Douglas Krantz - Technical Writer - Describing How It Works

NFPA's 7 Classes of Fire Alarm Paths

In fire alarm systems, a path is the route that signals and power travel between panels, and between panels and devices. The paths are classified by what happens when the pathway has a failure.

The NFPA used to describe the Class and the Style of wiring, getting into more and more detail. Now, just to be a little clearer, they have decided to classify pathways.
Rather than placing the emphasis on the techniques to achieve a Class or a Style, the NFPA is placing an emphasis how the pathway is going to function when things go wrong.

By Douglas Krantz

The NFPA has divided the signal paths in a fire alarm system into 7 classifications: Class A, Class B, Class C, Class D, Class E, Class N, Class X. These classifications don't show how to wire anything, these classifications show what happens when things go wrong.

To start with, as a life-safety system, a fire alarm system detects fire and lets people know about the fire. That, though, is what the input and output devices do.

Usually the fire alarm system includes with the input and output devices some sort of control system. This is the panel where the signals from the input devices (detectors and activation switches) go to and where the signals to the output devices (horns, strobes, relays, etc.) come from. Often, the control panel will also provide power to the devices.

Between the input devices, the output devices, and the control system is the communication and power infrastructure; this consists of the signal and power paths.

Fire Alarm Communication Paths

In Greece, over 2500 years ago, near the small town of Marathon, there was a battle. After the battle, one of the winning Athenians ran all the way from Marathon to Athens carrying the news.

He ran along a path.

Nowadays, the message could be carried by a person running along a narrow mountain road, a verbal telephone call, a news story over the microwave towers, a data signal carried over fiber optics, Etc.; so may choices.

The NFPA would consider all of these to be communication paths. The paths are no longer just copper wires, but wireless radio waves are also used, Ethernet data cables are also used, and fiber optic cables are also used in fire alarm systems.

Because of all the different types of communication paths for fire alarm systems being used nowadays, the NFPA is addressing them all differently than they did in the past.

Communication Paths and Their Classification

Before looking at the correct way to send signals along a path, we'll look at the wrong way. To wire up a fire alarm system, an electronic technician (but one who's extremely lazy can use lamp cord laying across the floor and under the rugs. That technician might not even include a self-checking system to let the panel know when a device is no longer connected.

When this system is first installed, it'll work; it'll detect fire and let people know about the fire --- at least for a while. If the NFPA addressed this kind of wiring path, though, the Class of wiring would be "Class-Don't".

A pathway classification describes more than that. When describing a Class, the NFPA is concerned with is Reliability, Fixability, and Survivability.

Reliability - The NFPA wants to make sure the fire alarm system continues to work in the long run.

Fixability - The NFPA wants to make sure any problems that do occur are found and fixed on a timely basis.

Survivability - The NFPA wants to make sure the fire alarm system will continue to work when fixing it on a timely basis isn't good enough.

In a fire alarm system, there are (at the moment) seven Classes of communication and power infrastructure (paths):
The letters after the word Class are not shown in the order of reliability or importance; the letters after the word Class are only the name of the particular classification.


There are three things considered by the NFPA with the carrying of the signals on the fire alarm system paths:
  1. Supervision - The method of self-checking for faults - the end-of-line resistor continuity check and handshaking using data signals are two common methods
  2. Redundancy - the continued operation of the whole fire alarm system, or a second path to carry signals around a problem is redundancy - an open wire or wire-to-wire short are some of the problems addressed with redundancy
  3. Protection from damage - conduit is one common method used to protect the path

Class of Paths

The Classes of the Paths are performance designations. Rather than describing how to build and use the communication paths (the NFPA is not a "How-To-Do-Anything-Book"), the NFPA uses Class types to describe what should happen in a fire alarm system along a path when things go wrong.

For each of the different types of communication paths, the classification types show the minimum requirements expected. If the minimum requirements of a Class aren't met, the Path is in a lessor Class.

The following descriptions show briefly the minimum requirements of each Class.

Ground Faults - All Classes

We'll start with a ground fault. In most cases, to qualify as a Class, a single ground fault will not be the cause of a failure in the system, and any single ground fault will result in a trouble showing on the panel. However, because wireless systems, fiber optic systems, and some data systems (like Ethernet) don't pass shorts caused by ground faults to the panel, a ground fault indication is not always needed.

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 and 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

  1. Signals from the fire panel to the monitoring company
  2. The use of IP (Internet Protocol), whether it's local communications or over the Internet
  3. The communicators over the telephone lines that are still in use

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:
  1. The fire door closes
  2. Emergency door locks release
  3. The damper closes
  4. The fans shut down

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

This is basically local Ethernet, Token Ring, or other network or IP infrastructure.
  1. 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
  2. These pathways are verified through end to end communication, like data handshaking
  3. Loss of communication between end points on any path show a trouble signal on the panel
  4. 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

Separation of Wiring

In order to be proper classified as Class A or Class X, the outgoing and return path routes for both Class A and Class X have to be separated by a certain distance; the two paths cannot be inside the same conduit, for instance.

The Class of the Path

Overall, the path used in a fire alarm system between devices and panels has become more of an idea than a specific type. The Class of the path is considered to be the quality of the path: the reliability, the fixability, and the survivability of the path.

Recommended Reading

I really, really recommend purchasing the "NFPA 72 National Fire Alarm and Signaling Code Handbook". It has the complete official NFPA Code, and included along with the Code are lots of explanations about what the Code means.

Basically, explaining what the fire alarm system should look like, the NFPA 72 Code itself is legal-talk; explaining what the NFPA means with the legal-talk, the Handbook has added comments (using terms that even I can understand) to help show what the Code means. They even use illustrations.

Yes, the Handbook is more expensive. However, to get an understanding of what is meant by the legal terms shown in the codebook, the added explanations are worth much more than the extra expense.

The Handbook is available in hardbound, in softbound, or even in electronic form for desktop or mobile devices on the official NFPA website ---
Life Safety
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