First off, NAC is an acronym. It's a bunch of letters making a short-cut to saying "Notification Appliance Circuit", which is a title giving a precise description as to what the wires of the circuit do.
- The word Notification means "Warn people using sound and lights" because there's immediate danger of fire, this is telling people to take action.
- The word Appliance means "The Device or Box being used to tell people something". When there's immediate danger of fire, this is the fire horns, strobes, bells, speakers, etc. being used to notify people to act.
- The word Circuit means "The Electrical Wires carrying signals or electrical power". These wires carry the signal or power to the devices that are notifying people to act
Well, even though the voltage is measured as reversing, the voltage reversal
you're talking about isn't really a voltage reversal, as such. It's the panel switching power sources when it goes into alarm.
To understand this, we have to consider the whole circuit (NAC) and how it functions, there are actually 4 things to consider:
- Wire, with the End-of-Line Resistor
- The Horns and Strobes
- The Panel
- The Power for the horns
Starting with the wire, the wire is the weak link of the fire alarm system - it's outside the panel. Because the wire is outside the panel, there's concern with water damage, loose or corroded connections, forklifts hitting the wire, people disconnecting devices or otherwise tampering with the system, etc.
When the panel isn't sounding the alarm, the panel is always running a continuity check of the wire (supervising the wire) to make sure the wire is continuous end-to-end and connected to everything. This means that at all times the panel is putting voltage on the loop to pass current through the wires and End-0f-Line Resistor. The actual voltage on this doesn't matter; it can range from 1.5 volts to 20 volts, depending on manufacturer. What matters is the panel can detect the electrical current this voltage pushes through all the wires. This supervision makes sure the wire is always continuous.
Horns and Strobes and the Panel
Now we have the appliances - the horns and strobes. The horns cannot be giving the alarm during normal times, so inside each fire horn or strobe, a blocking diode prevents this supervision current from passing through the horn or strobe itself.
Like I said, the panel isn't really reversing voltage, when it goes into alarm
it changes from a continuity check of the wires to pushing current through the horns and strobes. Inside the panel is physical relay (electrical switch operated by an electromagnet) that switches between the supervision circuitry in the panel and the power supply.
The voltage polarity of the power supply is opposite the voltage used to check continuity, so the blocking diode inside each fire horn will no longer block the electricity, but conduct, allowing the horn or strobe to sound the alarm.
Now we get to the interesting part: the power supply. The power supply can be internal (part of the fire alarm control panel) or external (a power source outside the fire alarm control panel).
The internal power supply used in smaller fire alarm panels is the internal 24 volt power supply. Some larger panels and any addressable fire alarm supervised output modules (NAC), however, have screw terminals for an external power supply.
When there is an alarm with any fire alarm system, the panel (or module) switches from the internal supervision circuitry to power. The power source can be an internal power supply, and external power supply, a 25 volt or 70 volt audio power amplifier or even a telephone line for a Firefighter's Telephone System.
Answers to Your Questions
I don't know of an specific name for the voltage reversal, except that one voltage is called "supervision" and the opposite voltage is called "alarm". It's not even a voltage reversal, as such; it's the panel switching from the supervision power supply to the alarm power supply. If the power coming in is the internal power supply or an external 24 volt power supply, then the voltage reverses; if the power coming in is a 25 volt or 70 volt audio circuit, then the voltage changes from DC to AC; if the power coming in is the firefighter's telephone line, then the voltage changes from DC to a telephone line.
The normal voltages on the circuit would be:
- Alarm voltage would be 24 volts Nominal DC from a power supply (Nominal voltage can be anywhere from 20 volts to over 27 volts, whatever the battery would be), or 25 or 70 volt Audio (25 or 70 volts is not what a volt meter will show, 25 or 70 volts is the maximum voltage of the audio power. With voice audio, that voltage is almost never attained.)
- Normal supervision voltage - Whatever voltage the manufacturer uses for checking continuity. That voltage can be anywhere from 1 or 2 volts to over 20 volts.
- Fault voltage - Any voltage that isn't alarm voltage or supervision voltage. These wires go all over the building so fault voltage would be 0 volts to 24 volts (or greater), AC or DC voltage, Common Mode or Differential Mode induced voltage, etc. Whatever voltage is not the alarm voltage or the normal voltage.
I wish I could be more precise on you question of what is normal, but because every manufacturer has a different idea of what is normal, nothing is really "normal". I hope this helps with your Mass Notification project.