In a fire alarm system, how a circuit is classified shows how the fire alarm system is going to be affected by something going wrong. Usually, the classification system isn't showing the type of signals are being carried, the classification system refers to the signal carrying capability.
It's mincing words, but the type of circuit isn't the real concern, it's the signal pathway that is being classified, not just the wires. When considering classification, the pathway may be:
- Wired
- Fiber Optic
- RF (Radio Frequency or Wireless)
The signals being carried may be:
- Conventional
- Addressable
- Analog Audio
- Digital Audio
Because circuits don't detect fire or sound the alarm for people, the circuit is really being watched (supervised) by the fire alarm control panel only to make sure that the detection devices and alarm devices are connected.
Class B Signal Carrying Pathway
One of the major concerns is the ability of a device to send signals to the panel and receive signals from the panel. If a wire comes loose from a connection, if a fiber optic cable breaks, if a radio transmitter or radio receiver (for wireless communication) fails, the pathway is broken.
If a Class B pathway is broken, all devices beyond the break will fail.
Class A Signal Carrying Pathway
Class A pathways go a step further than a Class B pathway. Class A pathways have a second, redundant path from the panel.
With Class A, this second, or redundant pathway gets switched into service if the panel detects a failure of the original Class B pathway.
Trouble Showing Up on the Panel
By the way, if anything goes wrong with any part of the fire alarm system, in the entire building, the panel is supposed to show a trouble. After all, this is a life-safety system, it's should always work. When the panel shows a trouble, the trouble can be fixed.
Class B and Class A
When something goes wrong, the Classification of a pathway shows what happens to the pathway for the Fire Detection and Alarm System.
Remember, conventional signals and addressable signals are both types of signals.
The Class A or Class B designation has to do with the circuit's pathway. In essence, the pathway is classified to show whether or not a fire alarm control panel can use the pathway to send and receive signals, including the alarms from the fire detection devices.
The signals are messages, the pathway is the route that the messages take.
Douglas Krantz
