NFPA Circuits & Pathways
by
Dan Horon
Cadgraphics Incorporated
RescueLogic.com
History
In the 2007 Edition, the Style Tables had become outdated.
They were prescriptive, and prevented new technologies.
Understanding them required a fire alarm engineer.
History
Even though they were less complex than previous Editions,
different tables represented types of circuits.
History
Even though they were less complex than previous Editions,
different tables represented types of circuits.
History
Throughout the years that Style Tables were in Code, the older terms
Class A and Class B never went away.
Everyone working in fire alarm knew Class B as being a single supervised
circuit, and that Class A has an additional redundant path.
History
In the 2010 Edition, we worked to replace the tables with
simple, easy to understand language. Basically, fire alarm
tradition: 1) requires a system to monitor itself for integrity;
and 2) recommends -- but not requires -- redundant paths.
History
In the 2010 Edition, Class B is described in basic terms.
History
In the 2010 Edition, simple performance-based language
also describes circuits that had never fit as Class A or B, even
though they had been commonplace in fire alarm systems.
History
Class 7, the most fault-tolerant, but also most expensive
type of circuit is now called Class X. Isolator modules are
added to prevent a short-circuit from affecting equipment.
History
The Annex of the 2010 Edition explains the pathway Classes,
and gives examples. More dramatically, it explains that a
single ground fault does not need to be reported if it does
not affect the circuit.
Current
Then, in the 2013 Edition, we added back the prescriptive
requirement to report a single ground.
Approved Fire Alarm Pathway
One familiar type of Class B pathway has wires with direct
current, and a resistor at the last device. The end-of-line
resistor offers a level of assurance that the all wires are
connected to the intended device.
(+)
(-)
(+)
(-)
End-of-Line
Resistor
End-of-Line
Resistor
FACP
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
If a short occurs at any point on a Class B notification
appliance pathway, the entire circuit becomes inoperable.
(+)
(-)
X
(+)
(-)
End-of-Line
Resistor
End-of-Line
Resistor
FACP
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
If an open occurs at any point on a Class B initiating device
circuit, every device thereafter is inoperable. This is true
even when a smoke detector is removed for maintenance.
(+)
(-)
(+)
(-)
End-of-Line
Resistor
End-of-Line
Resistor
FACP
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
Class B pathways report a single ground connection because,
a second ground may connection occur at some point later,
and act like a short-circuit. On a Class B IDC, a short-circuit
means alarm!
(+)
(-)
(+)
(-)
End-of-Line
Resistor
End-of-Line
Resistor
FACP
Wire touching ground
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
Class B pathways report a single ground connection because,
a second ground connection may occur at some point later,
and act like a short-circuit. On a Class B IDC, a short-circuit
means alarm!
(+)
(-)
(+)
(-)
End-of-Line
Resistor
End-of-Line
Resistor
FACP
Two wires touching ground
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
Class B pathways report a single ground connection because,
a second ground connection may occur at some point later,
and act like a short-circuit. And, on a Class B IDC, a shortcircuit means alarm!
(+)
(-)
(+)
(-)
End-of-Line
Resistor
End-of-Line
Resistor
FACP
Two wires touching ground
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
Up to this point, we’ve seen the required methods of
monitoring wires. We don’t know if any device is actually
functional, but we can be reasonably certain the intended
wires are connected.
(+)
(-)
(+)
(-)
End-of-Line
Resistor
End-of-Line
Resistor
FACP
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
The Class B, multi-drop signaling line circuit gives each
device a numerical address, and the control unit can
communicate with all attached devices on just two wires in
parallel.
(+)
End-of-Line
Resistor
(-)
(+)
(-)
001
002
003
004
Multi-drop End-of-Line
Communications
Resistor
FACP
101
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
An open on a Class B multi-drop SLC reports a trouble if the
control unit cannot communicate with every device.
(+)
End-of-Line
Resistor
(-)
(+)
(-)
FACP
001
002
003
004
Multi-drop
Communications
X
101
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
For decades, smart systems have been sold as an improvement over
conventional fire alarm systems. AHJs can see the obvious improvement
that communication with each device provides.
Yet, minimum Code requirements do not require operational capability
of each device to be known. We only require the wires to be monitored.
