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Fire Alarm -- Description

A Notification Appliance - Flashing Light or Noise Making Device - only does its thing while in alarm. When not in alarm, no power is applied because the blocking diode prevents the reverse voltage of supervision from sending current through the device.
During alarm conditions, when the voltage on the Notification Appliance Circuit (NAC) is as shown, the diode conducts to allow power to light or sounder. During non-alarm conditions, the voltage of the NAC opposite so the diode will not conduct.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

What is Notification Appliance Circuit (NAC) Supervision?

By Douglas Krantz

In the Notification Appliance Circuit (NAC), conventional fire horns, bells, strobes, chimes, and speakers, all have the Notification Appliance Riser Circuit Wiring in common. This circuit, under normal circumstances, is in either one of two different conditions: Supervision or Alarm.

Supervision is the normal, non-alarm, state; the devices aren't powered. The wires connecting all the devices to the panel are supervised (continuity tested) or watched to make sure they haven't been broken or lost connection. If the conventional fire alarm system is wired correctly, that means that the devices are all still attached.

In the supervision state, a low power electrical current is passed through the entire circuit and either goes back to the panel (Class A) or goes through an end-of-line resistor (Class B). If a wire breaks, a connection comes loose, or there is a ground fault or a short, the panel will indicate a trouble.

The panel is supervising the wires because:
  • If there's a broken wire, some or all of the horns or strobes won't work
  • If there's a short, the panel won't notify anyone of a fire
  • If there's a ground fault, the system may work properly, or the ground fault may short out the circuit if there's a fire

In the alarm state, the fire alarm system is getting people's attention. All the devices are powered to make noise or flash. The NAC wires from the panel are carrying the power to all the devices.

Current Blocking Inside the Notification Appliances

Fire horns, Strobes, Bells, Chimes are Notification Appliances. To prevent the devices themselves from shorting out the supervision, and to keep the supervision power from activating the devices, there is a current blocking diode inside the device that keeps the supervision current out of the device.

Fire Alarm Speakers are also Notification Appliances. The speakers, though, are a little different. Instead of the blocking diode, there is a Direct Current (DC) blocking capacitor. This capacitor prevents the NAC supervision DC from getting into the speaker, and it also prevents the speaker from shorting out the DC supervision of the NAC. The capacitor does, however, allow the alternating current (AC) of audio signals to get to the speaker.

Notification Appliance Power

Fire horns, Strobes, Bells, Chimes -- In the alarm state, the supervision is switched off and 24 volts is applied to the NAC, powering the devices. This voltage is in opposite polarity to the supervision voltage, so the diode that blocked the supervision current from getting to the device, now conducts the current into the device to operate it.

Speaker Audio -- In alarm, the speakers don't get this 24 volt DC power. Instead, just like any distributed sound system, the speakers are given power in the form of 25 volt or 70 volt audio, which is AC. The capacitor, that blocked the DC current in the fire alarm speaker, now has AC current which is passed on to the speaker.

Supervision and Power Switching

The switching between supervision and alarm power is usually done inside the panel with a relay. It physically switches between the supervision of the wiring and the riser power, activating the attention getting Notification Appliances.

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