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Fire Alarm -- Description

Class B Wiiring is constantly watched or supervised by the fire alarm panel
For signal communication outside the panel, Fire Alarm Systems use Loops of Wire. Because this is a Life Safety System, the Loops are Supervised to make sure the wires and connections are always secure.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

How Are Fire Alarm Loops Supervised to Make Sure They Continue to Work?

By Douglas Krantz

Because the fire alarm system is a Life Safety System (people's lives depend on the fire alarm system working in an emergency), using supervision, the panel makes sure the field devices are always connected to the loop. However, supervision isn't just the panel turning on its yellow light and sounding its local buzzer. When trouble occurs on the system, supervision is the owner doing something about the trouble.

  • If the panel's local sounder keeps making noise every 24 hours, for instance, the building management calls for service. That's supervision.
  • If a wire to a door holder breaks, the door won't stay open. The building management calls to get the door holder working again. That's also supervision.
To make sure the wiring is supervised, the wires are installed in the form of loops, according to strict standards, designed specifically for fire alarm systems.

Fire Alarm Loop

In a fire alarm system, a loop is a pair of wires. It carries power and signals between the circuit boards inside the control panel and the off-panel devices in the field.

The loop:
  • As an Initiating Device Circuit (IDC), carries signals from the input devices to the panel
  • As a Notification Appliance Circuit (NAC), carries signals and power from the panel to the fire horns, speakers, and strobes, causing the devices to make noise and flash lights, telling the occupants of the alarm
  • As a Signaling Line Circuit (SLC), carries signals in the form of data between the panel and the input and output devices
  • As a Power or Control Circuit, carries signals or power to specific fire safety controls and devices

IDC (Initiating Line Circuit)

The IDC is a pair of wires without t-taps, connecting the panel to the input sensing devices.

Fire Sensing Input Devices (Alarm)
  • Waterflow
  • Smoke Detectors
  • Heat Detectors
  • Manual Pull Stations
  • Fire Suppression System Alarms
Non-Fire Input Sensing Devices (Supervising other systems)
  • Duct Detectors (in older systems these were alarm)
  • Fire Suppression System Supervisories
  • Fire Pump Supervisories
  • Gatevalve Tamper Supervisories
To avoid signal confusion, Fire Sensing (Alarm) and Non-Fire Sensing (Supervisory) devices are not mixed on the same IDC loop.

The panel supervises the fire alarm wiring using an end-of-line resistor. This resistor allows small electrical current to pass through the wires of the loop, making sure the wires and connections are always complete. For a Class B loop, the end-of-line resistor may be in a distant part of the building, or for a Class A loop, the end-of-line resistor is part of the circuitry of the panel.

NAC (Notification Appliance Circuit)

Wired similar to the IDC Class A or Class B loop, the NAC carries power to operate the "Notification Appliances":
  • Fire horns
  • Strobes
  • Bells
  • Chimes
  • Speakers
In alarm, the voltage on the loop is forward biased; the electrical current provided by the panel goes through the notification appliances, letting people know about the alarm. They are notified.

During normal times, when there are no alarms, the voltage polarity is reversed, allowing the loop to be supervised as a Class A or Class B loop. Because the polarity is reversed, no current flows through the notification appliances.

When the fire panel goes into alarm, the panel stops supervising the loop and changes the voltage polarity to normal. Electrical current then goes through the devices, causing the devices to notify the occupants of the building.

SLC (Signaling Line Circuit)

The Signaling Line Circuit (SLC) carries signals both to and from the panel; it uses data to carry the signals out from the panel to field devices, and data to carry the signals back from the field devices to the panel.

The SLC also provides a small amount of power to operate the field devices.

Field Devices Connected to an SLC
  • Input
    • Smoke Detectors
    • Heat Detectors
    • Combination Smoke/Heat Detectors
    • Pull Stations
    • Alarm and Supervisory Input Modules
  • Output
    • Control Relays
    • NAC Riser Modules
To supervise the SLC and to make sure all devices are always connected, the panel sends data to each input or output device, polling the device and asking "are you there"? The device then answers by returning data to the panel saying "I am here."

With the constant polling of the field devices, the need to supervise the wiring itself isn't necessary; the "Class B" SLC may be t-tapped.

As well as the polling of the devices, Class A SLC loops are not t-tapped and are still supervised for wiring issues.

Power or Control Circuit

The control circuit, usually from an on-board relay in the panel, turns on or off devices and systems.

The on-board relays can also be used as part of another panel's IDC loop, sending alarm, supervisory, and control signals to that other panel. This is particularly useful when the fire alarm panel itself doesn't have an on-board communicator, to provide for off-site monitoring.

The actual wiring may be a little different from the standard IDC, NAC, and SLC loops, but the control loops still have to be supervised for integrity.

Some of the uses for the on-board relays:

Fire Alarm Supervision

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All wiring outside in a fire alarm system, whether it's an IDC, NAC, SLC, Power or Control loop, needs to be supervised, and supervision includes the call for service and repair of the system, once the panel indicates trouble with the system.


Douglas Krantz

Describing How It Works

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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