Class A and Class B wiring on a Fire Alarm
Most states have gone away from class A wiring for a few reasons: The cost of the wire, conduit and labor is quite a bit higher and with class B wiring you are notified immediately when there is a break or cut in the wire/cable. 
Utah and a few other places in the country still require class A wiring on certain jobs like schools, rest homes, certain public buildings etc...
Class B wiring when cut will indicate a trouble although all the devices or appliances after the cut in the wire/cable will not function. Class B wiring goes out but does not come back. There is an EOL or end of line resister at the last device or appliance. When the wire is cut the panel no longer sees the resister and it goes into trouble.
Class A wiring goes out from the panel and devices and or appliances are wired and then the wire comes back in a different conduit or via a different route. (does not go out and back in the same route or conduit)
Class A wiring when cut allows a trouble to be sent and all the devices and or appliances continue to work even after the cable has been cut.
Appliances = Horns, Strobes, Horn/Strobes, Speakers and Speaker Strobes
Devices = Modules, Smoke Detectors, Pull Stations, Heat Detectors etc...
Call Larry Love in Salt Lake City for more information or pricing on a fire alarm, security, access control or camera system. (801) 428 1384
larry@peakalarm.com