Douglas Krantz - Technical Writer - Describing How It Works
Get the book Make It Work - Conventional Fire Alarms

How Does a Booster Power Supply or Signal Power Expander Work?

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.

Even in an addressable fire alarm system, most booster power supplies are completely conventional, with conventional circuitry throughout the building for the Notification Appliance Circuits (NACs). Learn more about the conventional fire alarm system. Get the book Make It Work - Conventional Fire Alarms. This book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects all fire alarm systems.

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

Share This With Friends:

 Get your free diagram showing supervision for Class B wiring

Go to the Fire Alarm Description Map Page of Douglas Krantzs Technicians Corner
Go to the Residential Life Safety Map Page of Douglas Krantzs Technicians Corner
Go to the Residential Life Safety Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Alarm Description Map Page of Douglas Krantzs Technicians Corner
Go to the General Electrical Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Alarm Maintaining Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Suppression Map Page of Douglas Krantzs Technicians Corner
Go to the Guest Writer's Guidelines of Douglas Krantzs Technicians Corner
See Trivia on Douglas Krantzs Technicians Corner
This website uses cookies. See Privacy for details.
Get the book Make It Work - Hunting Ground Faults
Get the book Make It Work - Hunting Ground Faults
Get the book Make It Work - Conventional Fire Alarms
Reader's Favorite Articles

What is a Stair Pressurization Fan (SPF)?

Which Way Does Electricity Flow?

What's the Difference Between Class A and Class B?

What Makes the End of Line Resistor So Important?

What is a Flyback Diode?

What is a Fire Alarm System?

What is an RTU (Roof Top Unit)?

What Causes an Open NAC?

Recent Articles

When Testing, Why Isn't the LED Lit Continuously?

Is There a Procedure to Install EST Detectors?

Does it harm the panel if the trouble occurs during a weekend and lasts 2 or 3 days?

The fire alarm bell to my house is going off...

How is an Addressable Elevator Capture Panel Connected to a Conventional Panel?

Why Does Closing Some Gatevalves Show Trouble?

Home Wateflow Switch - Do you know how to reset it?

How is a Pathway Classified?

Just Who Is the Authority Having Jurisdiction (AHJ)?

What are Pull-Up or Pull-Down Resistors?

Do we need horns and strobes in a building shell?

Don't Breathe Smoke

The Pump was Very Hot - What Happened?

What's the Purpose of the Zener Diodes?

How do You Use the Switches for Addressing?


Will a check valve placed before a flow switch help with false flow alarms?

Will a check valve placed before a flow switch help with false flow alarms?

Essential Fire Safety Tips For Landlords

Is it possible the door holder relay is breaking down?

How Do I Connect Two Fire Alarm Panels Together?

Can I connect an addressable smoke detector to a conventional fire alarm control panel?

Learn about fire alarms, one article at a time -

Keep up on the latest article!




No Charge - Unsubscribe Anytime