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

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

70 volt speakers used in a public address overhead paging system do not have a built in DC blocking capacitor. Without this capacitor, the wires can't be supervised by the fire alarm system and looks like a short circuit to the fire alarm panel.

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.


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.

When 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).
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

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

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