Fire Alarm Circuit Diagram

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Understand how a conventional fire alarm circuit organises detection zones with end-of-line resistors, feeds notification appliance circuits, and distinguishes fault from alarm conditions before servicing or extending a system.

A fire alarm system's wiring architecture is a life-safety circuit, and its design is intentionally fault-tolerant. A key design principle is that the system must distinguish between three states on every supervised circuit: normal (no alarm, no fault), alarm (device activated), and fault (open circuit, short circuit, or missing end-of-line resistor). Understanding how the end-of-line resistor creates this three-state detection capability is fundamental to understanding any fire alarm wiring diagram.

Conventional fire alarm systems organise detection devices (smoke detectors, heat detectors, manual call points) into zones — typically one zone per floor or fire compartment. All devices in a zone are wired in a supervised loop. The end-of-line resistor (EOL resistor) is the key component: it is fitted at the far end of the zone loop, after the last device. During normal operation, the control panel monitors the small current flowing through the loop and through the EOL resistor. The resistance value (commonly 4.7 kΩ or 10 kΩ, panel-specific) sets this normal-state current. If the loop is broken (open circuit — a wire break or disconnected device), current drops to zero and the panel generates a fault condition. If the loop is shorted (damaged cable or fault inside a detector), current rises above the alarm threshold without the EOL resistor in the circuit, and the panel generates either an alarm or a fault depending on the panel design and the nature of the short.

When a detector activates, it presents a low-impedance path (effectively a short, but a controlled one via the detector's internal circuitry), reducing the loop impedance and raising current above the alarm threshold. The panel distinguishes this from a cable short by the specific current magnitude.

Addressable (intelligent) fire alarm systems use a fundamentally different architecture: each device on a two-wire loop has a unique address, communicates digitally with the control panel, and the panel can identify the specific device in alarm rather than only the zone. EOL resistors are not used in most addressable loop topologies, replaced by supervised protocol communication.

The Notification Appliance Circuit (NAC) drives alarm outputs — sounders, bells, and strobes. The NAC is also a supervised circuit with an EOL resistor, ensuring a wire break in the sounder circuit is detected as a fault rather than silently disabling the alarm outputs.

How to wire fire alarm circuit diagram

Consult applicable standards and the panel manufacturer's documentation before any work Fire alarm system installation and modification must comply with NFPA 72 (USA), BS 5839 (UK), AS 1670 (Australia), or the applicable national standard. The panel manufacturer's installation manual specifies EOL resistor values, zone wiring requirements, conductor specifications, and configuration procedures. Deviation from these documents voids approvals and creates legal liability.
Plan zone allocation and device placement Each zone should not cover more than one fire compartment. In most conventional systems, a single zone is limited to a maximum floor area (typically 2000 m² under BS 5839-1) or a maximum number of detectors per zone (typically 20–32 devices per zone, panel-dependent). Map detection device locations to zones before running any cable.
Run zone wiring from the control panel to the first detector Use the cable type specified in the panel documentation — typically 1.0–1.5 mm² two-core screened cable for conventional detection zones. Route cable away from power wiring to avoid interference. Maintain separation from mains cables in accordance with applicable wiring regulations.
Wire detectors in the zone loop in daisy-chain fashion Connect the incoming zone loop wires to each detector's terminal block, and connect outgoing loop wires to the next detector (loop-through / daisy-chain). Do not use T-connections (star topology) on conventional zone loops unless the panel and loop architecture specifically supports this. At each detector base, both the loop-in and loop-out conductors are terminated so the detector can be removed without breaking the loop — some detector bases include a short-circuit isolator for this purpose.
Fit the EOL resistor at the last device in the zone At the terminal block of the last detection device in the zone, connect the EOL resistor across the zone loop terminals (in parallel with the device terminals). Use the resistance value specified by the panel manufacturer. The EOL resistor should be the correct value (verify with a multimeter), a flame-proof or flame-retardant resistor, and secured inside the detector base so it cannot fall out and create an intermittent connection.
Wire the notification appliance circuit (NAC) Connect the NAC output from the control panel to each notification device in daisy-chain. Observe polarity — most NAC sounders and strobes are polarity-sensitive. Fit the NAC EOL resistor at the last notification appliance. Use only notification appliances listed/approved for the control panel in use.
Commission and test the system After wiring, commission the panel per the manufacturer's procedure. Test each zone by activating a device (using a test magnet for magnetic-test detectors, or aerosol detector tester for smoke detectors) and verifying the correct zone alarms at the panel. Test the EOL by opening the zone loop (disconnecting the EOL resistor) and confirming the panel shows a fault on that zone. Test the NAC by triggering an alarm and verifying all notification appliances operate.

