Douglas Krantz - Technical Writer - Describing How It Works
Go to the mobile version of this page
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Offers from Technician's Corner

Fire Alarm -- Description

Class A Wiring is providing an alternate route for the signals in case a wire breaks
During fire, if a wire breaks, Class A Wiring provides an alternate route for signals to pass between field devices and the fire alarm panel.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer






How Does Conventional Class A Fire Alarm Wiring Work?

By Douglas Krantz

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 and fix 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

The question is asked: In Class A wiring, how close to the feed wires are we allowed to get the return wires?

The answer is: It depends. During a fire, as the fire damages the wiring, how badly does one want the fire alarm system to continue to work?
  • The feed and return wires can be in the same bundle or conduit if there's no concern.
  • The feed will be in one part of the building and the return will be in the opposite part of the building if one is concerned about life safety.

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

The concern here is that whatever breaks a wire in the first part of the 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 feed and return 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.

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.

You can see how the fire alarm system works by ordering the book "Make It Work - Conventional Fire Alarms" by Douglas Krantz.




More Articles

Can More Than One Class A Circuit be in a Conduit? -- How many Class A Circuits are in a single conduit isn't really the issue. The issue is separating the feed part of any Class A Circuit from... Read More

What Makes the End of Line Resistor So Important? -- To allow the panel's internal ohmmeter to check the continuity of the wires (supervise the wires), the end-of-line resistor is at the end of the loop: the last device.Read More

What is Fire Alarm Class B Wiring? -- Fire alarm people are paranoid. They're afraid everything is working against them. They're in fear that that if anything goes wrong... Read More

How is an End of Line Resistor a Pull-Down Resistor? -- Sometimes when troubleshooting, the technician has to think inside the box. For instance, take... Read More



 Get your free diagram showing supervision for Class B wiring


This website uses cookies. See Privacy for details.

Get the book Make It Work - Conventional Fire Alarm Systems


Get help finding ground faults

I receive commissions for purchases made through links in this post




Advertisement