Circuits and Pathways in NFPA 72

 Circuits and Pathways in NFPA 72

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.

Beginning with the 2010 edition of NFPA 72, National Fire Alarm and Signalling Code, all of the “styles” of circuits were deleted, and four “classes” were added. A new chapter, Circuits and Pathways, was added during the reorganization of this code. In the past, the styles only described the operation during an alarm or fault condition for initiating device circuits, notification appliance circuits and signalling line circuits. NFPA 72, National Fire Alarm and Signalling Code, defines the performance of fire alarm circuit and pathways in Chapter 12. While the terms “circuit” and “pathway” are often used interchangeably, they are different. The styles of initiating device circuits (IDC), notification appliance circuits (NAC) and signalling line circuits (SLC) were eliminated, and four new classes were added. The intent was to add information about all circuits and pathways, not just IDCs, NACs and SLCs. A circuit is defined in Chapter 3 as “either means of providing power or a connection path between locations”, while a pathway is “any circuit, conductor, optic fiber, radio carrier or other means connecting two or more location.” Basically, a circuit is copper, while a pathway can be copper or any other type of connection.

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

Pathway performance is defined in Section 12.3, using Class A, B, C, D, E, N, or X based on the circuit performance. NFPA 72 does not determine what class is used on a particular pathway, but rather defines the performance of the pathway. The specification, designer, AHJ determine what class of pathway is used. NFPA 72, Chapter 24, “Emergency Communications Systems,” Section 24.3.14 is the only place I have found any requirements for survivability. Survivability for fire alarm pathways is typically only required for systems employing partial evacuation or relocation. Since building and fire codes require systems to be installed in accordance with NFPA 72, they do not have specific provisions for survivability. The Chapter 24 technical committee is proposing important changes to the 2022 edition of the code.

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):

• Class A
• Class B
• Class C
• Class D
• Class E
• Class N
• Class X

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.

Chapter 12 describes each class. Here are some examples of each you may find useful.

 

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:

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

Prior to the 2010 edition of NFPA 72, Class D pathways simply referred to as “fail-safe.” A typical application for a Class D pathway is a powered relay used for door release or fan control. Since the relay is powered in normal condition, operation of the relay is accomplished by removing power by control panel operation, or by a failure of the circuit.

 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

Class E pathways are not monitored for integrity and are used in applications where supervision is not required. Section 12.6 of NFPA 72 defines applications where monitoring for integrity is not required. Some examples include interconnection between equipment in an enclosure, interconnection between control equipment within 20 feet of each other where conductors are protected by conduit, and interconnection wiring of a stationary computer to its keyboard, monitor, or mouse where the wiring does not exceed 8 feet.

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

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.

Many designers and contractors do not include survivable cable or fire-rated enclosures in their designs for in-building fire emergency voice/alarm communications systems (EVACS) wiring as required in the code—or at least not until the authority having jurisdiction enforces the requirement because a designer missed it the first time around. This may be due in part to some confusion over where survivability is required in the first place. Pathway survivability is not required for all EVACS; it is only required where occupants will remain in the building during an emergency (i.e., relocation or partial evacuation). The intent is to ensure that we can still get subsequent instructions to those occupants if the situation worsens. For those systems where pathway survivability is required, it does not necessarily need to be applied throughout the entire system; it must be applied only where the circuit runs through multiple notification zones, so that a fire in one notification zone doesn’t impair the ability to communicate with occupants in another. Additionally, the current edition of NFPA 72 permits the installer to use Level 1, Level 2, or Level 3, meaning that a sprinklered building (i.e., Level 1 pathway survivability) does not require the use of fire-resistant cabling or an enclosure. No wonder a contractor or installer might not include fire-resistive cable in an initial design.

The reason behind survivability requirements is to maintain the operational reliability of the EVACS during a fire. Pathway survivability for emergency communications systems (including EVACS) is covered in 24.3.14; Section 12.4.5 includes the definition of Level 4 pathway survivability.

The previous requirements for survivability said only that the cable had to be two-hour-rated or installed in a two-hour-rated enclosure. The technical committee changed the requirement so that it is now related to the fire resistance rating of the building construction. This relationship is important because the listing of two-hour fire-rated cable is required to be attached to a concrete wall or floor assembly, constructed with a minimum of two-hour rating or as otherwise identified by the UL FHIT systems assembly.

Additionally, as stated in the Annex A of the code, “installing a pathway survivability Level 2 or Level 3 in a building that is constructed with less than two-hour construction would not be installed within the listing of the product.”

For building construction with a fire-resistance rating of one hour, but less than two hours, the technical committee has proposed a new Level 4 pathway survivability.

With the more common use of timber construction for larger buildings, the expectation is these buildings would be classified as one-hour fire rated. Two-hour cable, as it is presently listed, would not be allowed when using the proposed Level 4 wiring or with wiring that meets the proposed separation requirements. That prohibition is due to the concern that a fire causing an open, ground-fault, or short-circuit fault on the communication and control circuits between rooms or enclosures could affect the operation of the in-building fire emergency voice/alarm control equipment within these rooms or enclosures.

It is well known that the insulation on fire alarm cabling can melt when exposed to heat and flames, causing the conductors to short circuit. When this condition occurs, it is possible for an entire signaling line circuit to fail, seriously impacting communication and control circuits. To help guard against these kinds of catastrophic failures, the proposed code would require the use of fault circuit isolators to ensure that the entire circuit is not disabled during a fire.

These proposed changes to the 2022 code would provide guidance in Annex A to assist designers, contractors, and AHJs in the proper separation of circuits as well as the location of fault circuit isolators to ensure both compliance with the code and a high degree of operational reliability. It is important to remember that for fire alarm systems designed for relocation and partial evacuation, communication must remain operational on all floors other than the fire floor during a fire. The primary fire protection goal is reliable communication during the fire.