Fire Alarm Wiring Topologies: Conventional vs Addressable, Radial vs Loop and Cable Specification

Summary

The wiring topology of a fire alarm system determines how devices communicate with the panel, what happens when a cable breaks, and how much information the panel has about individual device status. Getting the topology right at design stage saves significant rework later — the topology choice also affects the cost and time of installation.

The shift from conventional to addressable systems has been the most significant change in fire alarm installation over the last 20 years. Addressable systems now dominate commercial installations above a certain size threshold (typically 20+ devices), while conventional systems remain cost-effective for small premises with simple layouts.

Understanding both topologies — and the cable requirements for each — is essential for any installer working on fire alarm systems. The cable specification is particularly important: using the wrong cable type on a circuit requiring fire-resistance integrity is a commissioning failure and a BS 5839-1 non-compliance.

Key Facts

• Conventional system — devices wired in circuits; all devices on one circuit = one zone; zone-level detection only; cheaper for small installations
• Addressable system — each device has a unique address (1–254 typically per loop); panel identifies individual device; loop or radial wiring
• Class B (radial/spur) — wire runs from panel to last device; end-of-line resistor at last device; a cable break creates an open circuit fault (zone/loop disabled beyond break)
• Class A (loop/ring) — wire runs from panel, through all devices on the loop, and returns to the panel at a second input; a single cable break is tolerated; loop continues to operate
• SLC (Signalling Line Circuit) — US-origin terminology for Class B or Class A addressable loops; used by some manufacturers (Simplex, Notifier, Kidde) in UK documentation
• ZDAL (Zener Diode Analogue Loop) — term used by some UK panel manufacturers for addressable loops
• Maximum devices per loop — typically 127–250 addressable devices per loop depending on panel manufacturer; always verify the panel data sheet
• Cable type — FP200 Gold — the most common fire-resistant cable for UK fire alarm applications; provides 1-hour circuit integrity at 1,000°C per BS 7629-1:2014; suitable for most circuits requiring integrity
• FP400 Plus — 2-hour integrity; used where extended circuit integrity is required
• PH30/PH60 cable — alternative fire performance cables; 30 or 60 minutes at 930°C; used for circuits not requiring the full 1-hour class
• MICC (Mineral Insulated Copper Clad) — highest integrity; used for critical circuits (emergency lighting, some fire suppression interfaces); withstands very high temperatures
• LSZH (Low Smoke Zero Halogen) — mandatory for fire alarm cable in most occupied buildings; produces minimal toxic smoke when burning; BS EN 50266
• Red sheath — UK fire alarm convention; fire alarm cables are red-sheathed to distinguish them from other services
• Earth screening — screened fire alarm cable used for long distances and areas with electrical interference; screen earthed at one end (panel end)
• BS 5839-1 Clause 26 — cable specification requirements; Clause 38 — cable installation requirements

Quick Reference Table

Detailed Guidance

Conventional Systems: Radial Circuits and EOLRs

A conventional fire alarm system consists of:

• Control panel (with zone cards)
• Detector/call point circuit per zone (typically 4-wire: signal pair + power pair, or 2-wire on older systems)
• End-of-line resistor (EOLR) at the last device on the circuit
• Sounder circuits (separate from detection circuits)

The EOLR is the monitoring mechanism. Under normal conditions, the panel sees a known resistance (e.g., 4.7 kΩ for a typical system). An open circuit (cable break or device disconnect) removes the EOLR from the circuit and the resistance goes to infinity — the panel generates an open circuit fault. A short circuit (cable damage, wiring error) takes the resistance to near zero — the panel generates a short circuit fault and may activate the zone.

Detection zones in conventional systems: Each detection circuit is one zone. All detectors on the circuit activate the same zone indicator. If zone 3 activates, the investigator goes to the area covered by zone 3 — but doesn't know which specific detector triggered until they physically check each one.

False alarm management in conventional: Limited. If one detector false-alarms, the whole zone activates. Isolating a faulty detector in a conventional system requires physically fitting an isolation resistor at the device, or disconnecting it (leaving the zone without that detector).

When to use conventional:

• Small buildings (shop, small office, single-floor premises)
• < 20 detectors total
• Budget-constrained projects where addressable premium cannot be justified
• Retrofits where existing conventional cable infrastructure is in good condition

Addressable Systems: Loops and Device Addressing

Addressable systems connect devices on a loop or radial circuit. Each device has a unique address — set by DIP switches, rotary encoders, or (in newer systems) automatically assigned by the panel. The panel polls each device in sequence, reading its status (normal/alarm/fault/tamper) and controlling its sounder output.

