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Fire Alarm -- Description

Horns, Strobes, Chimes, Bells, Klaxons, Speakers are notification appliances on the Conventional Notification Appliance Circuit or NAC.
Notification Appliances (Horns, Strobes, Chimes, Bells, Klaxons, Speakers, Etc.) are not supervised on a Conventional Notification Appliance Circuit. The wires that make up the circuit are what are being supervised.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






What Is a Conventional Notification Appliance Circuit or NAC?

By Douglas Krantz

NAC is the acronym for Notification Appliance Circuit.
  • Notification: Tells people of a Fire or other Life Threatening Emergency
  • Appliance: Horns, Strobes, Chimes, Bells, Klaxons, Speakers
  • Circuit: Physical wire loop carrying power to the Notification Appliances
The NAC circuit starts at the main fire alarm panel (or at power expander panels) and goes out to the notification devices in the building. Like the IDC circuit, the NAC circuit is a two wire circuit, and it has an end-of-line resistor.

Supervision and Alarm

There are two conditions for a working NAC: Supervision (Stand-by) and Alarm. To switch between supervision and alarm, inside the panel is a relay; it's a physical switch.

When the NAC is in standby condition, the relay connects NAC supervision circuitry inside the panel to the NAC circuit outside the panel. When the panel is in alarm condition, the relay connects NAC riser power from inside the panel to the NAC circuit outside the panel.

NAC Circuit Polarity

For the NAC circuit, the positive and negative polarity of the riser power is the opposite of supervision power, so for all practical purposes, the voltage on the NAC circuit reverses when the panel goes into alarm.

However, inside the panel, the relay is really just switching between the panel's supervision circuitry and NAC riser power.

Types of Power

In alarm, because the relay only switches the NAC circuit to the riser power, the riser power could be DC power for horns, chimes, bells, and strobes, or audio for the speakers, or even power for the firefighter's phone system.

Remember though, if it's an audio riser, because the audio voltage is always changing polarity, the audio is AC voltage and not DC voltage.

Supervision of the NAC

To make sure all the devices are connected to the panel through the building's wires, in stand-by condition, the NAC circuit is being supervised.

This supervision is similar to the Initiating Device Circuit (IDC), but unlike the IDC, the supervision is not used to power any devices.

The supervision for the NAC Circuit has three conditions:
  • Normal - - Measured at the panel, the voltage of the NAC circuit is lower voltage than alarm levels, but greater than shorted wire voltage. The supervision current passes through all the building's wires and the end-of-line resistor. Because the supervisory voltage is opposite polarity of the alarm voltage, the supervision current doesn't go through the devices.
  • Trouble Open - - Measured at the panel, the voltage of the NAC circuit is higher than normal supervision voltage. The supervision current is lower than normal because, somewhere in the NAC circuit, a wire is broken or a connection has become unfastened.
  • Trouble Short - - Measured at the panel, the voltage of the NAC circuit is lower than normal supervision voltage. The supervision current is higher than normal because it's taking a shortcut (wire-to-wire short) and bypasses the end-of-line resistor.

When the panel detects a short on the NAC circuit, keep in mind that the panel goes one step further. In order to protect power supply and the other internal circuitry from the shorted NAC, the panel won't apply riser power to the NAC circuit. In other words, if there's a short, the alarms won't sound.

Synchronization of Strobes and Horns

This explanation wouldn't be complete except to say that on many fire alarm systems nowadays, the power on the NAC circuit for the audible and visible devices (AVS) now carries with it synchronization signals.

Several different types of synchronization signals are in use, and these signals are proprietary. One manufacture's AVs are using a different kind of synchronization method than another's. When replacing broken AV devices, make certain the new devices are the same make and model as the old devices (or at least compatible). If the new ones aren't compatible, the new devices won't sync with the others on the system.

Sometimes, if the manufacturer of the new AV is different from the manufacturer the old device, the new AV devices won't even work.


P. S. Ya, I know. Using the word "circuit" after the acronym NAC (Notification Appliance Circuit) is redundant. But to me, the word NAC, like IDC and SLC, isn't an acronym anymore; it's just a name onto itself used to describe the type of circuit.


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