Douglas Krantz - Technical Writer - Describing How It Works
Get the Book "Make It Work - Convetional Fire Alarms"
This website uses cookies. See Privacy for details.

Is it Amplifier Currents and not Speaker Currents for Battery Calcs?

The quick answer is yes, the amplifier, and not the speakers, would be part of the battery calculations.

Is it Amplifier Currents and not Speaker Currents for Battery Calcs?


Greetings Douglas,

I recently purchased your e-book "Make It Work - Conventional Fire Alarms". I am an electrical designer and I am reviewing some shop drawings including battery calculations for an existing installation, where some new devices are being connected. We have some speaker/strobes and the designer of the system is showing zero (0) current for the speaker circuit.

Not having any experience physically wiring, the system physics dictate that speaker would have to be drawing current to produce sound during an alarm condition... Reading your book about speakers and AC Audio current (25 or 70 volt amplified audio), in an alarm condition, the panel switches to the audio amplifier...

I didn't see the original battery calculations... Would the amplifier have been part of the battery calculations therefore the speakers are not included?

Thanks in advance for any insight you can lend!

Thank you, TL

The quick answer is yes, the amplifier, and not the speakers, would be part of the battery calculations.

The problem with audio devices is that, unless the measurements are based on calibrated audio sources, and laboratory equipment is being used, audio voltages and current are impossible to measure accurately. Rather than looking at the speaker's electrical current use, the best that can be done is to base the amplifier's maximum standby current and alarm current on the actual use, under full load and full sound condition.

If the alarm condition current that you are supposed to use is for a 50-watt amplifier, for instance, the speaker load on the amplifier is considered to be added up to 50 watts, and the sound level is a constant "as loud as it gets".

Rarely, if ever, are these speaker power loads and continuous sound voltage levels achieved, but they're the battery current levels we have to calculate with.

Keep in mind that many fire alarm system amplifiers will fail-by-turning-off if the speaker load exceeds the 50 watts by even 1.0 watt. When designing fire alarm systems, commonly there is a 20% headroom given to the speaker's Notification Appliance Circuit (NAC) so the speaker load would only be 37.5 watts (or 40 watts, depending on how the 20% headroom is calculated).

Douglas Krantz

facpdoug@douglaskrantz.com
Index
Residential
Life Safety
Descriptions
Electronics
Maintenance
Suppression
This website uses cookies. See Privacy for details.
Books
Get the Book
Get the Book
Get the Book
Get the Book
Short Circuit Newsletter Sign-Up



No Charge - Unsubscribe Anytime