Summary

Cable installation is the invisible infrastructure of every security system. Done well, it is permanent, reliable, and unobtrusive. Done poorly, it causes interference, false alarms, failed connections, and requires ripping out walls years later when problems emerge.

The security industry has specific cable requirements that differ from general electrical wiring. Security cables carry low-voltage signals (6–24 V DC typically) and are susceptible to electromagnetic interference from mains power cables, fluorescent lighting ballasts, motors, and RF sources. Screening, segregation, and correct earthing are not optional extras — they are the difference between a system that works flawlessly and one that generates constant false alarms or fails intermittently.

Documentation is equally important. A security system cable schedule is an asset — it enables fault finding, system expansion, and maintenance by anyone (not just the original installer). An undocumented system is a liability.

Key Facts

  • BS 7671:2018+A2:2022 — IET Wiring Regulations; Section 528 governs segregation of low-voltage systems from mains power
  • Segregation distance — minimum 50 mm separation from mains power cables in open air/conduit; 100 mm at crossing points; 0 mm if segregated by earthed steel conduit, trunking, or partition
  • Screened cable — has an aluminium foil or braided copper screen around the signal conductors; drain wire connects screen to earth; used where EMI is a risk
  • Screen earthing — earth the screen at one end only (typically the panel/controller end); earthing both ends creates a ground loop which introduces interference, not eliminates it
  • Alarm cable types — standard: 6-core, 8-core unscreened; PIR/detector: 4-core 0.22 mm² screened; long-run or industrial: screened twisted pair
  • CCTV coaxial — RG59 (75Ω) for analogue CCTV up to 300 m; RG6 for longer runs or to minimise signal loss; Cat6 for IP cameras with PoE
  • RS-485 data cable — Belden 9841 (single pair screened) or equivalent; 120 Ω termination resistors at each end of the bus
  • Power cable sizing — calculate voltage drop: V = I × R × 2L (both ways); target < 5% of supply voltage at the furthest device
  • Cable concealment — all cables within secure areas should be concealed in walls, floors, or ceiling voids; surface-run cables in trunking where concealment is not possible
  • Tamper monitoring — cables in accessible/unsecured areas must be run in tamper-monitored conduit or have end-of-line resistors at each device to detect circuit interruption
  • Cable labelling — every cable must be labelled at each end and at junction points with a unique identifier matching the cable schedule
  • Fire-rated cable — use LSZH (Low Smoke Zero Halogen) cable throughout for fire alarm systems; recommended for security systems in occupied buildings
  • COSHH / Asbestos — do not drill or chase walls in buildings pre-1999 without asbestos survey confirmation; drilling asbestos-containing materials is a notifiable exposure

Quick Reference Table

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Cable Type Typical Use Cores × Area Screening Max Run
Alarm cable 6-core General detection zones 6 × 0.22 mm² Unscreened 100 m typical
PIR detector cable PIR detectors, vibration detectors 4 × 0.22 mm² screened Foil + drain 100 m
RS-485 data Keypad, expander, reader bus 1 × 0.22 mm² screened pair Foil + drain 1,200 m
CCTV RG59 coax Analogue cameras 75 Ω Braided 300 m
CCTV Cat6 IP cameras, PoE 4 × UTP pairs Unscreened (Cat6) / Screened (Cat6A STP) 90 m (PoE)
Door entry 2-wire Analogue intercom BUS 2 × 0.75 mm² Unscreened 200–300 m
Maglock cable Magnetic lock power 2 × 1.0 mm² Unscreened 50 m (size for current)
Stranded hook-up Panel internal wiring Various N/A N/A

Detailed Guidance

Screened vs Unscreened: When to Use Each

Unscreened cable is adequate for:

  • Short runs (< 30 m) in domestic properties with no significant EMI sources
  • Bell/sounder wiring
  • Magnetic contact (door/window) wiring
  • Internal panel connections

Screened cable is required for:

  • PIR detector wiring (the pyroelectric sensor is EMI sensitive)
  • RS-485 data buses (keypad, expander, reader communications)
  • Runs exceeding 50 m in any commercial environment
  • Runs parallel to mains cables for more than 5 m
  • Industrial environments with motors, drives, or fluorescent lighting
  • Vibration detector wiring (passive seismic detectors are sensitive to electrical noise)

Braided screen vs foil screen:

  • Foil screen (aluminium foil + drain wire): lower cost, lighter; adequate for most security applications
  • Braid screen (copper braid): higher screening effectiveness (90%+ coverage); used in demanding industrial environments
  • Overall screened + individually screened pairs: best for multiconductor data cables

Earthing the screen correctly:

