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.
