in trouble, the NAC is in one of two states or conditions: Alarm (making noise and flashing, notifying people of a fire) and Normal (silent and not flashing).

When servicing the fire alarm system, it's the difference between the two conditions that cause problems for the technicians. More on the problems later in this article.

Alarm Condition

To sound the alarm, the fire alarm panel's Notification Appliance Circuit gets people's attention (Notification) using devices like horns, chimes, bells, and strobes (Appliance), and does this by applying power over the building's wiring (Circuit).

Normal - Not Alarm Condition

Then again, under normal circumstances, to supervise the building's wiring, the panel uses a supervision voltage to drive electrical current through the same circuit.

Voltage Powers the Devices

There's a problem with the devices, though; when power is on -- they're turned on, when power is off -- they're turned off. The horns, bells, chimes, and strobes don't have switches, they're really simple devices.

Turning on the voltage turns on the horns and strobes, but voltage is also used to supervise the wiring. Because this is all sent over the same wires, the voltage is always on.

There has to be a way for the devices to tell the difference between the alarm condition, when alarm voltage is applied to their terminals, and the normal condition, when supervision voltage is applied to their terminals.

NAC Voltage Reversal

To solve this problem, the voltage produced by the panel on the NAC isn't just turned on and off. When the NAC in alarm, the voltage is forward; when the NAC is supervising the wiring, the voltage is reversed.

Polarity Sensitive Device

In order to react to this polarity reversal, the horn or strobe has an internal diode, making the device polarity sensitive.

When the voltage is forward (positive voltage applied to the + terminal of the device) the diode conducts and horn or strobe is on; when the voltage is backward (negative voltage applied to the + terminal of the device) the diode blocks the current and the horn or strobe is off.

Polarity Voltage Problems

The problem we have for the technician, though, is under normal circumstances, the polarity of the voltage on the wires of the NAC is backward.

Many times, before connecting the wires to the horn or strobe, installers, technicians, and other people who maintain the fire alarm system use their voltmeter to make sure of the polarity on the circuit. If the device is connected so the measured positive wire goes to the + symbol, the horn or strobe will never work.

At least from the device's point of view, supervision voltage on the NAC becomes the alarm voltage, and alarm voltage on the NAC becomes the supervision voltage.

Supervision voltage on the circuit has weak power that usually isn't enough to turn on the horn or strobe; the device won't turn on even though its internal diode conducts. When the alarm sounds, and the voltage changes to forward in the building wiring (NAC), the horn or strobe sees this forward voltage as backward and the internal diode won't conduct. In other words, the horn or strobe doesn't notify anyone.

Inspection Repairs

A number of times, I've had to reverse the wiring for the horns and strobes at the first annual inspection because the installers who put in the system didn't understand this voltage reversal, and never tested the horns and strobes to make sure they all worked.

Then the fire inspector, who should have been shown a fully functioning fire alarm system and not be required to troubleshoot the system for the installers, also missed their mistake.

Panel Trouble on the NAC

Unfortunately, most panels and booster power supplies don't supervise for this problem. They supervise the wire itself, but don't show trouble for a miss-wired device; they let an installer or technician connect the horn or strobe backward.

What's even worse, the supervision power isn't enough to activate the device, so false activation won't indicate anything wrong with the wiring to the installer, either.

The miss-wiring issue will not be discovered until either the next inspection, or when there's a fire and that device doesn't sound. This is, of course, unless the horns and strobes are turned on right then and there to test whether they work.

Voltage Reversal on the NAC

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It's important to remember that the polarity of the voltage, measured while the fire alarm system is normal, is backward; it is supervision voltage -- not alarm voltage.

Only when the system goes into alarm, to turn on the horns, strobes, chimes, and bells, does the voltage change to become forward.


Douglas Krantz

Describing How It Works

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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