Class A Fire Alarm Loops
By Douglas Krantz
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.
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
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.
Normal Class B wiring - All devices are supervised and working.
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
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.
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.
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.
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.
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.
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 188.8.131.52.2, 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.
I'll Send You the
Short Circuit Articles
Short Articles giving Tips and Suggestions for Fire Alarm Service Technicians
Fire Alarm Systems
Articles with Descriptions and Explanations, for Installation, Service, and Operation of Fire Alarm Systems
Electicity - - Conventional Flow or Electron Flow?
Electrical Engineers and Electronic Technicians each think electricity flows the opposite direction. Why don't they agree?
Friday Afternoon Repair
The fire alarm control panel looks normal after a Friday Afternoon Repair, but is it going...
Interfacing Elevator Capture
Elevator Installers and Fire Alarm Installers use similar words, but the words have different meanings
What does the Fire Alarm Control Panel do when the Notification Appliance Circuit (NAC) is shorted?
Why is Notification Appliance Circuit (NAC) supervision voltage opposite alarm voltage?
If there are repeated false waterflow alarms, air in the sprinkler system might be to blame
Class A and Class B Systems
If Class B supervison is good enough for most fire alarm systems, why use class A?
Fire Alarm Systems -- What are Class A Loops?
In case a wire breaks, 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.
Fire Alarm System 70 Volt Speakers
They look similar to distributed sound speakers, how are fire alarm 70 volt speakers different?
Electronics -- What is a Flyback Diode?
Manufacturers the world over spend good money installing these diodes, they must think they're important. What could be so important about a little diode?
Fire Alarm Systems -- What is a Waterflow Switch?
It doesn't turn on the water. It doesn't turn off the water. A waterflow just detects that water is flowing.
Fire Alarm Systems -- What is a Ground Fault?
Ground Faults are one of the simplest problems in a fire alarm system, an yet they're one of the hardest problems to find.
Fire Alarm Systems -- Wiring a Door Holder
The door holder looks innocent, and shouldn't be able to damage anything, so how does a door holder damage the Fire Alarm Control Relay that shuts off the current?
Fire Alarm Systems -- Wire Loop Supervision of the Fire Alarm System Helps Insure the System Will Work
Because the fire alarm system is a Life Safety System (people's lives depend on the fire alarm system working in an emergency), using supervision, the panel makes sure the field devices are always connected to the loop
Fire Alarm Systems -- What is an RTU (Roof Top Unit) and How is it Affected by Duct Detectors?
Help! I have an alarm on my fire alarm panel saying something about RTU, but it won't reset. I can't get any heat out of my furnace, either.
Fire Alarm Systems -- Battery Replacement
In a fire alarm system, where backup batteries are wired in series, if only one of them measures bad, why do both of them really have to be replaced?
Notification Appliance Circuit Voltage
Horns and Strobes get turned on when proper polarity voltage is applied. Normally, it's supervision voltage that's applied. The difference between the two can cause problems unless one pays attention.
Fire Alarm System -- Class B Wiring
How does the Fire Alarm Control Panel detect a wiring problem somewhere else in the building? System Wiring Integrity - That's what Class B Wiring is all about
Addressable Supervision of the Signaling Line Circuit (SLC)
Supervision of Addressable Fire Alarm Systems is done with Polling of the Devices. How does Polling of the Devices confirm that the wiring is intact?
Fire Alarm Systems -- What Should I Do When the Fire Alarm Sounds?
When the Alarm Sounds -- I would never suggest turning off the fire alarms, or in any way delay calling the fire department -- So says Brad Sveum, fire marshal ...
Fire Alarm Systems -- What is a Booster Power Supply?
A Booster Power Supply (BPS) or Signal Power Expander (SPX) provide extra power to horns and strobes when the horns and strobes require more power than the Fire Alarm Control Panel (FACP) can supply
Fire Alarm Systems -- Troubleshoot Using Supervision Voltage
You've got the supervision voltage right at the terminals on the panel, use it to troubleshoot what the panel is seeing.