Specifications

EOL resistor value (common)	4.7 kΩ or 10 kΩ — must match panel manufacturer's specification
Zone cable type	2-core screened, fire-resistant (e.g. FP200 or AS/NZS 1660 compliant)
Zone cable conductor size	1.0–1.5 mm² (panel and current-load dependent)
Maximum devices per conventional zone (indicative)	20–32 (panel-specific; verify with manufacturer)
Insulation resistance minimum (before commissioning)	Greater than 1 MΩ at 500 V DC between each conductor and earth
System standard — USA	NFPA 72 National Fire Alarm and Signaling Code
System standard — UK/Europe	BS 5839-1 (detection and alarm), BS 5839-6 (domestic)
System standard — Australia	AS 1670.1 (fire detection and alarm systems)

Safety warnings

Fire alarm system installation, modification, and commissioning is life-safety work that must be performed by qualified fire alarm technicians licensed under applicable regulations: NFPA 72 (USA), BS 5839 (UK), AS 1670 (Australia). Unqualified or incorrect installation creates a serious risk to life.
Do not disable, bypass, or inhibit any zone or system output without notifying the building owner, fire safety officer, and local fire authority. A bypassed zone leaves a fire compartment unprotected — implement alternative precautions if a zone must be taken out of service.
Always perform an access-controlled isolation procedure before working on a live fire alarm system — an inadvertent activation during testing may trigger a fire brigade response and result in penalties.
Using a non-listed or non-approved detection device on a fire alarm circuit invalidates the system's listing/certification and may prevent insurance claims in the event of a fire. Only use devices approved for use with the specific control panel.
Ground faults and insulation failures on fire alarm cables can cause spurious alarms and are a common cause of unwanted fire brigade call-outs. Use a megohmmeter to verify insulation resistance of all zone wiring before commissioning — minimum 1 MΩ between each conductor and earth.

Tools needed

Digital multimeter
Megohmmeter (insulation resistance tester, 500 V DC)
Smoke detector test aerosol and extension wand
Magnetic test tool (for magnetic-test detectors)
Fire alarm panel configuration software (addressable systems)
Screwdrivers and cable preparation tools
Zone test resistor set (for bench-testing panel zone inputs)
Fire alarm system documentation and as-built records

Common mistakes

Using the wrong EOL resistor value for the panel — a 10 kΩ resistor on a panel expecting 4.7 kΩ will cause the panel to report a persistent zone fault or failure to alarm, depending on panel design.
Using T-junction (star) wiring topology on a conventional zone loop — this means a cable break between the panel and the junction leaves all detectors beyond the junction unsupervised without generating a fault.
Omitting the NAC EOL resistor — the panel will then be unable to detect a break in the sounder circuit wiring, and the sounders could fail silently without the panel generating a fault.
Mixing addressable and conventional devices on the same loop without the appropriate interface module — addressable control panels cannot supervise conventional detector bases directly.
Routing zone cable alongside mains power cables without maintaining the required separation — induced interference can cause false alarms and difficulty passing commissioning tests.

Troubleshooting

Panel shows zone fault immediately after wiring but before any device is connected: Cause: EOL resistor not fitted or incorrect value; open circuit in zone cable Fix: Verify that the EOL resistor is installed at the last device position and measure its resistance with a multimeter before installation. Test cable continuity from the panel zone terminals to the EOL location. An open-circuit reading at any point indicates a broken cable or connection.
Zone goes into alarm when no detectors have activated: Cause: Short circuit on zone loop wiring; incorrect zone wiring (active device connected across zone terminals); EOL resistor value too high causing low normal-state current that is misread as an alarm Fix: Disconnect all devices from the zone and measure loop resistance from the panel terminals — high resistance is expected. If resistance is near zero without any devices connected, there is a short in the cable. Re-introduce devices one at a time to isolate the fault source.
Panel shows ground fault on zone: Cause: Zone cable insulation damaged, conductor making contact with earth (metal conduit, trunking, or building structure) Fix: Disconnect the zone from the panel. Using a megohmmeter set to 500 V DC, measure insulation resistance from each conductor to earth — a healthy cable measures above 1 MΩ. Progressively isolate cable sections to locate the point of insulation breakdown.

Frequently asked questions

What is the end-of-line resistor and why is it required?

The end-of-line (EOL) resistor is fitted at the physical end of each supervised zone loop, after the last detection device. It allows the control panel to monitor zone wiring integrity continuously by measuring loop current. Without the EOL resistor, the panel cannot distinguish between a normal zone (no detectors activated) and an open-circuit fault — both would show zero or very low current.

What is the difference between a conventional and an addressable fire alarm system?

A conventional system groups detectors into zones; the panel knows which zone has an alarm but not which specific detector activated. An addressable system assigns a unique address to each device; the panel identifies the exact device in alarm. Addressable systems are more complex to wire initially but easier to maintain and locate faults in larger buildings. EOL resistors are used in conventional zone loops; addressable loops use supervised digital communication.

What value EOL resistor should I use?

The EOL resistor value is specified by the fire alarm control panel (FACP) manufacturer — it is not generic. Common values include 4.7 kΩ and 10 kΩ, but some panels use 6.8 kΩ or other values. Using the wrong resistance will cause the panel to generate a fault or fail to detect an alarm condition correctly. Always use the value specified in the panel installation manual.

What is the NAC circuit?

The Notification Appliance Circuit (NAC) is the panel output circuit that powers and drives alarm notification devices — horns, bells, sounders, strobes, and voice evacuation speakers. Like detection zones, the NAC is a supervised circuit with its own EOL resistor to detect wiring faults in the sounder circuit. A fault on the NAC would silently disable alarm sounders, which is why supervision is critical.

Why does a fire alarm panel show 'Ground Fault' on a zone?

A ground fault occurs when a conductor in the zone loop makes unintended contact with the building's earthing system or metal conduit. On low-voltage supervised circuits, a ground fault changes the loop impedance unpredictably and can cause intermittent faults and alarms. Locating a ground fault requires a megohmmeter to measure insulation resistance of the zone conductors to earth while progressively isolating sections of the loop.

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