EN 54-7 · BS 5839-7 · 2-wire loop · 4-wire addressable · 9 min read

Smoke Detector Wiring: 2-Wire and 4-Wire Loop Configurations

Correct wiring of fire detection loops is fundamental to reliable alarm operation. This guide covers conventional 2-wire zone loops with end-of-line resistors, addressable 4-wire loops with Hochiki ESP and Notifier FlashScan protocols, Class A and Class B loop configurations, detector spacing rules and the cable specifications required under BS 5839-7 and EN 50200.

Safety notice: Fire detection system wiring must be carried out by engineers competent in BS 5839-1 (UK) or EN 54 (EU). Incorrect wiring — wrong loop resistance, missing end-of-line resistor, mixed cable types — will cause the panel to flag a fault or, worse, fail to detect a genuine fire. All wiring must be tested and witnessed by a qualified commissioning engineer.

2-wire conventional loop: how it works

A conventional 2-wire zone loop uses the same pair of conductors for both power supply to the detectors and the alarm signal back to the panel. The zone card on the fire panel energises the loop at 24V DC; detectors draw a small standby current and a much larger current when in alarm.

Electrical characteristics

• Loop supply voltage: 24V DC from panel zone card
• Standby current per detector: 50–100 μA (varies by model)
• Alarm current per detector: ~60 mA (causes voltage drop — panel detects alarm)
• End-of-line (EOL) resistor: typically 10 kΩ fitted at the last detector
• Open circuit (cable break): panel reports zone fault — EOL resistor current absent
• Short circuit: panel reports zone fault — current exceeds alarm threshold without detector trigger

Zone current calculation

• Maximum zone current (standby): limited by zone card — typically 300–500 mA total
• 32 detectors at 100 μA each = 3.2 mA standby — well within limit
• Maximum detectors per zone: 32 conventional detectors (BS 5839-1 guideline)
• Zone card alarm current capacity: must handle all detectors in alarm simultaneously
• Always confirm zone card maximum current from panel datasheet before designing loop
• Use detector manufacturer's standby and alarm current specifications for precise sizing

Conventional 2-wire loop wiring diagram

Zone card (panel)
    +24V ─────┬─────── Detector 1 ───── Detector 2 ─── ... ─── Detector N
              │            │                 │                       │
             COM ──────────┴─────────────────┴───────────────────────┤
                                                              EOL resistor
                                                              10 kΩ (typical)
                                                              fitted at last
                                                              detector base

Normal (standby):
  Current path: +24V → detectors (50-100μA each) → EOL 10kΩ → COM
  Panel measures: expected standby current (loop healthy)

Alarm (detector triggered):
  Detector internal circuit: reduces loop resistance → current rises to ~60mA
  Panel measures: current spike above alarm threshold → ALARM

Fault (open circuit — cable break):
  EOL resistor disconnected → no current → panel detects OPEN CIRCUIT → FAULT

Fault (short circuit):
  +24V shorted to COM → excess current → panel detects SHORT → FAULT

4-wire addressable loop: Hochiki ESP and Notifier FlashScan

Addressable loops use a 4-core cable — two conductors for power (24V and 0V) and two for a data bus (A and B). The panel polls each device's unique address every 3 seconds; each detector responds with its current status, analogue sensitivity reading and any fault conditions. This gives device-level identification instead of zone-only.

Hochiki ESP protocol

• Address range: 1–127 devices per loop (rotary switch or software addressing)
• Poll cycle: every 3 seconds per device — panel receives analogue value
• Data format: proprietary ESP protocol — panel reads sensitivity drift over time
• Detector types: DCP-E (optical), DFJ-E (heat), DCSP-E (multi-sensor)
• Max loop resistance: 40Ω total (A+B conductors combined)
• Cable: 4×0.75mm² screened, BS 5839-7 CPC fire-rated

Notifier FlashScan protocol

• Address range: 1–99 devices per loop (rotary switch physical addressing)
• Poll cycle: scan every 3–4 seconds, faster in pre-alarm state
• FlashScan CLIP mode: compatible with older one-man-one-vote detectors
• Detector types: System Sensor FAPT951, FAOT951 optical, FAHT951 heat
• Max loop resistance: 40Ω total
• Class A loop: both ends return to panel (loop card with 2 × Class B connections)

