Douglas Krantz - Technical Writer - Describing How It Works

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.

Class B wiring is meant to send a trouble to the panel if there's a broken wire or a bad connection in the system. Class A wiring takes that a step further by backfeeding communication on the return loop.
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.
Life Safety
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