Important Circuitry

Fire Alarm Voltage Standards

Since a fire alarm is an electrical appliance, it of course needs power, and there are several things to note here:

  • In the early days of Fire Alarm: High Voltage AC was used to power Notification Appliances, typically bells.
  • When there was a shift to horns, lower voltage levels came to be the standard. 24VAC was the standard for a little while.
  • Eventually, more modern systems came out to be powered by DC. Some were 12VDC, and others were 24VDC. Certain horns needed to be put on accordingly.
  • Currently: Standard voltage is 24VDC for commercial fire, and 12VDC for burglary panels that allow for fire detection.
  • Though we are seeing it go away in some areas, it's very common for high voltage bells to be in place for water-flow applications.

Circuit Supervision

Supervision of a fire alarm circuit is relatively simple: There is a resistor in the circuit somewhere. The fire alarm control panel, or module of an Intelligent FACP, is always looking for its resistor by ensuring there is a proper current flow though it at all times. If there is a break in the circuit, current cannot flow, and that will put the control panel into trouble on the circuit. In the event of a short circuit, the panel will see this as an increase in current. This can cause an alarm or trouble depending on the circuit in question. Since each panel and brand requires different resistors, a user should refer to the manual for their device to determine what resistor is needed.

Detecting an Alarm

This again ties into our current supervision mentioned above. A pull station, for example, will cause a short circuit that the panel or intelligent module will see. This causes the panel to go into alarm. But what about a smoke detector? Conventional smoke detectors work a bit differently, but share similar concepts. Upon detecting smoke, a detector will cause an increase of current, but still have enough voltage to operate and latch in its alarm state. Usually, when a conventional detector is in alarm, it drops the entire circuit to about 6VDC, but this will vary.

In whatever situation, the panel sees the increase of current and uses that to determine if it should be in alarm.

What Powers What?

So with the basics aside, it's important to note what kind of voltage readings we see in different areas of a modern Fire Alarm System. Below is a breakdown of the typical voltage readings for different applications:

  • Auxiliary Power: 24VDC
  • NAC (Not Active): Generally Less than 1VDC, but some older panels do full reverse polarity
  • NAC (Active): 24VDC
  • Initiating: 24VDC
  • Active 2-Wire Smoke: ~6VDC
  • Addressable SLC: ~16VDC

Keep in mind, this is very generalized, and may not be accurate in all situations. Check panel specs for more accurate readings