Circuits and Pathways in NFPA 72
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.
- 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.
