Notification Appliances & Circuits
NAC fundamentals, strobes, horns, speakers, voice evacuation, synchronization, voltage drop calculations, audibility requirements, and intelligibility.
- Explain NAC fundamentals including polarity, supervision, and voltage drop
- Describe strobe candela ratings and synchronization requirements
- Identify audibility requirements for horns in sleeping and non-sleeping areas
- Explain voice evacuation speaker circuits and intelligibility testing
Lesson 1
NAC Fundamentals - Polarity, Supervision & Voltage Drop
Notification Appliance Circuits
A Notification Appliance Circuit (NAC) powers the audible and visual devices that alert building occupants during an alarm. The NAC is normally in a supervisory state (low current, monitoring for faults) and switches to alarm state (full current, activating devices) when the FACP initiates an alarm.
How NACs Work
In the supervisory state, the FACP applies a reverse polarity voltage to the NAC. The notification appliances are designed to block reverse polarity, so no current flows except through the end-of-line (EOL) resistor. The FACP monitors this small supervisory current to verify the circuit is intact.
When an alarm occurs, the FACP switches the NAC to forward polarity. The notification appliances activate and draw full operating current.
Voltage Drop
The wire run from the panel to the last device on the NAC creates voltage drop. If the voltage at the last device drops below its minimum operating voltage, it may not activate properly. Calculate voltage drop using:
V drop = I x R (total wire resistance)
Important
The voltage at the last device on the NAC must remain above the device's minimum rated voltage. Always calculate voltage drop for long runs and high-current circuits. Increase wire gauge if drop exceeds limits.
NACs use reverse polarity for supervision and forward polarity for alarm activation. An EOL resistor at the end of the circuit allows the panel to monitor circuit integrity. Always calculate voltage drop to ensure the last device receives adequate voltage.