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Fire Alarm -- Description

Class B wiring is designed to tell the fire alarm panel if there is a broken connection or wire.
Class B Wiring is meant to make sure that the wires are always connected to all the devices on the loop outside of the panel.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

What is Fire Alarm Class B Wiring?

By Douglas Krantz

Fire alarm people are paranoid. They're afraid everything is working against them. They're in fear that that if anything goes wrong, like a wire breaks or a connection comes loose, lives will be lost, or property will be destroyed, or both.

Wiring, for example, is considered to be unreliable. Even when installed perfectly -- it's in conduit and all connections are properly made -- wiring can be damaged by external forces: a smoke detector is broken by a forklift, a mini-horn is disconnected in an apartment so the resident doesn't have to listen to it, a connection is corroded by water leaking from the roof, etc.

This of course doesn't include poorly made connections, squirrels chewing on the wire (I've seen that), other vendors removing the fire alarm wire by mistake, and anything else that breaks the wire.

Keeping the Fire Alarm System Working

A broken wire or connection keeps the fire alarm system from working: detectors don't detect, fire horns don't sound the alarm, strobes don't flash.

That is why fire marshals and other Authorities Having Jurisdiction (AHJ) want any problems fixed early; once a fire starts, it's too late to troubleshoot and fix the fire alarm system.


To make sure the wire throughout the building is always working, the wire is supervised, or continually checked for continuity, by the fire alarm control panel using a small electrical current.

The current, as it leaves the panel, goes out one of a pair of wires, goes through a current limiting resistor called an End-of-Line resistor (EOL), and returns on the other wire. This pair, along with the EOL, makes up a fire alarm loop. Devices are connected to this loop so their connections, also, receive the supervision current.

If this current stops for any reason, the supervision circuitry in the fire alarm panel assumes that the whole loop is not working, and calls out a trouble condition. The trouble condition on the panel, of course, cannot be reset; the cause of the trouble can only be fixed.

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Besides the devices themselves being connected, there are no "T" taps; no wire runs to another device not supervised by this loop. This includes short runs within a box that doesn't run the supervision wire through any devices on the loop.

Class B Wiring

Class A Loops can be used in fire alarm systems, but Class B Loops are the most common for non-addressable fire alarm loops.


Douglas Krantz

Describing How It Works

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Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at