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Fire Alarm -- Description

Auxiliary Smoke Power Supply to proved power to 4 Wire Smoke Detectors and Duct Smoke Detectors
Fire Alarm Panels often have a resettable power supply or a smoke power supply that is used to provide power to 4 Wire Smoke Detectors and Conventional Duct Smoke Detectors. The terminals on the FACP circuit board can be called Smoke Power Supply, Interruptible Power, SMK, 24VS, 24V Smoke Power, Etc.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

What is Smoke Power on a Fire Alarm Panel?

By Douglas Krantz

2 Wire Smoke Detectors are connected to the Zone Inputs of a conventional Fire Alarm Control Panel (FACP), and so too are 4 Wire Smoke Detectors. However the operating power for each type of detector comes from different sources, and it's the interruption (turning off and on) of the operating power that resets both types of detectors.

In order to have a functioning fire alarm system, there has to be a way to reset both the 2 Wire and the 4 Wire Smoke Detector.

Most conventional Duct Smoke Detectors are 4 Wire Smoke Detectors with extra relay contacts.


Conventional manual pull stations are just switches. To indicate an alarm, the switch shorts the zone input wires of the FACP, and the FACP considers this to be a fire alarm. After the alarm is over, the pull station is reset by putting the switch back, basically turning off the short.

Smoke Detectors are a little different.

Standard method of wiring for a Class B Initiating Device Circuit
To reset a 2 Wire Smoke Detector, the power on the Zone is turned off for a few seconds and turned on again.

2 Wire Smoke Detectors

Because they use the zone wires for their power, conventional 2 Wire Smoke Detectors can't short the wires like a pull station to send in their alarm. If they shorted the wires, the zone voltage would drop to zero and immediately reset the smoke detector.

Instead, when going into alarm, the 2 Wire Smoke Detector creates a partial short; it draws a little more current which lowers the supervision voltage a little, sending in the alarm. Because there is still zone supervision voltage, the detectors can latch into alarm.

The reset button, when pressed, not only resets the fire alarm control panel to a normal state, but the power to the zone, which is the supervisory voltage, is also turned off and on. This resets the smoke detector.

Standard method of wiring for a Class B Initiating Device Circuit in a 4 wire configuration
A 4 Wire Smoke Detector or Duct Smoke Detector uses auxiliary 24 volts for power. To reset, the 24 volt auxiliary power to the smoke detector is turned off and on. Panels with the resettable smoke power do this automatically when the reset button is pressed.

4 Wire Smoke Detectors

When going into alarm, a 4 Wire Smoke Detector shorts its alarm contacts and latches them. This shorts the Zone Input wires to the FACP, sending in the alarm.

The power comes from an Auxiliary 24 Volt Power Supply, which is a separate set of terminals on the FACP.

One other thing, the smoke power that provides power to the smoke detector, needs supervision as well as the zone wiring for the detector. This is usually done with an end of line device like a relay, which when the smoke power is disconnected, opens up the end-of-line resistor.

To reset the smoke detectors though, the power to the 4 Wire Smoke Detector has to be interrupted, or turned off and on. This, of course, could be done with a switch on the 24 Volts power to the Smoke Detector, or the Auxiliary 24 Volt Power Supply itself could be interrupted by the reset process.

Of course, the interrupted power supply, if it is available on the FACP, is the easy way (and the method accepted by the manufacturer) of resetting the 4 Wire Smoke Detector.

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Auxiliary Interruptible 24 Volt or Smoke Power Supply

The interruptible power supply isn't really usable for much else, because most other uses require stable power. The only real purpose of the Auxiliary 24 Volt Interruptible Power Supply or the 24 Volt Smoke Power Supply is to provide resettable power to the 4 Wire Smoke Detectors.


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Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at