What Does SLC Stand For in Fire Alarm?

In the fire alarm system, a common term that you will mostly see is the SLC. If you have zero ideas about this, then this guide will be beneficial.

Well, the SLC is a pathway that makes communication possible among devices and control panels. By doing so, it helps the addressable devices to signal immediately in the event of fire or smoke.

In this guide, I’ll discuss about what SLC stands for in fire alarm systems along with its definition, purpose, devices, and other details. Stay Tune!

What Does SLC Stand for in Fire Alarm

What Is SLC Fire Alarm Systems?

The SLC in the fire alarm system stands for Signaling Line Circuits. It’s a circuit that helps devices like the fire alarm panel, transmitter, and other devices communicate with each other.

It’s like a pathway that lets input, output, or both signals flow between different parts of the system. In other words, it is a path that allows input, output, or both signals to pass between circuit interfaces, control units, and transmitters.

Key Takeaways:

  • The SLC has 3 types of wiring circuits such as Style 4 (Class B), Style 6 (Class A), and Style 7 (Class X).
  • It is advised to use a twisted-pair shielded wire (18 AWG to 12 AWG size) on SLC to lessen electrical interferences.
  • The SLC Performance can differ based on the wiring Class (A, B, and X).

The Purpose of the SLC in Fire Alarm Systems

The intent of the SLC in a fire alarm system is to supply power to devices. To simplify it, the SLC sends power to the addressable (input and output) modules and other devices attached to it.

Without the power of SLC, these devices won’t work. Technicians usually can see the power or voltage of the SLC using the voltmeter when fixing electrical issues.

Another aim of the SLC is to make communication among devices. Well, it offers signals between the control panel and other addressable devices connected to the circuit.

FYI, the signals of digital SCL are primarily digital data, while it’s a mix of digital and pulse-width data for analog SCL signals.

While the SCL passes signals, it makes an erratic voltage reading (varying up and down by 9 – 10 volts or more) which the mechanic can see.

The Addressable Devices for SLC!

Under this segment, I’ll break down all the SLC addressable devices in the fire alarm system that you should know:

1. Smoke Detector

It’s a device that can detect smoke, especially in the event of a fire. This works when the control panel processor sends signals to the detectors and checks their status.

Smoke Detector
Image: wikipedia.org

After that, the detector gives Normal, Trouble, or Alarm conditions based on the situation and communicates to the control panel.

In a fire alarm system, commercial smoke detectors transmit a signal to a central fire alarm control panel. While they don’t typically sound alarms on their own, certain units may incorporate built-in sounders for added functionality.

2. Heat Detector

This is a device that senses heat or high temperature that occurs in a fire incident. It works by intermixing on the circuit for zones that detect heat or fire.

Heat Detector
Image: globalspec.com

And this type of device is set to work in 2 modes−

  • Rise Operator: The heat detector activates when the temperature rises rapidly within a specified range.
  • Fixed Temperature: The heat detector triggers when the temperature reaches a predetermined fixed threshold.

Thanks to this programming, the heat detectors can sense better.

3. Addressable Output Modules

As the SLC is a pathway that works both ways, it can be used to send signals to devices (input) and get currents from FACP (output). The addressable output modules are the output relays.

These operate when the circuit of other devices (heat or smoke detectors) communicates with the addressable monitor and control modules. Here’s how it works:

  • If the FACP requires turning on an output module (waterfall device or fire alarm), it offers the signal to the module using the SLC.
  • If the module detects an issue, it sends the signal to the FACP via the SLC.

Now that you know about its basic working, it’s time to understand its types:

Addressable Monitor Relay

The monitor module allows remote devices that open or close a circuit to activate specific events in the system. It works a lot like the NAC but there are differences. The monitor relay works outside the FACP while the NAC works inside the FACP.

The FACP sends a little voltage to the addressable devices to know whether smoke or heat is detected. To do this, the control panel uses the addressable monitor relay to observe the entire circuit condition of initiating devices (manual pull stations, smoke detectors, and so on).

Addressable Control Relay

The control module lets the FACP selectively turn on the system remote relay outputs or NAC (stands for Notification Appliance Circuits).

As the FACP gets the signals of smoke or heat in a zone, it immediately sends signals using the control module to NAC so that the activation signals go to Notification Appliances (horn or fire alarm).

4. Manual Pull Station

Also known as a call point or firebox, A pull station is a manually activated part of the fire alarm system. It’s mostly a red box located on the wall with clear instructions on how to use it.

To use the manual pull station, you only need to trigger the alarm system by pulling it down or breaking the glass to hit the button. It works by intermixing on the circuit with the right response programmed into the FACP (stands for Fire Alarm Control Panel).

Manual Pull Station
Image: blog.koorsen.com

There are 2 types of manual pull stations. These are:

  • Single-action Pull Station: They are exposed with no covering or special activation mechanism. To use them, you’ll need to just pull the handle down.
  • Dual-action Pull Station: They are protected with a glass covering to stop mistaken activation. In order to use it, you’ll require to break the glass and then hit the button to activate the fire alarm.