Panel polling cycle: A panel typically polls all devices on a loop every 1–3 seconds. An alarm event is communicated within one polling cycle of the device triggering.

Device capacity per loop: Varies by panel manufacturer. Common limits:

• 127 devices (1–127 addressing) — many conventional addressable panels
• 200 devices — mid-range commercial panels
• 250 devices — larger commercial panels (Apollo, Hochiki, Advanced)
• 254 devices per loop — typical upper limit for legacy 8-bit addressing

For large buildings, panels support multiple loops (2, 4, 8 loops) to provide the required device capacity.

Sounder control in addressable systems: In addressable systems, sounders are individually addressable — the panel can activate specific sounders while leaving others silent. This enables staged evacuation strategies (alarm only on the affected floor initially) that are not possible in conventional systems.

Class A vs Class B Wiring

Class B (radial/spur):

• Wire runs from panel output to each device in sequence; EOLR at the last device
• Simple and cheaper to install
• A single cable break disables all devices beyond the break
• Appropriate where cable fault tolerance is less critical, or where cables are well protected
• Also called "Style B" in some international documentation

Class A (loop/ring):

• Wire leaves panel on one terminal, passes through all devices, and returns to a second terminal on the panel
• The panel can communicate with every device via either path — a single cable break is transparent to operation; both halves of the loop continue to function
• More resilient than Class B
• Requires twice the cable if the loop is routed back along the same path (same physical route); ideally the return cable is routed via a different physical route for maximum resilience
• Required by BS 5839-1 for many commercial installations and for circuits requiring high integrity

When Class A is required:

• Large commercial buildings where a single cable fault should not disable a significant part of the system
• High-risk premises (sleeping risk, hospitals, large public buildings)
• Circuits between multiple panels in networked systems
• Where the fire safety strategy relies on the system remaining operational throughout a fire scenario

When Class B is acceptable:

• Small premises with simple layouts
• Short, easily accessible cable runs where cable fault probability is low
• Individual room or small zone extensions where cable break risk is acceptable

Cable Specification for Fire Alarm Systems

Circuit integrity requirement: BS 5839-1 Clause 26 requires cables on circuits that must continue to function during a fire to have fire-resistant properties (circuit integrity). These circuits include:

• Detector loops (fire could affect the detector circuits it needs to report on)
• Sounder circuits (must remain operational to alert occupants)
• Control circuits to interfaces (suppression release, door holders, etc.)
• Power supply cables from the PSU to the panel

FP200 Gold (General Electric type, Prysmian FP200 Gold, or equivalent) is the most common UK fire alarm cable:

• Construction: MICC-principle mineral-wrapped conductors within a red LSZH sheath
• Integrity: 60 minutes at 850°C under EN 50200 (Annex E) — equivalent to approximately 1 hour in a standard fire scenario
• Available in 0.5 mm², 1.0 mm², and 1.5 mm² (most common for detector circuits), 2.5 mm² for sounder and power circuits
• Voltage rating: 300/500 V

Choosing conductor size:

• Detection circuits (4-wire analogue, addressable loop): 1.5 mm² standard; 1.0 mm² for short runs
• Sounder circuits: 1.5 mm² or 2.5 mm² depending on sounder current draw and run length
• Check voltage drop: sounders must receive adequate voltage at the end of the circuit — calculate drop before specifying conductor size

Non-integrity circuits: Ancillary circuits that do not need to function in fire (remote indication displays, ancillary relay outputs) can use standard LSZH cable. This is cheaper than FP cable but must be clearly documented — never mix integrity and non-integrity circuits in the same conduit.

Conduit and trunking for fire alarm cable:

• Fire alarm cables should be in dedicated red conduit or trunking, separate from other services
• Metal conduit provides mechanical protection and can substitute for circuit integrity for short runs within enclosures, but is not typically used as the primary integrity method for fire alarm circuits in UK practice
• Minimum 25 mm separation from mains power cables (BS 7671:2018+A2:2022 Section 528)

Cable Installation Principles

Route selection: Route fire alarm cables to avoid areas of high fire risk where possible — the cable serving a detector in the server room should not pass through the kitchenette. Where unavoidable, ensure the cable has appropriate integrity rating for the route.

