The National Fire Protection Association (NFPA) has changed their definitions for pathways little in the NFPA 72.
In the past, the NFPA used Class A and Class B wiring for both the Initiating Device Circuit (IDC) and also for the Notification Appliance Circuit (NAC). Then they added a whole bunch of Style numbers and letters for the variations on Class A and Class B. Then they tried fitting in fiber optics, wireless systems, RS485 wiring methods, RS232 wiring methods, etc. into the Style numbers and letters. This type of classification and style systems for the all the forms of communication got very cumbersome.
They have now boiled down all the different Styles and Classes of the various signal pathways to a total of seven classifications. These classifications don't really show how to wire anything, these classes show how the building wide fire alarm system is going to work when something goes wrong with the alarm's communication routes. The pathways are now classified as Class A, Class B, Class C, Class D, Class E, Class N, and Class X.
A Pathway used for the IDC or NAC can be classified under Class A or Class B. In essence, the old Class A and Class B pathways are still Class A and Class B pathways.
A pathway used for the SLC can be classified as Class A, Class B, and the new Class X. The old Style 6 SLC is now Class A, the old Style 4 SLC is now Class B, and the old Style 7 SLC is now Class X.
Pathway
When they ask about the pathway, they are looking for is the classification of the pathway being used for the fire alarm system. They want you to write down how well the pathway is supervised, and if there's a problem with the circuit, whether there's a redundant path to make sure signals still get through.
What you need to put down as a pathway is the classification of the pathway.
Classification
The following is a summary of the classifications for the various pathways that is now used by the NFPA.
Class A
- 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 to the devices that were cut off
- If wires are used, a wire-to-wire short may shut down the whole path
- Both conventional and addressable systems fit into this
- The IDC (Initiating Line Circuit), the NAC (Notification Appliance Circuit), and the SLC (Signaling Line Circuit) fit into this
- The panel shows a trouble signal when there is a problem
Class B
- There is no redundant path
- Any device beyond a break won't work
- If wires are used, a wire-to-wire short may shut down the whole path
- Both conventional addressable systems fit into this
- The IDC, the NAC, and the SLC fit into this
- The panel shows a trouble signal when there is a problem
Class C
- Uses Handshaking (equivalent to "I have received the signal") to supervise the path
- Can have more than one pathway
- The panel shows a trouble signal when there is a problem
Examples:
- Signals from the fire panel to the monitoring company
- The use of IP (Internet Protocol), whether it's local communications or over the Internet
- The communicators over the telephone lines that are still in use
Class D
- Fail-Safe operation - If there is a failure, the device that is controlled by the fire alarm system goes into fire mode
- 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:
- The fire door closes
- Emergency door locks release
- The damper closes
- The fans shut down
Class E
- These pathways are not supervised at all
- No trouble signal will be shown on the panel if the path fails
Class N
This is basically local Ethernet, Token Ring, or another network or IP infrastructure.
- 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
- These pathways are verified through end to end communication, like data handshaking
- Loss of communication between end points on any path show a trouble signal on the panel
- Problems with one pathway won't affect the other pathway
Class X
- 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 for the devices that were cut off
- Devices on both sides of an open will continue to communicate with the panel
- 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)
- The SLC fits into this
- The panel shows a trouble signal when there is a problem
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. The trouble is being shown because of all the false activations, pathway failures, or failures of the whole system if a second ground fault occurs to short out something through the building's ground system.
In other words, with a second ground fault, the building ground system becomes an unwanted, interfering pathway that prevents the system from working properly.
However, because wireless systems, fiber optic systems, and some data systems (like Ethernet) don't pass shorts caused by ground faults to the panel or other devices, a ground fault indication is not always needed.
Douglas Krantz
