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I have a question on how something works.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer




I have a Question

Dear Sir,

I saw your article http://www.douglaskrantz.com/BlogClassAWiring.html. So I am looking for answers for SPEAKER wiring, not alarm units...

Our client is insisting on Class A wiring on the speaker lines: Class A => Loop wiring on the speaker lines, instead of Class B standard EOL lines.

Also can you please inform me which PA/VA brand can support Class A speaker wiring?

Yes, client and me is aware about A/B wiring - but the client is insisting on this Class A...

With best regards, Z D

An IDC (Initiating Device Circuit) is wired the same as a NAC (Notification Appliance Circuit). It's the wires and not the devices attached to the wires that are talked about with Class A or with Class B.

The only wiring difference between Class A and Class B is that with Class B, the end-of-line resistor can be anywhere in the building, while with Class A, the end-of-line resistor is back at the panel, usually on the main circuit board. (It's there, usually you aren't told about it.)

With Class B, any break in the wire will show up as a trouble on the panel, and any device beyond the break in the wire is useless. This break, of course, will be fixed in a timely manner.

In a fire, a timely manner isn't good enough. To keep the devices working that are beyond the break, the panel switches to feeding power or audio on both the Class B feed and the Class A return part of the loop. When there's a fire, this back-feeding on the Class A to get power or audio to the devices might just save lives.


Besides the length allowed, and the diameter of the wire, there is no difference between an input circuit (IDC) or an output circuit (NAC).

Any input device (pull station, smoke detector, waterflow switch, etc.) is attached to the Initiating Device Circuit (IDC). Any output device (horn, strobe, speaker, etc.) is attached to the Notification Appliance Circuit (NAC).

The only difference between a horn circuit and a speaker circuit is that the horn circuit is fed 24 volt power when there's an alarm, and the speaker is fed amplified audio (like any distributed sound system) when there's an alarm.


There are addressable modules that can be used to send audio over Class A circuits, but I haven't seen any complete panels yet that are specifically designed to do Class A. They probably exist, but I have never had to research them. You'll have to research to find them yourself.

One other place you can get information is the technical support people at the fire alarm system manufacturer for the rest of the system.

Douglas Krantz
See how Class A Wiring works

More Articles

Why Can't a 70 Volt Speaker be Used With a Fire Alarm System? -- When the fire alarm system is active and is being used for EVAC or voice, there is no difference between the speaker audio used... Read More

Can Two Different NAC Circuits be in the Same Conduit in Class A? -- Keep in mind that the purpose of Class A isn't so any problem with the wiring can be repaired in a timely manner; the purpose of Class A is to keep the system working once ... Read More

Can a Person Just Add a Horn or Strobe to an Existing Notification Appliance Circuit (NAC)? -- In a building that has been remodeled and remodeled, when a fire drill is sounded, have you ever found that a few strobes are running slow or won't even work? Then, when troubleshooting... Read More

Can I Relocate a Strobe Without Reprogramming? -- Do the technicians have to reprogram the fire alarm system after relocating devices just a few inches from their original position? Read More

PDF Book
PDF of Make It Work - Conventional Fire Alarm Systems
PDF of Make It Work - Conventional Fire Alarm Systems


Douglas Krantz

Describing How It Works
facpdoug@douglaskrantz.com

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Electrical Flow


On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at www.douglaskrantz.com/
ElecElectricalFlow.html
.