What's the Difference Between Conventional Class A and Class B?
In conventional fire alarm circuits, if a wire breaks in a Class B circuit, some of the devices are disconnected from the panel, if a wire breaks in a Class A circuit, the devices that would have been disconnected use an alternative wire path to the panel.
During a fire, in Class B wiring style, if a wire breaks, the devices beyond the break won't communicate with the panel. In Class A wiring style, the panel can back-feed communication on the return loop, so most if not all devices still communicate. Class A Wiring will survive better than Class B Wiring.
By Douglas Krantz
The Fire Alarm System is a Life Safety System - so the occupants of a building can escape quickly, the idea behind a Fire Alarm System
is that it will provide a warning that there is a fire.
The trouble is, if something is wrong with the system, like a wire is broken somewhere in the building, the Fire Alarm System might not work. It might not provide a timely warning, and people who aren't warned in time might die.
Given this as a premise, the Fire Alarm Control Panel is always supervising
the building wiring. Using supervision, the panel makes sure the wires are always connected; they will always be able to carry the alarm detection and the notifying signals around the building.
If a connection or wire is broken, Class B Wiring Systems
stop working properly. But because the wiring is supervised by the fire alarm panel, however, as soon as the connection is broken, the panel indicates there is trouble
. The broken wire can be fixed right away.
Class A Wiring Systems
go a step further. Not only do they indicate there is trouble in the wiring, but if a fire breaks out in the building, before a single broken wire or connection has been fixed, all devices will still work.
Properly installed Class A Wiring Systems, therefore, are more reliable than Class B Wiring Systems. This is because, with a single broken wire, in case a fire breaks out before the wiring can be fixed, Class B Wiring Systems won't always work while Class A Wiring Systems still work.