4-wire addressable loop — Class B (spur) vs Class A (loop)

CLASS B (spur) — EN 54-2 Clause 13.1:
  Panel loop card
    A+ ──── Det.1 ──── Det.2 ──── Det.3 ─── [end of spur]
    A- ────────────────────────────────────── (return)
    B+ ──── [open end — fault if cable break between any detectors]

  Fault: cable break → detectors beyond break go offline
  Use: small buildings, short spurs, low risk of cable damage

CLASS A (loop) — EN 54-2 Clause 13.2:
  Panel loop card
    A+/A- ──── Det.1 ──── Det.2 ──── Det.3 ──── Det.N ──── B+/B-
                                                               │
                                           Loop returns to panel (second pair)

  Fault: cable break → panel detects break AND isolates section
         Remaining detectors on both sides of break still operational
  Use: commercial buildings, escape routes, high-risk areas
  Note: requires addressable loop isolator modules between detectors

Cable specification: BS 5839-7 and EN 50200

Cable selection for fire detection systems is governed by BS 5839-7 (UK) and EN 50200 (EU). Both standards require fire-rated cables that maintain circuit integrity under fire conditions — allowing the system to continue operating and signal alarms even as the building structure is affected.

Application	Cable type	Cross-section	Screening	Fire rating
Conventional 2-wire zone	FPLR or BS 5839-7 CPC	2×1.5mm²	Not required	FE180/E30 minimum
Addressable 4-wire loop	BS 5839-7 CPC screened	4×0.75mm²	Overall screen required (data integrity)	FE180/E90 (90 min integrity)
Sounder circuits	FPLR or BS 5839-7 FP200 Gold	2×1.5mm²	Not required	FE180/E30 minimum
Panel power supply	Fire-rated 2-core	2×2.5mm² or per load	Not required	FE180/E60 minimum
Relay output to KNX	Fire-rated (fire side) + LSZH (KNX side)	2×1.5mm²	Not required on KNX side	FE180/E90 on fire side

Cable laying rules

• Minimum separation from power cables: 50mm clear air gap, or fire-rated partition between
• Separate conduit or trunking from all other cable types — never share with data, AV or power
• Fire-resistant fixings: use ceramic or stainless steel clips, not standard plastic P-clips
• Cable penetrations through fire-rated walls: use intumescent fire-stop collars rated to at least the cable's fire performance rating
• Screen earth: addressable loop cable screen must be earthed at one end only (typically panel end) to avoid earth loops

Smoke detector types and EN 54 standards

Different detector technologies respond to different fire signatures. Specifying the correct detector type for each environment is critical to both detection performance and false alarm rate.

Ionisation smoke detector

Formerly EN 54-7

• Technology: Americium-241 radioactive source ionises air — smoke particles disrupt ion current
• Best for: fast-flame fires with little visible smoke (paper, thin wood)
• Not suitable for: slow smouldering fires, dusty environments
• Regulatory status: banned in many EU states due to radioactive source — check local law before specifying
• Replacement: optical/photoelectric detector now standard in most EU markets

Photoelectric / optical smoke detector

EN 54-7 certified

• Technology: infrared LED scatter — smoke particles scatter light onto photodiode
• Best for: slow smouldering fires, most commercial and residential applications
• Standard choice for the majority of KNX-integrated fire projects
• Lower false alarm rate than ionisation in most environments
• Example: Hochiki DCP-E, Apollo Discovery optical, System Sensor FAOT951

Heat detector

EN 54-5 certified (Class A1R)

• Technology: thermistor or fusible alloy — triggers at fixed temperature or rate-of-rise
• Class A1R: fixed threshold 58°C + rate-of-rise ≥ 3°C/min
• Best for: kitchens, garages, boiler rooms — environments where smoke detectors false alarm
• Not suitable for areas where early warning of smouldering fire is needed
• Do not use as substitute for smoke detector in occupied escape routes