5. Fire Sprinkler Switches

The sprinkler water flow switch is a vital device of the fire sprinkler system. It senses the flow of water via the pipes of the fire sprinkler system.

Fire Sprinkler Switches
Image: douglaskrantz.com

When it isn’t activated, the stagnant water remains in the pipes and won’t flow. However, when it is turned on by fire-induced temperatures, the stagnant water starts to flow via the system and comes out of the fire sprinkler.

And, it detects this change in the water activity and then signals the changes to the FACP. After this, the sprinkler system gets turned on.

Understanding the Styles or Classes of SLC!

The Styles or Classes of the SLC do vary which can be understood if you know the basics of FACP electrical work. Let me share it briefly and show the differences below:

Style 4 (Class B)

It’s the traditional wiring style of SLC using 2 wires only. The SLC Style 4 or Class B contains no unnecessary pathway return to the control. Plus, this kind of wiring style is susceptible to wiring faults or issues.

Style 4 Class B

In this type of wiring, the Branch Line Resistor from the End of the Line (Line End) to the FACP panel should not be over 50 ohms per Branch.

Style 6 (Class A)

The SLC Style 6 or Class A is a commonly used wiring circuit that uses 4 wires to make an SLC loop. It has a benefit of a redundant circuit that returns to the control panel. This kind of wiring ensures you get the alert even if one wire breaks apart thanks to the looping system.

Style 6 Class A

Due to this pathway, it gets easier for the unit (FACP) to receive any signals immediately if it detects fire or smoke. Based on the NFPA 72 (2013) rules, it is suggested to use Fiber Optics cable as this doesn’t affect by ground and short.

Style 7 (Class X)

This kind of wiring style contains the same elements as Style 6 (Class A) and uses the redundant pathway. It is supposed to let the circuit stay functional even if a wire-to-wire short occurs.

Style 7 Class X

Explaining the SLC Performance and Faults!

Based on Class (A, B, and X) and wiring, the SLC performance can give different outputs. If the wiring is physically mismatched, the performance of the SLC will be shown in the Trouble Message.

FYI, the SLC in fire alarm can be in 3 states in Trouble: Open, Short, and Ground. Let me explain it in a table so you can understand better:

Fault TypesClass BClass AClass X
Single OpenTroubleAlarm, TroubleAlarm, Trouble
Single GroundAlarm, Trouble (Ground)Alarm, Trouble (Ground)Alarm, Trouble (Ground)
ShortTroubleTroubleAlarm, Trouble
Short and OpenTroubleTroubleTrouble
Short and GroundTroubleTroubleAlarm, Trouble
Open and GroundTroubleAlarm, TroubleAlarm, Trouble
Communication LossTroubleTroubleTrouble

To understand it better, here are a few things that you’ll need to know:

  • Alarm means the control panel signals a trouble condition for a specific fault type.
  • Trouble means the control panel needs to process an alarm input signal in the state of a specific fault type.
  • Open means the failure of one or more active field devices that can’t communicate properly with the control panel.
  • Short means the devices not passing signals to the regular path via addressable devices. Instead, the devices use a shortened electrical pathway.
  • Ground means the failure of earth connection in field devices.
  • Communication Loss indicates the devices can’t communicate or pass signals to the control panel.

The SLC Loop Wiring − Style 4 and Style 6 or 7!

Based on the NFPA (National Fire Protection Association), 2 types of SLC loop wiring are used. And these are:

The SLC Loop Wiring

2-Wire SLC Loop (Class B/Style 4)

In this kind of SLC loop, the total wire is used in two. For that, it either uses Channel A or B to make the SLC loop by connecting negative and positive lines to the proper control module’s terminal block.

4-Wire SLC Loop (Class A or X /Style 6 or 7)

It contains 4 wires in the control module’s terminal block. In this type of SLC loop, Channel B is an output loop and Channel A is a loop return. For that, it required connecting negative and positive lines to the terminal block.

Frequently Asked Questions (FAQs)

1. What is a fire alarm supervisory signal?

The supervisory signal means there is an error with one of the connected systems and is monitored by the fire alarm system.

2. What does a fire alarm system consist of?

The fire alarm system is made of devices like smoke detectors, heat detectors, manual stations, audible warning devices, and so on.

3. What is a fire alarm SLC loop?

The SLC loop is a 2-wire or 4-wire connection that makes it possible for addressable devices to communicate with the control panel.

4. How far can I run my addressable loop wire?

Based on the wire size, wire types, and device number, you can find the right distance to run your addressable loop wire. Check the fire alarm panel’s manual to know the exact distance detail.

5. What is the difference between SLC and NAC in fire safety?

The main difference between SLC and NAC is the purpose. Well, the SLC is a circuit or pathway that lets communication between devices and the control panel. Then again, the NAC is a pathway that sends signals from the control panel to devices that notify people.

Wrapping Up

Now, you know what SLC stands for in fire alarm systems! It is a vital part of the fire alarm system as it needs to send signals or powers to ensure communication between the control panel and other devices (like fire alarms and heat or smoke detectors).

Next time when the technician talks about SLC, you can understand what is being said. Hope you find it helpful!