Douglas Krantz - Technical Writer - Describing How It Works
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Offers from Technician's Corner

Fire Alarm -- Description

Wired as Class B or Class A, fire alarm speakers are a distributed sound system. The only difference between fire alarm and distributed sound speakers is the DC current blocking capacitor inside the fire alarm speaker.
Wired in either Class A or Class B configuration, in supervision the fire alarm speaker is treated just as if it was any other fire alarm device. When active, the fire alarm speaker is treated as if it was any other public address sound distribution speaker.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






Why Can't a 70 Volt Speaker be Used With a Fire Alarm System?

By Douglas Krantz

When the fire alarm system is active and is being used for EVAC or voice, there is no difference between the speaker audio used in voice EVAC and the speaker audio used in a distributed sound overhead paging system.

It's still the same 25 volt or 70 volt AC audio power sent out on a two wire sound system.

hen the system is not being used to send out audio, the fire alarm system is testing the building wiring using a direct current supervision voltage. The difference is the paging system cannot handle this feature.

When the fire alarm system is being supervised, if a paging speaker is used on the system, the transformer in the paging speaker will short out the supervision voltage. The fire alarm speaker won't short the system.

The reason is inside the fire alarm speaker assembly. The assembly uses a Direct Current (DC) Blocking Capacitor to keep the DC supervision voltage out of the transformer.

Besides the DC Blocking Capacitor and other than being tested, approved, and listed by UL, CE, or some other testing agency, there is no difference between the regular overhead paging speaker and the fire alarm speaker.

Just remember though, even though it is meant for the same voltage audio system, don't use an overhead paging speaker on a fire alarm system. It will short out the supervision current, and when a fire occurs, the fire alarm panel won't send EVAC audio down that Notification Appliance Circuit (NAC).


More Articles

How Do I Shut Down Auditorium Sound on Fire Alarm? -- I have an auditorium that needs to have the sound shut down when the fire alarm is activated. The sound board or DSP (Digital Signal Processing) has an emergency input that I can connect to, but the prints are engineered to use the SLC loop to shut it down ... Read More

How Does the Fire Panel Know Addressable Detectors are OK? -- It's assumed that fire alarm systems have to work all the time because fires happen anytime, but, then again, this is the real world. Problems with... Read More

What is a Fire Alarm Klaxon? -- Before there were DC fire horns, and as an alternative to clanging bells, klaxons were used to notify... Read More

Just What Is a Signaling Line Circuit (SLC)? -- The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss ... Read More



 Get your free diagram showing supervision for Class B wiring

I receive commissions for purchases made through links in this post



This website uses cookies. See Privacy for details.

Get the book Make It Work - Conventional Fire Alarm Systems


Get help finding ground faults



Advertisement