Cable fixings: Use proprietary cable clips or saddles at 400–500 mm intervals on surface runs. Never rest cables on suspended ceiling tiles unsupported — they must be clipped or on a cable tray.

Penetrations: All cable penetrations through fire compartment walls and floors must be fire-stopped. Intumescent sealant or proprietary fire-stop collars are required. The fire alarm cable itself does not provide the compartment seal — the stopping material does.

Conduit bends: Do not exceed 4× the conduit internal diameter for bends. Over-tight bends damage fire-performance cable and compromise the integrity of the insulation.

Frequently Asked Questions

Can standard white twin-and-earth cable be used for fire alarm circuits?

No. Standard twin-and-earth cable (T+E) is PVC insulated and sheathed — it provides no fire-resistance integrity. When PVC cable burns, it loses conductivity rapidly. All fire alarm circuits requiring integrity must use fire-resistant cable (FP200 or equivalent). T+E may be used for non-integrity ancillary circuits only, clearly documented as such.

What is the maximum loop length for an addressable system?

Loop length depends on the cable specification and the panel's power supply capacity. A typical 1.5 mm² FP200 loop can run to approximately 1,500–2,000 m total loop length (combined distance, going and returning). Beyond this, voltage drop reduces detector operating voltage below the minimum specified. Many manufacturers specify a maximum loop resistance (e.g., 40 Ω). Calculate actual resistance for the planned run before committing to the design.

Can I use the same cable run for detectors and sounders?

Detectors and sounders should be on separate circuits. This is because:

• Sounders may draw significantly more current than detectors, requiring larger conductor size
• A cable fault that takes out both the detector and the sounder for the same area is a serious single-point failure
• The alarm strategy may require independent control of sounders (staged evacuation)
• BS 5839-1 recommends separate circuits for detection and alarm

In very small systems (a single-zone domestic-scale commercial installation), combined circuits may be acceptable, but it is not best practice.

What is meant by "short-circuit isolation" on an addressable loop?

Addressable loop devices often include a built-in short-circuit isolator (SCI). If a short circuit occurs on the loop cable, the isolator in the adjacent devices automatically isolates the fault, allowing the rest of the loop to continue operating. This is distinct from Class A wiring — an SCI limits the impact of a short circuit, while Class A limits the impact of an open circuit (cable break). Premium addressable systems have SCI capability in every device; more basic systems have SCI at zone boundaries only.

Does the choice of conventional vs addressable affect the BAFE SP203-1 assessment?

No directly — both technologies can be used in SP203-1 certified installations. The assessor checks compliance with BS 5839-1, not which technology was used. However, if the design required device-level identification (for example, an L1 system in a large hospital where precise detector location is essential), and you used conventional wiring, the assessor would question whether the system meets the design intent even if it technically complies with the minimum zone size rules.

Regulations & Standards

• BS 5839-1:2017 — Clause 26 (cable specification), Clause 38 (cable installation); primary reference for wiring requirements

• BS 7629-1:2014 — Specification for fire-resistant electric cables; relevant for FP200-type cables

• BS EN 50200:2015 — Method of test for resistance to fire of unprotected small cables; the test standard for FP200 classification

• BS 7671:2018+A2:2022 — IET Wiring Regulations; Section 528 (segregation), Section 411 (SELV)

• BS EN 54-2:1997+A1:2006 — Control and indicating equipment standard; defines class A and class B circuit requirements

• BS EN 50266 — Common test methods for cables under fire conditions; LSZH classification

• Prysmian FP200 Gold Technical Data — UK manufacturer data for the most common UK fire alarm cable

• FIA Wiring Guidance — Fire Industry Association guidance on cable specification and installation

• Apollo Fire Detectors Wiring Guide — Example addressable loop wiring documentation

• Advanced Electronics System Design Guide — Class A and Class B loop design for addressable panels

• BS 5839-1:2017 — BSI standard; Clauses 26 and 38

• bs 5839 1 fire alarm standard — The standard governing all wiring requirements

• fire alarm zoning design — Zone structure that the wiring topology implements

• fire alarm commissioning procedure — Circuit testing during commissioning

• fire alarm detector types — Detector types connected to the wiring topology

• cable installation security systems — Parallel cable installation guidance for security systems