How Fire Panel Know my Detectors are OK?

How Fire Panel Know my Detectors are OK?

Wishing you a very Happy New Year 2022.

The exact design of a commercial fire alarm system is determined by the commercial space’s occupancy classification and the local codes mandated by the Authority Having Jurisdiction (AHJ) for that specific occupancy classification. But regardless of the system design, all commercial fire alarm systems must have a Fire Alarm Control Panel (FACP).

An FACP is the “brain” of the fire alarm system to which all other devices are connected. When an initiating device (such as a smoke detector or a manual pull station) transmits an alarm signal to the FACP, it activates the notification devices to alert the occupants via audible and visual alarm devices.

An FACP also has a digital display that provides the current status of the fire alarm system. In modern “addressable” fire alarm systems, every device connected to the FACP has a unique address (for example: “57 - smoke detector basement electrical room”) which allows the FACP to display the specific device that initiated the signal. This level of detail allows the authorized personnel to quickly locate the source of the alarm. The building’s authorized personnel also use the FACP’s functional switches to acknowledge signals, silence alarms and reset the system once the alarm condition has been cleared by fire department personnel.

It's assumed that fire alarm systems have to work all the time because fires happen anytime... but, then again, this is the real world. Problems with the fire alarm system do occur, and that's when it may not go into alarm. To know when it needs to be fixed, the whole fire alarm system needs to be supervised. 

Smoke detectors look for the presence of smoke. The most common type used today are photoelectric smoke detectors. A photoelectric smoke detector operates by projecting a small beam of light across an internal chamber. If that beam of light becomes obscured beyond a preset threshold (a percentage of light obscuration) by smoke, dirt or other small particles, the detector will transmit an alarm signal to the

If there's trouble inside the fire alarm panel, the panel does its self-diagnostics and indicates trouble. However, when there's a problem in the wiring or devices outside the panel, that's another story. 

Addressable Supervision is the Fire Alarm Control Panel polling or asking a device "Are You OK?", and the device answers "I'm OK!"

Polling is an I'm OK Check

Conventional Class A and Class B wiring directly supervises only the wires; it does not really supervise the devices. The Signaling Line Circuit (SLC), on the other hand, is different; it supervises the devices. 

Unlike the conventional methods, the SLC is a kind of two way street. To supervise, the SLC uses Polling of the Devices. Polling is similar to an I'm OK survey of the entire fire alarm system outside the panel. 

To Poll the devices, the panel uses a signal to say 'device 27 report', and device 27 reports back with a signal that says essentially, 'I'm OK.' If device 27 doesn't report back, or if it does report back but says it isn't working correctly, the message on the fire alarm panel indicates trouble on device 27. 

Failure of the device to report back could be trouble with the device, or trouble with the wiring. However, because the panel specifies the device in trouble, the troubleshooting process of the fire alarm system is sped up. 

Supervising the Wire

With conventional Class A or Class B wiring methods, in order to directly supervise the wire, a current is passed through the entire wire loop. 

The devices connected to the loop aren't supervised; the panel never checks the devices to see if they work. Unless a detector in the field opens the loop, or a horn or strobe shorts (almost never happens), the fire alarm panel won't indicate trouble. 

On the other hand, with the exception of Styles 6 and 7 (equivalent to the conventional Class A), the wires in an addressable loop aren't directly supervised. 

Instead of passing a current through the wire, the SLC wiring is indirectly supervised. If, during the polling process, the returning I'm OK signal is received at the panel, the panel assumes that the wiring is complete. That's the indirect supervision of the wire in the loop. 

Alarm Signals -- Input and Output Device

For input devices, of course, if smoke detector 27 goes into alarm, the panel gets the signal from device 27, and using words on the display, identifies smoke detector 27 as the source of the alarm.

Then again, to turn on the horns or strobes is a specific area, the panel can also send signals along the same SLC to the specific output modules it needs to turn on. 

Addressable I'm OK Supervision -- It's All About Confidence

Using the "I'm OK" signals of the polling process, the addressable fire alarm system directly supervises the devices on the SLC. With this direct supervision of the devices, the fire alarm system has confidence the building wiring is intact.

According to NFPA 72, fire alarm initiating devices are defined as devices used to manually or automatically signal a fire alarm system to initiate responses from equipment and people. These devices connect to the alarm system’s control panel and are under the control panel’s surveillance. When triggered, the control panel identifies the location and then goes into the alarm stage, sounding alerts throughout the building and sending commands to emergency responders.

SSA Integrate offers NFPA inspection programs to our customers. Regular fire equipment inspections will ensure the ongoing reliability and safety of your fire protection systems. As part of the fire protection inspection process, detailed reports will be provided identifying any system deficiencies and recommended action to bring the system back up to applicable fire codes.
We provide a comprehensive selection of fire protection inspection services through our Preventive Maintenance Agreement (PMA). Your company can save valuable time and money by having your fire safety needs addressed with one PMA.

Working of Fire Alarm Wiring

Working of Fire Alarm Wiring

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

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.

A Signaling Line Circuit (SLC) carries information in the form of data to and from the field devices for the fire alarm system, and also carries power from the control panel to the devices.

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


Class B Loops

Normal Class B wiring - All devices are supervised and working

In conventional / non-addressable Class B Loops / Zone Wiring, 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. 

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. 

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 B sometime called Style 4 loop.

Class A Loops

Normal Class A wiring - All devices are supervised and working.

Under normal conditions, Class A Loops  sometimes called Style 7 loop are similar to Class B Loops, but with an important difference. 

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. So loop makes addressable type, now the word came SLC.

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?

1.      The feed and return wires can be in the same bundle or conduit if there's no concern.

2.      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.