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How to Classify a Signal Pathway

The classification is determined by what the pathway isn't or can't do - the classification is a process of elimination...
One way to find out a fire alarm pathway's classification is to use a process of eliminaton


By Douglas Krantz

A pathway is the route that a signal uses to get from one device or panel to another device or panel. The National Fire Protection Association divides the different types of pathways into classifications.

The classification for a pathway is not determined by the method of carrying a signal or the type of signal the pathway is carrying:
Yes, the NFPA classifies a pathway by what it does, but when trying to figure out if a pathway is Class A, or Class X, or whatever, one has to look at the pathway from the direction of what the pathway isn't or can't do, or how it fails; the classification is a process of elimination:

NFPA's Classifications

Class A

  1. This will include a redundant signal path - If the path is interrupted, the system feeds both ends of the paths so there are now two paths; the original outgoing path which is now cut shorter, and the return path which is now being used as an outgoing path
  2. If wires are used, a wire-to-wire short may shut down the whole path
  3. Both conventional and addressable systems fit into this
  4. Both the IDC (Initiating Device Circuit) and the NAC (Notification Appliance Circuit) fit into this
  5. The panel shows a trouble signal when there is a problem

Class B

  1. There is no redundant path
  2. Any device beyond a break won't work
  3. If wires are used, a wire-to-wire short may shut down the whole path
  4. Both conventional addressable systems fit into this
  5. Both IDC and NAC fit into this
  6. The panel shows a trouble signal when there is a problem

Class C

  1. Uses Handshaking (equivalent to an I'm OK signal) to supervise the path
  2. Can have more than one pathway
  3. The panel shows a trouble signal when there is a problem

Examples:

Class D

  1. Fail-Safe operation - If there is a failure, the device that is controlled by the fire alarm system goes into fire mode
  2. No trouble shows on the panel

This is annoyance supervision - people get annoyed when things don't work right and they want the system fixed.

Example of a device going into fire mode when a wire breaks or a signal is lost:

Class E

  1. These pathways are not supervised at all
  2. No trouble signal will be shown on the panel if the path fails

Class N

  1. This is basically local Ethernet, Token Ring, or other computer style network or IP infrastructure.
  2. Unless a single device is connected, or the path is short (less than 20 feet) and really protected in something like conduit, two pathways are used
  3. These pathways are verified through end to end communication, like data handshaking
  4. Loss of communication between end points on any path show a trouble signal on the panel
  5. Problems with one pathway won't affect the other pathway

Class X

  1. This will include a redundant signal path. Like Class A, if the path is interrupted, the system feeds both ends of the circuit so there are two circuits, the original outgoing path which is now cut shorter, and the return path which is now being used as an outgoing path
  2. Devices on both sides of an open will continue to communicate with the panel
  3. If wires are used, devices on both sides of a wire-to-wire short will continue to communicate with the panel (basically the short has to be isolated on both sides of the short)
  4. The panel shows a trouble signal when there is a problem

Ground Faults

According to the NFPA, ground faults are included in the pathway classifications. Ground faults, though, can be thought of as a "rule-of-thumb" issue: if there is a ground fault, the ground fault will show up as a trouble on the panel. Period.

There is a common sense exception to this. If there is absolutely no possibility, ever, of the ground fault affecting the operation of anything in the whole Fire Detection and Alarm System (FDAS), then that ground fault does not have to show up as a trouble on the panel. This would be something like a ground fault in a single, wireless, battery operated detector that is connected to the panel only through radio waves.

Get help finding those Ground Faults - Buy the book "Make It Work - Hunting Ground Faults" by Douglas Krantz - Ground Fault Hunting becomes easier when you know what causes the ground fault and what is needed to "see" the ground fault.
Get help finding those Ground Faults - Buy the book "Make It Work - Hunting Ground Faults" by Douglas Krantz - Ground Fault Hunting becomes easier when you know what causes the ground fault and what is needed to "see" the ground fault.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

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Get the book Make It Work - Hunting Ground Faults
Get the book Make It Work - Hunting Ground Faults
Get the book Make It Work - Conventional Fire Alarms
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