(+)
End-of-Line
Resistor
(-)
(+)
(-)
001
002
003
004
Multi-drop
Communications
FACP
101
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
Shorting the two wires at any point stops communication
with every device. Not an indication of alarm as with an IDC,
a short on the Class B multi-drop SLC reports a trouble.
(+)
End-of-Line
Resistor
(-)
(+)
(-)
001
002
003
004
Multi-drop
Communications
FACP
X
101
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
While every device is attached to the same two wires, a single
ground reports a trouble. But, the single ground is not allowed to
impact communications.
(+)
End-of-Line
Resistor
(-)
(+)
(-)
001
002
003
004
Multi-drop
Communications
FACP
101
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Approved Fire Alarm Pathway
A second ground on a Class B, multi-drop signaling line
circuit stops all communication, as it is a short-circuit.
(+)
End-of-Line
Resistor
(-)
(+)
(-)
001
002
003
004
Multi-drop
Communications
FACP
Version 3
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Comparison 1
Every single connection on an Ethernet network is required
by IEEE 802 (an ANSI standard) to be galvanically isolated.
Traditional Fire Alarm
Report a
single
ground
A single ground must report
even if it has no impact on
communication. Wireless or
fiber optic paths are excepted
because they cannot be
grounded, but IEEE 802
isolation means nothing.
Cat 5/6 Ethernet
A ground has no impact on IEEE 802
equipment. Every connection is
galvanically isolated, which prevents
propagation of transients, shorts, and
grounds. (IEEE 802 is a verifiable ANSI
Standard that can be referenced in
Code.)
Comparison 2
Class A and X paths have exactly two paths that will work if
the other fails. A mesh network usually has more than two
paths, not knowing exactly where a failure might occur.
Redundant
paths
Traditional Fire Alarm
Cat 5/6 Ethernet
Redundant paths do not exist
unless they are separately
monitored for integrity.
We must know the status of
opens, grounds, and shorts on
every connected wire.
A mesh network design remains
operational even after segments fail.
Critical paths have redundancy, and
failures of essential equipment report
to responsible personnel.
Comparison 3
Buildings are increasingly dependent on the network
infrastructure.
Traditional Fire Alarm
Building
networks
No fire alarm equipment
should ever be connected to a
building network
infrastructure. Building
networks are not as reliable as
traditional fire alarm systems.
Cat 5/6 Ethernet
Building networks are a new
infrastructure. Modern building
systems have standardized on LANs,
WANs, and VLANs to interconnect
intelligent equipment. A pair of
parallel wires connected to multiple
addresses is inherently unreliable.
Wiring Comparison
Every single connection on an Ethernet network is required
by IEEE 802 (an ANSI standard) to be galvanically isolated.
Wiring
Traditional Fire Alarm
Cat 5/6 Ethernet
Terminations
Wire nuts, junction boxes,
Encased cables, RJ45 ends
stripped wires, screw terminals
Length
Wire length is only limited
when voltage drop and
capacitance prove
communication failure.
The same wires touch every
device in parallel. Like an old
party-line telephone system, all
devices talk on the same wires.
Paralleled
Version 1
Segmented every 300 feet or less.
Packets are refreshed by forwarding
equipment.
Multi-layer design. Each device is an
individually addressable endpoint.
Devices are not physically wired in
parallel with each other.
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Proposed Fire Alarm Pathway
Network equipment can be thought of in two basic
categories: Data Endpoints and Data Forwarding Equipment.
Data Endpoint
Version 1
Data
Forwarding
Dan Horon © 2013 DanHoron@RescueLogic.com
Data Endpoints
Permission required for distribution.
Category 5 Ethernet Cable
In between Data Endpoints and Data Forwarding Equipment,
a standardized cable is used. ‘Cat 5’ is an example.
Transmit
Data
Data Endpoint
generates
Receive or acts
Data
on alarm
events
IEEE requires
1
2
3
4
5
6
7
8
Transmit Data +
Transmit Data Receive Data +
NC
NC
Receive Data NC
NC
IEEE requires
galvanic isolation on
each pair of wires
Version 1
Data
Forwarding
1
2
3
4Network hub
5or switch that
6
7forwards data
8to endpoints
galvanic isolation on
each pair of wires
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Category 5 Ethernet Cable
Each Cat 5 cable is galvanically isolated at each end, inside the
equipment. The isolation helps prevents transient grounds and
shorts on one cable from affecting other components.