Fire Alarm Systems -- Understanding the Firefighters Plain Old Telephone System or POTS
In a large building, so they can talk to each other while battling fires, firefighters use their two-way radios. But when the radios just don't work, the firefighter's phone system is the next best way to communicate.
Eletronics -- Ohms Law - E=IR Isn't Just a Word
If it isn't just a word, and it isn't really something to be memorized in order to pass a test, what do the letters in Ohm's Law really mean?
Fire Alarm Troubleshooting -- Smoke Detector Reset Using Fire Alarm Auxiliary Smoke Power Supply
Fire alarm devices have to be reset. Conventiinal 4 Wire Smoke Detectors take a little more wiring to be able to reset
Fire Alarm Systems Showing Ground Fault -- Why Doesn't the Ohmmeter Show the Ground Fault?
Resistance is not futile; resistance is fickle. When looking for ground faults in a fire alarm system, the voltage inside the ohmmeter can affect the number of ohms one measures.
Fire Alarm System -- Is Testing Smoke Detectors using a Magnet Really Testing Smoke Detectors?
Many fire alarm technicians test smoke detectors using the magnet test. What does the magnet test?
Fire Alarm Systems -- How to Clean a Dirty Duct Detector
An air duct smoke detector is a regular photoelectric smoke detector, only set up to monitor the air in an air duct. If it's dirty, the smoke chamber still has to be cleaned.
The Fire Alarm System -- It Raises the Alarm, Gets People's Attention
A Fire Alarm System Raises the Alarm. Raising the Alarm is the seizing of people's attention; It's a Warning -- Now is the Time to Take Action.
Fire Alarm Systems -- Why does the Fire Alarm System Turn on the Stair Pressurization Fans?)
The stairways may not burn, but they can still fill up with smoke. The smoke is kept back...
Electronics -- Induction or Switching Power Supply Transformer, TV Flyback Transformer, Automobile Ignition Coil
What do the Induction Transformer, the Ignition Coil, and the Switching Mode Power Supply Transformer have in common with the Old Fashioned Television Set Flyback Transformer?
Fire Sprinkler Systems -- How Does a Dry Sprinkler System Keep Water Out of the Pipes?
If the pipes don't have water in them that can freeze, how does the sprinkler system spray water?
Why do Pull Stations Break?
Ever wonder why they break? One of the keys to figuring this out is to look at the early fire alarm systems.
Frie Sprinkler Systems -- These Were False Waterflow Alarms
The fire department had already been there and left, but because of waterflow issues, this was the fourth night in a row the boys in the little red truck had turned up at this 6 story downtown department store.
Fire Alarm Systems -- Dear Firefighter - Please Don't Reset the Fire Alarm Panel
I know the fire alarm system has to be operating all the time, but if a smoke detector is bad and calling out false alarms, resetting the panel before finding the bad detector only makes for more false alarms.
Fire Alarm System -- What is it?
A fire alarm system is a call to action. It detects fire and tells people to get away from the fire, and may even help control the spread of smoke and fire...
Sprinkler Systems -- Which is Worse, A Fire or Water Damage?
Excess water to put out the fire, or water from a broken sprinkler system damages the property it's supposed to protect
What to Look For in a Fire Alarm Panel
A fire alarm panel is there to be used in an emergency -- When the person using it doesn't have the time to refer to the operating manual.
Fire Alarm System Evacuation -- Total, Staged, or Defend-in-Place?
When the Fire Alarm sounds off, should everyone all at once get out of the building, wait until those near the fire get out first, or defend themselves where they're at from the fire? Some buildings are just easier to get out of than others.
Area of Refuge -- Newer Buildings Have Them - What Are They?
To get out of the building from the higher floors, the building is on fire, the stairs are the only way out. However, some people just aren't physically able to take the stairs. What can those people do, other than seek refuge from the fire?
Electrical Arc Flash -- These Things Can Kill, Why Do They Happen?
Burning, blinded, deafened, an electrician is thrown across the room; having reached into a live electrical panel, all he did was to drop his screwdriver...
