Notification Appliance Circuit
Simply said, to sound the alarm, the fire alarm panel (in a Conventional System) turns on the fire horns and strobes on a Notification Appliance Circuit (NAC) by turning on the power (in the alarm voltage polarity); to check the wires (Supervise the Wires) when the panel is not in alarm, the fire alarm panel is constantly performing a continuity check of the wires by applying a small amount of power (in the supervision voltage polarity).
On an addressable fire alarm system, the NAC (Notification Appliance Circuit) screw terminals on a Supervised Output Module exactly the same as the conventional NAC screw terminals on a fire alarm control panel.
Output Modules (Supervised Output Modules for the Horns and Strobes) are very similar to the NAC terminals on the fire alarm panel. External power is provided to the Output Module, and the Signaling Line Circuit (SLC) tells the Module when to turn off the supervision of the NAC and turn on the external power to the Notification Appliance Circuit.
Point of View
There are two conditions to the conventional Notification Appliance Circuit (NAC): Supervision of the wires and providing power to the horns and strobes. There are three different points of view for the NAC: The blocking diode inside the horn and strobe, voltmeter measurements made by the technician, the relay inside the panel switching power sources between supervision and alarm.
Because each condition and point of view are all examining different parts of the circuitry, engineers and technicians trying to explain the circuit seem to be conflicting each other. Even though they're actually talking about the same thing, this produces a lot of the confusion that makes the Notification Appliance Circuit hard to understand.
The Horns and Strobes have a Blocking Diode
Inside all conventional horns, strobes, bells, chimes, etc. is a blocking diode.
A blocking diode isn't a diode specially made just to block electricity; a blocking diode is just a regular diode that is used to block electricity. Basically, that's what a diode does; like a check valve used in plumbing to keep water flowing only one direction, a diode blocks electrical current that is trying to flow one direction, and lets electricity through that is flowing the other direction.
A horn or strobe will sound the alarm or flash if electricity is flowing through it, but it won't sound the alarm or flash if electricity isn't flowing through it. Simple, yet effective.
When the panel isn't sounding the alarm, the blocking diode keeps the current from flowing through the horn or strobe because the polarity of the supervision current is reversed. The horn or strobe is not "powered" so the horn or strobe stays off.
When the panel is sounding the alarm, the blocking diode is no longer blocking the current because the polarity of the powering current is forward. The horn or strobe is using the current to sound the alarm or flash.
The technician doesn't see what is going on inside the horn or strobe, or what is going on inside the panel. The technician can only see what is happening to the voltage on the Notification Appliance Circuit (NAC).
When the panel is not in alarm, the voltage measured at the horn or strobe is reversed: the measured negative voltage is on the "+" terminal of the horn or strobe and the measured positive voltage is on the "-" terminal of the horn or strobe. This is normal because the horn or strobe isn't supposed to sound the alarm or flash.
This is the time that the panel is supervising the wiring of the circuit. Because the panel is performing a continuity check of the wires to supervise the wires, the voltage is going to be lower than what is needed to power the horns and strobes. The voltage could be as low as 1.5 volts and as high as 20 volts.
When the panel is sounding the alarm, the voltage measured at the horn or strobe is forward: the measured positive voltage is on the "+" terminal of the horn or strobe and the measured negative voltage is on the "-" terminal of the horn or strobe. This is normal because the horn or strobe is supposed to sound the alarm or flash.
To the technician, when the panel goes into alarm and changes power sources, it appears to be a voltage reversal on the Notification Appliance Circuit (NAC).
The Panel Switches Sources
Inside the panel, and inside the Supervised Output Module is an electrically operated mechanical switch called a Relay. This has contacts similar to a switch on the wall. It is operated by an electromagnet.
See "How does a Relay Work?"
When the panel is not in alarm, the relay is relaxed. It is normally switched to put out supervision voltage on the NAC (Notification Appliance Circuit) to drive electricity through one of the wires all the way to the end of the NAC, through the end of line resistor, and back to the panel on the other wire of the NAC.
If the current stops because of an open connection or a broken wire, the panel says there's an "Open NAC". If there's a short so the electricity flow bypasses the end of line resistor, the panel says there's a "Shorted NAC".
That is "Wire Supervision".
In the Notification Appliance Circuit, the voltage polarity of the supervision circuitry inside the panel is reversed. This is so that when the horn or strobe is not supposed to sound the alarm or flash, the blocking diode will prevent the current from going through the horn or strobe.
When there's a fire alarm, by running electrical current through the coil the panel turns on the electromagnet in the relay. When that happens, the arm of the relay disconnects the supervision circuitry from the NAC, and connects the NAC almost directly to the power supply. (There are some safeties in there, so it's not quite a direct connection.)
The voltage polarity of the power supply in the panel is normal. This is so that when the horn or strobe is supposed to sound the alarm or flash, the blocking diode will allow the alarm current to pass through the horn or strobe.
One concern with all this is a "Shorted NAC". If the supervision circuitry is detecting a short in the NAC, the relay will not be energized to prevent the shorted NAC from shorting out the fire alarm system's power supply. In other words, fix a "Shorted NAC" right away or the occupants of the building won't be warned if there is a fire.
Point of View
When looking at all three points of view at once, the blocking diode, the reversing voltage, and the relay inside the panel, a technician can see how the Notification Appliance Circuit works to supervise the wires, and to sound the alarm or flash the lights.