Signaling Line Circuit
The Signaling Line Circuit (SLC) for the building wiring for a fire alarm system is the communication and power circuit that is connected between the fire alarm panel and the Addressable Input Modules (AIM) and devices, and the Addressable Output Modules (AOM).
Signaling Line Circuit Power
The purpose of the SLC (Signaling Line Circuit) starts with its being a power supply. It supplies power to the addressable modules (both Input and Output) and to the other addressable devices attached to the SLC. These are electronic devices and require power to operate. The Signaling Line Circuit being a power supply is not listed in the title, but without the SLC's provided power, the addressable modules and addressable devices wouldn't operate.
Detecting this power is what the technician is seeing when connecting a voltmeter to the circuit while troubleshooting.
Signaling Line Circuit Signals
The other purpose of the SLC is also to carry communication signals between the control panel and the addressable devices attached to the circuit. These signals can be primarily digital data (for a digital SLC), or a mix of digital data and pulse-width data (for an analog SLC).
Making the voltage readings very erratic (sometimes varying up and down by 10 volts or more), the technician can identify the circuit by the unstable voltage readings.
Both Input and Output Signals on the Signaling Line Circuit
The SLC is a communication path, it sends signals both ways along the pair of wires making up the SLC.
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Addressable Input Module (AIM)
- When there's an alarm, the input module or device sends the signal to the panel using the SLC
- When there's a problem with the input, the input module or device sends the signal to the panel using the SLC
- When the panel resets the input module or device, the panel sends the signal to the input module or device using the SLC
Addressable Output Module (AOM)
The most common use of an Addressable Output Module is to actually operate a separate NAC circuit, somewhere in the building away from the control panel.
- When the panel needs to turn on an output module, the panel sends the signal to the module using the SLC
- When the module has a problem or sees a problem, the module sends the signal to the panel using the SLC
Polling (Attendance Taking)
Making sure all modules are connected to the fire alarm system at all times, the panel polls or takes attendance of all modules (Addressable Input Modules, Addressable Output Modules, Addressable Devices).
The panel sends a device address out using the Signaling Line Circuit, in essence saying "Device 27, are you there?". Device 27 had better call back with "Here, and Normal" or the panel assumes that there's something wrong with device 27, and turns on its trouble light.
Two Types of Addressable Output Modules
In essence, all addressable output modules are relays. When the panel has sent a signal over the Signaling Line Circuit to output device 37 to turn on, output device 37 turns on its relay.
Addressable Control Relay
If the output module is a Control Relay module, the control relay is just relay contacts that are normally open (normally it is not connected to the common contact), normally closed (normally it is connected to the common contact), and common contact.
When the relay is turned on, the relay changes to connect the normally open contact to the common contact, and removes the normally closed contact from the common contact.
Addressable Supervised Output Module and the Notification Appliance Circuit
Functionally, the main difference between an Addressable Supervised Output Module connected to the SLC and a Notification Appliance Circuit (NAC) on the main circuit board of the control panel is its location. The Output Module is outside the panel while the NAC is inside the panel.
The screw terminals of both provide power to a conventional NAC (Notification Appliance Circuit). The two are so similar that to move the wires so they land on one or the other's NAC terminals (the control panel or the NAC terminals of the Supervised Output Module), the only thing that needs to be changed is the end of line resistor at the end of the circuit.
In both the output module and the panel's NAC output, if the panel has not gone into alarm, the relay has connected internal supervision circuitry to the NAC terminal output. The voltage polarity on the supervision circuitry is reversed from what would power the horns and strobes, so the horns and strobes remain off.
When there's an alarm so the relays turn on, the relays switch from the supervision circuitry to a high-power source to run the horns and strobes. For the panel, the relay switches in its own power supply, for the Addressable Supervised Output Module, the relay switches in an external high-power source.
In both cases, the panel's power supply and the external high-power source, the voltage polarity is correct to power the horns and strobes on the NAC.
When there's an alarm, at least from a practical point of view, when the relays are turned on, the voltage polarity on the NACs for both types of circuit is reversed.
NAC is for Horns and Strobes
When there's been a fire, once the fire is out, the fire alarm system is often silenced. At this time, the building can be still full of smoke. When the fire alarm system is silenced, the horns and strobes are no longer powered.
By silencing the fire alarm system, if a smoke extractor fan or exhaust fan is connected to the same circuit as powers the horns and strobes, the extractor fan or exhaust fan is also turned off. This leaves the building full of smoke.
The fans, door holders, door locks, dampers, and other smoke control devices should always be connected to their own control relays and not be connected to the Notification Appliance Circuit (NAC). By not connecting the smoke control devices to the NAC, the panel can operate smoke control system separately from the Alarm Silence button.