Douglas Krantz - Technical Writer - Describing How It Works
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How Does a Booster Power Supply or Signal Power Expander Work?

A Booster Power Supply is a power supply for horns, strobes, bells, etc. It is turned on using a "trip circuit". When it is not turned on, it performs a continuity check of the output circuits to the horns, strobes, bells, etc.

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.


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:

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.

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.

Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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