Douglas Krantz - Technical Writer - Describing How It Works
Articles on Fire Alarm Systems

Class A Fire Alarm Loops

An example of the 4 wires coming out of a panel, using Class A wiring techniques
Photo Courtesy Integrated Fire & Security
Class A Loop wiring can be used for conventional loops, or addressable loops. A Detection loop can be Class A, or a Notification Loop (fire horns and strobes) can Class A.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer


Fire alarm systems save lives and protect property. Fire alarm systems also break down because they're electrical.

Class A or Class B wiring loops help the fire alarm panel to find these breakdowns (faults) before a fire, while there is time for repairs.

Class B Loops

Diagram showing the schematic for Class B Wiring
Normal Class B wiring - All devices are supervised and working.
In conventional Class B Loops, all devices are daisy-chained together. By watching a small electrical current passing through the wires, the panel supervises them, and to limit this supervising current, at the end of the daisy-chain is an end-of-line resistor. The panel constantly watches for this current.

Diagram showing the schematic for Class B Wiring
Open Fault in the Class B wiring. Supervision tells the panel that the wiring does not go through, but also the devices further from the panel don't work.
If the supervising current stops flowing, the panel assumes a wire is broken (an open fault), and displays a trouble. When a wire breaks in Class B, the devices closest to the panel will still work, but because of the wire break, the devices further from the panel are cut off.

Class A Loops

Diagram showing the schematic for Class B Wiring
Normal Class A wiring - All devices are supervised and working.
Under normal conditions, Class A Loops are similar to Class B Loops, but with an important difference.
Diagram showing the schematic for Class B Wiring
Class A wiring takes error detection further than Class B. If a wire breaks, the panel uses a redundant wire path to maintain communication with devices beyond the break. Here even though a wire is broken, all devices work.
To keep more devices working, Class A uses a second path from the fire alarm panel; a redundant wire loop goes around the broken wire. A fire can still be detected, because, using this redundant path, most, if not all, devices on the loop remain connected to the panel.

Basically, when the fire alarm panel detects an open wire in the Class A Loop, it automatically switches to using two separate un-supervised Class B loops. The first one is the original Class A loop, and second one back-feeds on the separate pair of wires to make the second Class B loop.

Most of the devices on the original Class A loop will be on either the first or the second Class B loop.

Separation on Class A Wiring Routes

True Class A wiring schemes, though, make sure to protect this redundant loop path by routing it through the building on a separate route.

The concern here is that whatever breaks a wire in the first loop might break all the wires in the same bundle. An example: A forklift tears through all the wires in a bundle at once. If both wiring routes use the same wire bundle, and the whole bundle of wires is broken, and all the devices beyond the break will not communicate with the panel.

In that case, Class A wiring will not be any better than Class B.

The NFPA Code does allow for some exceptions, but mostly the code says the outgoing wiring path and the incoming wiring path should be separated by some distance.

Resetting Class A Troubles

Most fire alarm panels automatically restore trouble messages when the trouble is repaired. However, because the Class A Loop isn't supervised the same way as Class B Loops, the fire alarm panel can't detect corrections.
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With Class A faults, after correcting the open fault, resetting the panel will clear the trouble message.

Bottom Line for Class A

Class A Loop wiring uses both a primary wire path, and a redundant secondary wire path.

When a wire breaks, by using both paths, devices are still able to communicate with the fire panel.

Questions or Comments
Ken said - On a class a system, how far can you run a SLC circuit in the same conduit (there and back) to pick up 1 smoke device? I believe it's 10' from panel to j-box to where you split and go your different ways, but not positive about a single device. Also, redundant conduit has to be either 4' horizontal or 1' vertical? Is that correct?

Douglas Krantz - According to NFPA72 2007, the wiring to and from devices from the fire alarm panel cannot run in the same cable, enclosure, or raceway. The idea behind using Class A over Class B is a higher level of survivability: even when a single wire is broken during a fire, Class A wiring provides the fire alarm system the ability to fully function.

The exceptions talked about in the question above, are for the wiring inside conduit, not for free-wiring inside walls or ceilings. The wiring to a single smoke detector does not seem to have a distance limit, because the loss of all four conductors in a single conduit during a fire would spell the loss of only a single smoke detector.

Allowing for practical wiring while still providing the protection of Class A, the 1', 4', and 10' limits are common sense exceptions.
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Douglas Krantz | Technical Writer -- Describing How It Works
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