Fire Alarm -- Description
Klaxons are A. C. devices. They are wired in series and the Fire Alarm Control PaneL (FACP) is adjusted so its Notification Appliance Circuit (NAC) voltage is equal to the total added voltage rating of all the Klaxons on the circuit.
What is a Fire Alarm Klaxon?
By Douglas Krantz
Before there were DC fire horns, and as an alternative to clanging bells, klaxons were used to notify people of fire alarms
The individual klaxons operated on 3 to 24 VAC, and when activated, they gave a very loud continuous buzzing sound. The klaxons were extensively used before the 1970's, and some klaxons are still in use today.
When klaxons were being installed in fire alarm systems, the fire alarm systems were AC only. Converting to DC wasn't necessary, and besides, the DC power supplies weren't reliable enough. Because relays were reliable, fire alarm systems used relays.
One problem is that without DC there was no battery backup.
Bypassing most of the fuses inside the building, the trouble power source (to sound the trouble buzzer) was often AC from an emergency power panel. The emergency panel, however, still used the same power lines coming onto the property as the normal electrical power.
The input circuits were from manual pull stations, automatic waterflow switches, and automatic heat detectors. No smoke detectors could be used because, unlike the manual stations, a smoke detector requires DC voltages to operate.
The klaxons were series wired, so if any klaxon failed, or if the wire broke anywhere, the panel could sense the loss of supervision current and then sound the trouble alarm.
Then again, because klaxons were series wired
, if any of them failed, all of them failed.
Giving different loudnesses, klaxons also came in various voltage ratings: greater voltage would mean a louder klaxon. The voltage at the terminals of any klaxon could not exceed the rating of the klaxon.
If all of the voltage ratings of a series string of klaxons didn't add up to 120 volts, the applied voltage had to be reduced so the klaxons would exceed their rated voltage. To reduce the voltage, a variac (an output voltage adjustable transformer - looks like a very large wire wound potentiometer) had to be used.
It may seem that this is just history, but because this history is still being used in some installations, we as fire alarm technicians still have to know about klaxons.