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Fire Alarm -- Description

The Booster Power Supply boosts or expands the power for the horns, strobes, chimes, etc.
Think of it as a single zone, non-latching fire alarm panel. When a signal turns it on using the trip input, the outputs turn on. When the signal used to trip it on stops, the outputs turn off. It's an extra power source for the Horns, Strobes, Chimes, Etc.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

What is a Booster Power Supply or Signal Power Expander?

By Douglas Krantz

A fire alarm system consists of input devices to detect a fire (automatic detectors and manual pull stations), output devices to notify occupants of a fire (fire horns and strobes), and a control system to turn on the outputs when the inputs detect a fire (Fire Alarm Control Panel or FACP).

The problem is that fire horns and strobes use a lot of electrical current from the FACP. Unless the fire alarm system is for a small building, the horns and strobes use more current than the FACP can provide. Something is needed to provide this extra electrical current.

Known by many different names from many different manufactures, the Booster Power Supply (BPS) or Signal Power Expander (SPX) provides this extra current.

There can be many BPSs connected to a single FACP. Because the only reason for the BPS is to provide extra electrical current, a greater number of horns and strobes can be used than the FACP could otherwise handle on its own.

Operation of the BPS or SPX

When the FACP goes into alarm, it reverses its output voltage to turn on the fire horns and strobes. The BPS, having been connected to a notification output of the FACP, sees this voltage reversal and reverses its output voltage to turn on its fire horns and strobes.

Location of BPSs or SPXs

Rather than congregating them next to the FACP, the BPSs are usually located closer their horns and strobes. This keeps the wires shorter, which is important because, as the current carrying wires get longer, the wires loose more voltage.

The BPSs can be found almost anywhere in a building. They can be located:
  • In janitor closets
  • In data rooms
  • In electrical rooms
  • Above the ceiling (not supposed to be, but I've seen them there)
  • In water rooms
  • On the walls of manufacturing floors
  • Etc.

Battery Backup

To keep the BPS working during a power outage, batteries provide backup power. They keep the BPS standing by and ready for up to 24 or 72 hours under normal power out conditions, and then provide extra power so the BPS can operate the horns and strobes in case of fire, after that.

Auxiliary Power

In some cases, when an output of the BPS is programmed to be always turned on, the BPS can also be used for auxiliary power to operate door holders, control relays, and other circuits and devices.

Troubles on the Booster Power Supply

When the BPS is AC powered, a green LED inside the BPS lights up. When this LED is not lit, the BPS is in trouble.

Under normal conditions, the BPS supervises its outputs the same way the FACP supervises its inputs and outputs. When this supervision detects a short or open in the loop wiring, the BPS goes into trouble. The detection of a ground fault or a problem with the batteries are also common troubles for the BPS. When the BPS detects a problem with these circuits, LEDs turn yellow to indicate the trouble.
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When the BPS goes into trouble, it changes the condition of its trouble relay contacts, sending a trouble to the FACP. Also, depending on how it is wired, it also can open the wiring in the trigger loop so the FACP supervision doesn't see its end-of-line resistor. This will provide a trouble to the FACP.


Douglas Krantz

Describing How It Works

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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