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Ask the Technician

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





J M, PE Asked:

Hi Douglas

I miss the north sometimes...lived up in the Cities years ago.

Can an SLC loop (Signaling Line Circuit), whether Class B or A, also carry the power to activate (turn on) a horn strobe?

If yes, then the same pair of wires can be characterized by NAC (Notification Appliance Circuit) and SLC? If no, then would separate pair characterized as NAC?

Douglas Krantz Answered:

The Signaling Line Circuit (SLC) is meant to carry signals between addressable devices (Modules and Detectors) and the panel.

In some ways, the addressable devices can be thought of as extensions of the panel itself. The SLC can be thought of as the data/power buss connecting the modules to the CPU in the panel. Power to run the modules and detectors is a secondary purpose of the SLC; the only purpose of the power that rides on the SLC (data bus) is to power the modules and detectors.


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Because the power (electrical current) is so low, the devices on the buss can manipulate the current to produce huge voltage swings in the data that is sent back and forth. This makes sending the data between the devices and the panel very reliable.

On the other hand, the power to run the horns on the Notification Appliance Circuit (NAC), the strobes, chimes, bells, or other notification devices, is huge; the current isn't measured in micro-amps, it is measured in amps.

If the SLC powered a bunch of strobes, the SLC would have to be able to carry hundreds to thousands of times more current than it normally does now.

Because of the high current of the horns and strobes, when sending data to and from the panel, the devices themselves wouldn't be able to affect the voltage very much. The data sent by the devices would not be as reliable.

Also, as a result of the higher current draw of the notification appliances on the circuit, the wires themselves have to be much larger.

True, a few manufactures have put data on the NAC to make the horns and strobes addressable. However, unlike the SLC, the distances for the NAC can only be a few hundred feet.

What all this means is no, the SLC cannot power a horn/strobe, and yes, a separate pair of wires has to be used to carry power to the notification appliances (horns and strobes).







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Articles

How Does Class A Fire Alarm Wiring Work?-- Fire alarm systems save lives and protect property. Fire alarm systems also break down because... 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

How is a Buffer Relay Wired Into a Door Holder Circuit? -- Like a door stop, a door holder keeps a fire door open. When smoke is detected, the door holder releases, allowing the door to shut. The door holder looks simple and innocuous enough... Read More

How Does One Find a Soft Ground Fault? -- Normally, we think of resistance like that of a resistor. The amount of resistance is built-in; no matter what voltage is used to drive the electrical... Read More

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More



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
.