Douglas Krantz - Technical Writer - Describing How It Works

How are the IDC, the NAC, and the SLC Supervised?

By Douglas Krantz | Descriptions | 8 minutes of reading

How are the IDC, the NAC, and the SLC Supervised?


How are the IDC, the NAC, and the SLC Supervised?


Greetings Douglas,

It's been my experience that in an SLC circuit as opposed to an IDC circuit, the communications are needed to be supervised as well. The digital signals are still being applied however in a dual purpose. Please elaborate.

Thank you, ME

To understand supervision in a fire alarm system, we have to step back and look at the overall concept of a fire alarm system. A fire alarm system is a Fire Detection and Alarm System (FDAS).

The detection part consists of devices like smoke detectors, pull stations (MCPs or Manual Call Points), heat detectors, waterflow switches, etc., and the alarm part consists of devices like horns, strobes, bells, chimes, etc. Everything else is support for these devices, and the supervision method used for everything else varies wildly.

Supervision - Not Just a Light and a Buzzer

The intent of supervising a fire alarm system was never to just turn on a light and a buzzer; the intent of supervising a fire alarm system is to keep the fire alarm system working to detect fires, and, by sounding the alarm, to warn people. If it wasn't important to always be able to detect fires and warn people, there would be no need for supervision.

The goal of supervision is to get the fire alarm system to resume looking for fire, should somethig go wrong.



Each type of wired circuit can be wired as Class B or Class A.

Class B IDC or Initiating Device Circuit



Wire supervision is checking continuity of the wire circuit.



This is Wire Supervision.

The words Initiating Device Circuit is a technical term for the input circuit to a conventionally wired fire alarm system. A Conventional IDC has a very specific method of wiring.

A switch in a pull station can short out the IDC, sending an alarm, but a switch cannot be supervised; when it is not sending an alarm, no current goes through the switch.

The best that a fire alarm panel can do to supervise the switch is to make sure it is always connected. The panel forces a small current through one of the wires, which connects to one of the inputs to the switch, through the end of line resistor, back to the other input to the switch, and back through the other wire. As long as this current goes through the circuit, the panel considers the circuit to be normal.

When the current stops, the panel shows a trouble.



When the current stops, the panel shows a trouble.
  • When a wire breaks, the current stops
  • When a wire comes loose from one of the terminals of the switch, the current stops
  • When a person removes the switch, the current stops

Any IDC that has a broken wire or loose connection is no longer being supervised.

Class B NAC or Notification Appliance Circuit



Wire supervision is checking continuity of the wire circuit.



This is Wire Supervision.

The words Notification Appliance Circuit is a technical term for the output circuit from a conventionally wired fire alarm system. A Conventional NAC has a very specific method of wiring.

A horn or strobe has a current blocking diode. When the circuit is sounding the alarm, a Positive Voltage is applied to the Plus Terminal of the horn or strobe, and a Negative Voltage is applied to the Minus Terminal. When supervising the circuit, the voltage reverses.

When the circuit is in the supervision mode, a Negative Voltage is applied to the Plus Terminal of the horn or strobe, and a Positive Voltage is applied to the Minus Terminal. When supervising the circuit, the current is being blocked by the diode.

The best that a fire alarm panel can do to supervise the horn or strobe is to make sure it is always connected. The panel forces a small current through one of the wires, which connects to one of the inputs to the horn or strobe, through the end of line resistor, back to the other input to the horn or strobe, and back through the other wire. As long as this current goes through the circuit, the panel considers the circuit to be normal.

When the current stops, the panel shows a trouble.



When the current stops, the panel shows a trouble.
  • When a wire breaks, the current stops
  • When a wire comes loose from one of the terminals of the switch, the current stops
  • When a person removes the switch, the current stops

Any NAC that has a broken wire or loose connection is no longer being supervised.

SLC Polling or Signaling Line Circuit Polling



Devcie supervision is like taking attendance



Whether it's a Class A circuit or a Class B circuit, all devices on an SLC circuit are supervised by polling. Like a teacher taking attendance, the electronic name of a fire alarm detector or module is called out, and the device should answer. Whether the device says "Here and Normal", "Here and in Alarm", or "Here and in Trouble", the panel can assume that the wiring is complete between the panel and the device.

Class B SLC or Signaling Line Circuit



This is Device Supervision. Commonly, this is referred to as an Addressable System.



This is Device Supervision. Commonly, this is referred to as an Addressable System.

With a Class B SLC circuit, it doesn't matter how many T-taps are there. So long as all the devices on a Class B SLC circuit respond to the polling with "Here", all devices are assumed to be connected to the Fire Detection and Alarm System.

There are a few manufactures that have a limit to how many T-taps can be on the circuit, but that number is usually quite high.

Class A Circuits

Class A Conventional Circuits supervise the wiring exactly the same way as Class B Conventional Circuits. The difference is that Class A Circuits do something when a wire breaks, Class A provides a work-around-path or redundant path around the broken wire.

By placing the end of line resistor inside the circuit board, if the circuit has a trouble, the panel can switch to feeding the circuit from both ends.

Class B wiring means that nothing communicates beyond a wire break, Class A wiring means that all devices communicate even if a wire breaks.



Whether it's an IDC circuit, or a NAC circuit, the end of line resistor is inside the circuit board.

Class B wiring means that nothing communicates beyond a wire break, Class A wiring means that all devices communicate even if a wire breaks.



SLC Circuits (Addressable)

A Class B SLC doesn't have an end of line device to supervise the wires, however, a Class A SLC has an end of line device to supervise the wires. The end of line device is located inside the control panel's circuit board. If a wire breaks or comes loose from a connection, the panel can switch to feeding the SLC from both ends of the circuit.

When the control panel switches to using a redundant path, the control panel is no longer supervising the wires, but the panel is still supervising the devices.

Class B wiring means that nothing communicates beyond a wire break, Class A wiring means that all devices communicate even if a wire breaks.



Loss of Supervision

Any time a wire breaks or comes loose in a IDC, NAC, or SLC circuit, wire supervision is no longer working.

Multi-Purpose Supervision

Checking to make sure that input devices and output devices can communicate with the panel is the primary purpose of supervision. From the Fire Detection and Alarm System FDAS) point of view, the alarm signals are what're important; from a communication point of view, supervision is what's important.

Douglas Krantz

facpdoug@douglaskrantz.com
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