Troisième année Lesson 13 of 13 · about 13 min
Fire alarm basics
What you’ll learn
- The parts of a fire alarm system: panel, initiating devices, notification appliances, power.
- Supervision — how a panel watches its own wiring — and what the end-of-line device does.
- The idea behind Class A and Class B circuits, and conventional versus addressable panels.
- The arithmetic: supervisory current and notification-circuit voltage drop.
Before this: Ohm's law · Second year
A fire alarm system detects a fire, tells the people in the building, and tells someone outside — and it has to keep checking that it can still do all three. That last part, supervision, is what makes fire alarm wiring different from anything else you install. This lesson is concepts only. How it is installed is set by the code and the fire alarm standards — CAN/ULC-S524 (installation), S536 (inspection and testing) and S537 (verification) in Canada, NFPA 72 in the US — and by who your jurisdiction allows to do the work.
The parts
- Control panelthe brain: watches every circuit, decides alarm or trouble, drives the outputs
- Initiating devicespull stations, smoke and heat detectors, sprinkler flow switches
- Notification applianceshorns, bells, strobes, speakers
- Puissancea dedicated supply plus standby batteries
- Outputsmonitoring connection, door holders, fan shutdown, elevator recall
Supervision and the end-of-line device
The panel never assumes its wiring is fine; it measures it. On a conventional circuit, an end-of-line resistor (or other EOL device) at the far end lets a small current flow around the loop. The panel knows the current it expects. If a wire breaks, the current stops and the panel shows a trouble. When a detector or pull station operates, it puts a low resistance across the pair, the current jumps, and the panel goes into alarm. A ground fault on the wiring is watched for separately.
Because the proof is the current passing through each connection, the wiring goes in and out of each device and ends at the EOL. A tap that hangs off the side, or an EOL left in the panel “to make it clear”, means some devices can be disconnected without anyone knowing — the circuit looks fine and will not work.
Class A and Class B, conventional and addressable
A Class B circuit runs out from the panel and ends at its EOL: a single break reports a trouble, but devices past the break are lost. A Class A circuit runs out and returns to the panel, which can feed it from both ends, so one break reports a trouble while every device keeps working. Which is required where is set by the standard and the code.
A conventional panel groups devices into zones and tells you which zone is in alarm. An addressable panel talks to every device on a data loop (a signalling line circuit), each with its own address, and tells you exactly which device operated — and which one has a fault or is dirty. Addressable systems use isolator modules so a short on one part of the loop does not take the whole loop out.
Worked: supervisory current and notification voltage drop
- A 24 V conventional initiating circuit with a 4.7 kΩ EOL: I = 24 ÷ 4,700 = 5.1 mA when all is normal. An open wire drops it to zero.
- A notification circuit feeds eight horn/strobes at 0.1 A each (practice figure — the real draw is on each appliance’s data sheet): total 0.8 A.
- The loop resistance of the circuit wiring is 3.2 Ω (practice; from the length and the conductor’s data). Worst-case drop = 0.8 × 3.2 = 2.56 V.
- Voltage at the last appliance ≈ 24 − 2.56 = 21.4 V. Compare against the lowest voltage the appliance is listed to work at, and remember the panel may be running on batteries at a lower voltage when it matters.
Sur le chantier
Tips
- Before working on an existing system, tell the owner and the monitoring company and get it put on test.
- Label every circuit at the panel and at the EOL location; the next person will thank you.
- Wire in and out of every device — cut the wire, do not loop it under one screw.
- Keep a copy of the riser and the device addresses on site as you install.
- Keep fire alarm wiring in its own raceway or cable as the code requires, clear of power.
Common mistakes
- T-tapping a conventional Class B circuit.
- Leaving the EOL at the panel so the circuit reads normal with devices missing.
- Mixing up initiating and notification circuits on the panel terminals.
- Forgetting to account for voltage drop on long notification circuits.
- Starting work on a monitored system without calling it in.
Check yourself
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1.Which of these is an INITIATING device?
Initiating devices start an alarm or signal: pull stations, smoke and heat detectors, sprinkler flow switches. Notification appliances (horns, strobes, speakers) tell people about it.
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2.What is the end-of-line (EOL) device for on a conventional Class B circuit?
Supervision is the heart of fire alarm: the panel constantly checks its own wiring. With an EOL resistor at the far end, a small current flows when everything is normal; an open wire stops it (trouble), and a device in alarm changes it (alarm). That only works if the EOL is at the END — after the last device.
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3.A 24 V initiating circuit has a 4.7 kΩ end-of-line resistor. Roughly what supervisory current flows in the normal state?
I = V ÷ R = 24 ÷ 4,700 = 0.0051 A ≈ 5 mA. The panel watches for that current; if it drops to zero, the circuit is open somewhere and the panel reports a trouble.
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4.An installer T-taps a branch off a conventional Class B initiating circuit and puts the EOL at the end of the main run. What is wrong?
Supervision works because the supervisory current passes every connection on its way to the EOL. A tap that the current does not pass through can break without the panel seeing it. Wire in and out of each device, with no T-taps on circuits that do not allow them.
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5.What is the main difference between a conventional and an addressable system?
Conventional panels group devices into zones and report the zone. Addressable panels poll each device on a signalling line circuit and report "smoke detector, room 214" — faster to find, easier to program, and each device reports its own trouble.
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6.Eight horn/strobes each draw 0.1 A (practice figure) on a 24 V notification circuit whose loop resistance is 3.2 Ω. About what voltage reaches the far end?
Total current = 8 × 0.1 = 0.8 A. Drop = 0.8 × 3.2 = 2.56 V (a worst case, as if all the current flowed the full length). 24 − 2.56 ≈ 21.4 V. Compare that against the appliance's rated operating range from its data sheet.
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7.In broad terms, what does a Class A circuit add over Class B?
A Class A circuit runs out and back to the panel, so the panel can feed from both ends: one break is reported as a trouble but every device keeps working. Class B runs out to an EOL only, so devices past a break are lost. Where each class is required is set by the fire alarm standard and the code.
Questions fréquentes
Can any electrician work on a fire alarm system?
Rules differ by province and state. In many places installation can be done by electricians, while verification, inspection and testing must be done by technicians certified for it (in Canada, verification is done to CAN/ULC-S537 by qualified people). Ask before you touch an existing system, and always tell the monitoring company and the building owner first — or the fire department will be the one who tells you.
Why do fire alarm circuits have their own wiring rules?
Because the system has to keep working during a fire — often in the building it is protecting — and has to tell someone when it cannot. That drives supervision, circuit classes, dedicated circuits, cable types and survivability requirements. The details are in your code book and the fire alarm installation standard (CAN/ULC-S524 in Canada, NFPA 72 in the US).
What is the difference between an alarm, a trouble and a supervisory?
An alarm means fire: a device has detected or someone has pulled. A trouble means the system itself has a problem — an open wire, a ground fault, a missing device, low battery — and may not work. A supervisory means something that protects the building is off-normal, such as a sprinkler valve being closed. Each has its own indication and its own response.
Voir aussi
- Reading commercial and industrial prints — finding the FA riser
- Les bases de la basse tension et du câblage de données
- Emergency and standby power
- Ohm's law practice — the arithmetic behind EOL and voltage drop
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