# What is the Difference in Power Supplies?

Greetings Douglas,

What is the difference between an auxiliary power 24-volt connection from the control panel and other additional power 24-volt power supplies like the booster power supply?

Thank you, T F

## Utility 120 Volt or 230 Volt Power

It doesn't matter whether the power supply is in the fire alarm control panel, or in a different box somewhere else, no power supply produces power; all power supplies only convert power. They require utility power (120 or 230 volts AC) and convert this utility power to the 12 volts or 24 volts DC that the fire alarm system uses. This includes the Auxiliary Power Supplies and the Booster Power Supplies (BPS).

Yes, in fire alarm systems there are batteries, but the batteries get their stored power from chemical reactions in order to produce their 12 volts or 24 volts power. In other words, as an alternative power source, the batteries give their previously stored power to the Fire Alarm Control Panel, the Auxiliary Power Supply, or the NAC Power Supply.

Because the power supply only converts power, the Primary Source of power for the Power Converters (Power Supplies) is the Utility 120 or 230 Volts, and the Alternate Source of power for the Power Converters is the 12-volt or 24-volt batteries.

## Auxiliary Power from the Fire Alarm Control Panel

Most fire alarm panels are 24 volts (nominal), but a few are 12 volts (nominal). Be careful, 24 volts (nominal) is not 24 volts, and 12 volts (nominal) is not 12 volts. The word "nominal" (which is defined as "in name only") after the word "volts" means the voltage is whatever voltage the battery has at the moment. From a power supply named 12 volts nominal, the true voltage is really somewhere between 10 and 13.6 volts. From a power supply named 24 volts nominal, the true voltage is really somewhere between 20 volts and 27.4 volts.

Read the manual to find out what current can be used from the power supply.

## Smoke Power or Reset Power from the Fire Alarm Control Panel

Not all power supply connections on a fire alarm panel are the same; be careful about power supply connections. If the connection says something like "S Power" or "Reset Power", the power supply is not just an auxiliary power supply. This is a power supply that gets interrupted every time the reset button on the control panel is pressed. The overall current on this power supply is usually less than the main auxiliary power supply.

Read the manual to find out all power supply limitations and capabilities.

## Fire Rated Auxiliary Power Supply

A Fire Alarm System Auxiliary Power Supply is a power converter that uses utility power as its primary power source, and uses backup batteries as its alternative power source. A Fire Rated power supply has been tested by a "Third Party", Nationally Known, Testing Laboratory like UL, CE, ULC, FM, etc. The laboratory has the power supply listed, and there is a label on the power supply that says it is tested and listed.

In addition, because it's designed to be used in a Life-Safety fire alarm system, it self-monitors. It has a means of telling the fire alarm control panel that something is wrong. Things like a shorted output, loss of voltage on its output, weak batteries, or other internal problems send the signal.

Usually, the internal troubles will cause a "trouble relay" to send the trouble signal back to the fire alarm control panel.

How much current can be used is shown in the manual that comes with the power supply.

## Non-Fire Rated Auxiliary Power Supply

A Non-Fire Rated Auxiliary Power Supply is a power converter that uses utility power as its primary power source. Sometimes it has the capability of charging and using backup batteries as its alternative power source. The batteries, however, are almost never monitored, so don't count on the batteries for life safety purposes.

Because it has not been tested to be used in a fire alarm system by UL, ULC, FM, CE or other testing laboratory, it should never be counted on for anything important in a fire alarm system.

Usually, there are no trouble relays so if there are internal troubles, or even outright failures, no one will know until something else in the building fails due to lack of power.

How much current can be used is shown in the manual that comes with the power supply . . . Except that the current rating for a non-fire rated power supply was only tested under ideal laboratory conditions. "Ideal Laboratory Conditions" are not reasonable measurements for something that is going to be used for life safety.

Unlike a Fire Rated Power Supply, the greatest current level that can be used is half of what it's rated for; it gets too hot inside the box if the rated current is used continuously. Under continuous current use, and especially in poorly ventilated areas, the real allowable current from a non-fire rated power supply is, at best, only half of what the power supply is rated for.

## NAC Power Supply on the Main Panel or Separate Booster Power Supply (BPS)

The acronym NAC stands for Notification Appliance Circuit. The NAC is the wiring or circuit in the building that carries the needed power to the horns, strobes, chimes, speakers, etc. that warn people of fire. The power supply outputs to connect to the building's NAC circuit are usually labeled as NAC 1, NAC 2, NAC . . .

The panel has these power supply outputs, and there are separate panels designed for additional outputs. The separate panels are named things like BPS (Booster Power Supply), XPS (Expansion Power Supply), NAC Power Supply, and so forth. Whatever it's called, it's a Fire Alarm System Auxiliary Power Supply - with a Plus.

The Plus is that the NAC power supply is normally supervising the wires on anywhere from two to four horn / strobe circuits. On command, the power supply switches from the wire supervision (checking continuity) of the circuits to providing 12 volts or 24 volts alarm power to the horns / strobes attached to the NAC circuits.

To make things interesting, normally the voltage polarity for the wire supervision (continuity checking) is backwards so the horns and strobes don't turn on. When the power supply is switched from wire supervision to providing the 12 volts or 24 volts power, the voltage polarity on the wires is switched to forward so the horns and strobes can use this power to turn on.

If the supervision circuitry finds something wrong, it will also activate the trouble relay.

How much current can be used is shown in the manual that comes with the power supply.

## Specifications

To see the capabilities and limitations of the power supplies (specifications), read the manuals that come with the fire alarm control panel, the auxiliary power supplies, and the BPS power supplies. In particular, check out the current limits of each output, and check the total current limits of the power supply.

The manuals are there for you, the installer and technician. Read the manual.

Douglas Krantz
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