Fire Alarm Systems -- Power for the Strobes is Always Constant
P = I x E -- If the light output of a Fire Alarm Strobe stays constant (it's power remains the same), then when voltage goes down, current has to go up.
False Alarms in the Fire Alarm System -- Are False Alarms as Dangerous as Real Fires?
False Alarms in the Fire Alarm System -- Are False Alarms as Dangerous as Real Fires?
Each false fire alarm increases the severity of a real fire. This is a progression - with no false alarms everyone acts as safely as possible...
Fire Alarm System Service -- Follow the Wire, See Where It Goes
Problems are in the middle of the wire, or at the end of the wire. The trouble is that the wire leads many places, and is installed in many styles. For fire alarm systems, the wire...
Electronic Troubleshooting -- Rubbing Off the Tarnish
The circuit isn't working. You take it apart and look at it. After reassembling, it works. You know you didn't do anything to fix it. Why does it work now when it didn't work before?
Fire Alarm Systems -- Why Does a Somke Detector Stop the Elevator?
Those smoke detectors outside the elevator doors stop the elevator from working at all until the elevator company comes on site to fix it. Why?
Mixed Assembly -- Assembling a PCB with Mixed Lead Components
Printed circuit boards (PCBs) with surface-mount devices (SMDs) can be wave or reflow soldered, while PCBs with pin-through-the-hole devices (PTHs) can only be wave soldered. PCBs with a mixture of Smds and PTHs require both wave and reflow soldering, and have to be soldered in several steps.
Four Wire Duct Smoke Detectors
Why 4 Wires and All the Relay Contacts?
Fire Suppression -- The Gas Shunt Solenoid was so Hot it Started to Smoke - What Happened?
The solenoid smoked so much the fire alarm system sounded off. How could removing the solenoid from the valve be bad for the solenoid?
Fire Alarm Systems -- The System Doesn't Call the Fire Department so the Residents are Supposed to Call 911...
When the halls are full of smoke, will a resident even notice the sign after going past it for years?
Fire Alarm Systems -- When Servicing, the As-Builts Often Aren't Available - Why?
Design Engineers and Contractors will all tell the fire alarm technician that As-Built Drawings are always available. In reality, for the technician arriving on site, the as-built drawings are rarely available. There are just too many...
Fire Alarm Systems -- Why Combine Ion and Photo Smoke Detectors in the Same Detector Housing?
A flaming fire tends to produce small particles in its smoke while a smoldering fire tends to produce large particles in its smoke. A smoke detector or smoke alarm, though, has to be sensitive to both types of smoke.
Multiple Ground Faults
When the ground fault light comes on again and again, if the ground faults are fixed in a different place each time, the returning technican isn't the problem, the returning technician is the solution.
Fire Alarm Systems -- Can Two Companies Working on the Same System Really Keep Each Other Honest?
Some building owners use two different companies to service different parts of the Fire Alarm System. If one company doesn't care enough to service properly, can the other one make up the difference?
Fire Suppression Releasing Systems -- Should They be Activated by the Building Wide Fire Alarm System?
The code says yes, combining the common fire alarm system with the release system is allowed -- but is it wise to combine them?
Fire Alarm Systems -- When The Alarm Sounds, What Should I Do?
When you're in charge as a caretaker, what should be done at 3:00 AM when the fire alarms are waking everyone?
Mass Notification Systems -- What Are They?
Mass Notification Systems -- What Are They?
Unlike a fire alarm system, which only notifies the people of a fire inside a single building, a Mass Notification System reaches people in a community or set of buildings, both inside and out.
Fire Alarm Panel Wiring -- Someone has to Service it Later - Make it Purdy
So many times fire alarm panels are so messy they're hard to service. Seems like this is obvious, but many installers don't think about why they should take the extra time to make the panel look good.
Douglas Krantz | Technical Writer -- Describing How It Works
6520 Irving Ave So, Richfield, MN, 55423, USA | email@example.com | Phone / Text 612/986-4210
Copyright © 2011-2015 Douglas Krantz -- All Rights Reserved