Multi-sensor (combination) detector

EN 54-29 certified

• Technology: optical + heat combined — both sensors must reach threshold for alarm
• Significantly lower false alarm rate than single-technology detectors
• Standard specification for commercial KNX projects in the EU and UK
• Addressable analogue panels track optical AND heat values independently
• Example: Hochiki DCSP-E (optical + heat), Apollo Soteria multi-sensor

Detector spacing rules: EN 54-7

EN 54-7 defines the maximum floor area per optical smoke detector and adjusts coverage for ceiling height, obstructions and roof geometry. These rules are mandatory — insufficient coverage invalidates the EN 54-2 system approval.

Condition	EN 54-7 rule	Practical implication
Open area coverage	60m² per detector maximum in flat ceiling open areas	Grid layout: 7.8m × 7.8m spacing for flat ceiling office
Ceiling height ≤ 6m	Standard 60m² coverage applies	No reduction required for standard commercial floor-to-ceiling height
Ceiling height 6–9m	Coverage area reduced — consult manufacturer spacing table	Detector spacing must be reduced; use manufacturer's fire engineering data
Ceiling height > 9m	High-sensitivity or aspirating detector required (EN 54-20)	Standard optical detectors cannot detect smoke reliably at very high ceilings
Beam obstruction	Beams > 300mm deep create separate detection zones below the beam	Detectors required in each bay formed by beams — do not space across structural beams
Pitched roof	Detector within 600mm of apex (ridge)	Apex is highest smoke accumulation point — always place first detector at apex
Partitioned areas	Each partitioned space > 0.1m² requires a detector	Office pods, server rooms, storage rooms each need at least one detector

False alarm management (AFD)

Analogue addressable panels continuously track each detector's sensitivity drift and can apply confirmation delays before triggering the alarm output. This Automatic Fire Detection (AFD) logic significantly reduces unwanted alarms without compromising genuine fire detection.

Sensitivity drift monitoring

• Panel reads optical chamber value every 3 seconds
• Tracks drift over time — dirty chamber raises baseline
• Panel flags detector as 'dirty' before it causes false alarm
• Maintenance alert before unwanted alarm — not after
• Addressable only — conventional panels cannot do this

Cause and effect delay (confirmation)

• Single detector alarm → 30-second confirmation delay
• If alarm persists after 30 seconds → full alarm output
• If alarm clears within 30 seconds → investigate mode, no output
• Two detectors in same zone alarm → immediate output (no delay)
• Delay configured per zone in panel software — BS 5839-1 Annex D guidance

Cross-zone detection

• Two detectors in the same zone must both alarm before output
• Prevents single burnt toast or steam event triggering evacuation
• Requires at least 2 detectors per zone for cross-zone to work
• Common in commercial kitchens, laundries, dusty environments
• Fire engineer must confirm cross-zone logic meets BS 5839-1 L-grade

Maintenance schedule

BS 5839-1 Section 9 and EN 54-2 require a documented maintenance programme. The schedule below is the minimum — high-dust or high-use environments require more frequent inspection.

Frequency	Task	Method	Record
6-monthly	Sensitivity test of all detectors	Addressable: read analogue value from panel log. Conventional: aerosol smoke test at each head	Log sensitivity readings — compare to baseline at commissioning
6-monthly	Test all sounders and visual indicators	Trigger test alarm — verify all sounders operate in each zone	Record any failures; replace immediately
6-monthly	Verify all relay outputs operate	Trigger test mode — confirm KNX integration relay outputs operate and reset correctly	Log relay output test results in O&M record
Annual	Full function test under BS 5839-1	Simulate fire in each zone — verify cause-and-effect actions including KNX outputs	Issue annual service certificate
Annual (high-dust)	Replace detector heads in high-dust areas	Kitchen extract, workshops, garages — high contamination rate reduces detector life	Record head replacement in panel logbook
Every 5 years	Replace all detector heads (BS 5839-1 guidance)	Detector sensitivity drifts irreversibly after 5–10 years — replace preventively	Update commissioning record with new head serial numbers

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