Transmit
Data
Data Endpoint
generates
Receive or acts
Data
on alarm
events
IEEE requires
galvanic isolation on
each pair of wires
Data Endpoint
1
2
3
4
5
6
7
8
Transmit Data +
Transmit Data Receive Data +
NC
NC
Receive Data NC
NC
IEEE requires
Cat 5 Cable
Four wires are used
for data transmission
generates or acts
on alarm events
Version 1
Data
Forwarding
1
2
3
4Network hub
5or switch that
6
7forwards data
8to endpoints
galvanic isolation on
each pair of wires
Data
Forwarding
Network hub
or switch that
forwards data
to endpoints
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
If Transmit Data (+) and (–) or Receive Data (+) and (-) are shorted
together, communication stops. In fire alarm systems, a fault
condition must be reported within 200 seconds.
Transmit
Data
Receive
Data
IEEE requires
1
2
3
4
5
6
7
8
Transmit Data +
Transmit Data Receive Data +
NC
NC
Receive Data NC
NC
X
IEEE requires
galvanic isolation on
each pair of wires
galvanic isolation on
each pair of wires
Data Endpoint
Data
Forwarding
generates or acts
on alarm events
Version 1
1
2
3
4
5
6
7
8
Network hub
or switch that
forwards data
to endpoints
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
A ground connection on any one signal wire does not block
communication. IEEE requires isolation at each end of every Cat 5
cable. Every data packet is checked for errors and re-transmitted
until verified.
Transmit
Data
Receive
Data
IEEE requires
1
2
3
4
5
6
7
8
Transmit Data +
Transmit Data Receive Data +
NC
NC
Receive Data NC
NC
IEEE requires
galvanic isolation on
each pair of wires
galvanic isolation on
each pair of wires
Data Endpoint
Data
Forwarding
generates or acts
on alarm events
Version 1
1
2
3
4
5
6
7
8
Network hub
or switch that
forwards data
to endpoints
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
A second ground connection, on the other wire of a
matched pair, will impair communication if it causes a short.
A fault condition must be reported within 200 seconds.
Transmit
Data
Receive
Data
IEEE requires
1
2
3
4
5
6
7
8
Transmit Data +
Transmit Data Receive Data +
NC
NC
Receive Data NC
NC
IEEE requires
galvanic isolation on
each pair of wires
galvanic isolation on
each pair of wires
Data Endpoint
Data
Forwarding
generates or acts
on alarm events
Version 1
1
2
3
4
5
6
7
8
Network hub
or switch that
forwards data
to endpoints
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Again, each Cat 5 cable is galvanically isolated. Grounds and
shorts have no direct path of electrical connection to other
equipment or devices.
Transmit
Data
Receive
Data
IEEE requires
1
2
3
4
5
6
7
8
Transmit Data +
Transmit Data Receive Data +
NC
NC
Receive Data NC
NC
IEEE requires
Data Endpoints
galvanic isolation on
galvanic isolation on
each pair of wires
each pair of wires
Data Endpoint
Data
Forwarding
generates or acts
on alarm events
Version 1
1
2
3
4
5
6
7
8
Network hub
or switch that
forwards data
to endpoints
Dan Horon © 2013 DanHoron@RescueLogic.com
Permission required for distribution.
Here
Goodyou
network
can see
design
a potential
prevents
vulnerability.
a fault on any
If asingle
singlecable
path
were
from making
to become
more
impaired,
than one
multiple
device data
inoperable.
endpoints would
not communicate with essential equipment.
Data Endpoint
Version 1
Data
Forwarding
Dan Horon © 2013 DanHoron@RescueLogic.com
Data Endpoints
Permission required for distribution.
A proposed Class N path would require alternate
communication pathways whenever more than one device
would be impacted by a fault.
Data Endpoint
Version 1
Data
Forwarding
Equipment
Dan Horon © 2013 DanHoron@RescueLogic.com
Data Endpoints
Permission required for distribution.
Class N
Ethernet cables do not report grounds. Therefore, to allow
Ethernet network wiring in the Code:
• Any segment of a path that affects more than one device
must be the equivalent of Class X.
• Equivalent of Class B paths may be used when only one
device is dependent on the path.