  • The drain wire from the screen connects to the signal earth at the controller/panel — never the mains earth
  • The far end of the screen should be left unconnected or connected to signal earth only if local earth potential is known to match the controller end
  • Ground loops (screen earthed at both ends) can actually increase interference by allowing circulating currents through the screen

Segregation from Mains Power

BS 7671:2018+A2:2022 Section 528 specifies minimum segregation distances. For security systems carrying SELV (Safety Extra Low Voltage — typically 12–24 V DC):

Condition Minimum Separation
Open routing alongside mains 50 mm
Crossing mains at right angles 100 mm gap or insulated crossing
In same conduit/trunking as mains NOT PERMITTED (unless Category 3 separation)
Separated by earthed metallic partition 0 mm (partition provides Category 3 separation)
In separate compartment of multi-compartment trunking 0 mm (compartment provides Category 2 separation)

Practical routing:

  • Run security cables on the opposite side of the ceiling void or wall cavity to mains cables
  • Where crossing is unavoidable, cross at 90° — this minimises the inductive coupling length
  • In trunking, use dedicated low-voltage compartments or a separate trunking run
  • Never bundle security cables with mains in the same conduit

Voltage Drop Calculation

For DC-powered devices (PIR detectors, panels, locks), voltage drop on the cable is critical. A PIR at the end of a long cable run may not receive enough voltage to operate reliably if the cable is too light.

Formula:

Voltage drop (V) = (2 × cable length in metres × current in amps) ÷ (cable CSA in mm² × 56)

(56 = conductivity of copper in m/Ω·mm²)

Example: 100 m run, PIR drawing 15 mA, 6-core 0.22 mm² cable:

V = (2 × 100 × 0.015) ÷ (0.22 × 56) = 3 ÷ 12.32 = 0.24 V drop

On a 12 V supply, 0.24 V drop is 2% — acceptable. But a maglock drawing 0.5 A on the same 100 m cable:

V = (2 × 100 × 0.5) ÷ (0.22 × 56) = 100 ÷ 12.32 = 8.1 V drop

Unacceptable — the maglock receives only 3.9 V. Upsize to 1.0 mm² and the drop becomes 1.8 V (acceptable on a 12 V supply).

Cable Mapping and Documentation

Every professionally installed security system requires a cable schedule. This document is an asset that facilitates:

  • Fault finding by any competent engineer
  • System expansion without disruption
  • Insurance and due diligence documentation
  • Compliance with NSI/SSAIB audit requirements

Minimum cable schedule content:

  • Cable reference number (must match the label on both cable ends)
  • Origin (panel zone, controller port, power supply terminal)
  • Destination (device type, device reference, room/location)
  • Cable type and specification
  • Screen connection method
  • Installed date and installer name

Additional documentation:

  • Floor plan with cable routes marked
  • Device location plan showing all detectors, cameras, keypads, panels
  • Zone schedule for intruder alarm panels
  • Camera ID plan for CCTV systems (camera number, view direction, field of view)

For NSI/SSAIB approved installations, as-fitted drawings are a compliance requirement and must be provided to the customer at handover.

Common Installation Faults and Prevention

Intermittent fault — cause: poor termination Use appropriate ferrules (bootlace crimps) on stranded conductors before inserting into screw terminal blocks. Bare stranded wire in screw terminals loosens under vibration over time. For PIR detectors, use 0.22 mm² ferrules; for power cables, use the correctly sized ferrule.

False alarms — cause: unscreened PIR cable in EMI environment Retrofit screening: separate the PIR cable from EMI sources, or replace with screened cable. As a temporary fix, add a 100 nF ceramic capacitor from each conductor to the signal earth at the PIR (damper capacitors against RF pickup) — but proper screened cable replacement is the correct fix.

Short circuit — cause: cable damage during installation Always draw a cable (pull the figure 8 slack out) before cutting to length. Use a cable puller for long runs in conduit rather than brute force. Never pinch cable in door frames, under skirting, or at penetrations. Use appropriate grommet/bushes at all penetrations.

Intermittent connection — cause: cable movement stress Dress cables with adequate loop at each device to allow for movement. Secure all cable runs at 300–400 mm intervals. At devices that may be removed for maintenance (PIRs, keypads), leave 150–200 mm service loop.

Frequently Asked Questions

Does security cable need to be red like fire alarm cable?

No mandatory colour requirement applies to intruder alarm cabling in UK standards. However, industry convention uses white or grey for security cables and red for fire alarm cables. Using colour consistently within your installations makes identification easier. Fire alarm cables to BS 5839-1 are a different matter — cabling for fire alarm systems has specific requirements for cable specification and route protection.

Can I run CCTV and alarm cables in the same conduit?

Yes, provided both are SELV (≤50 V AC or ≤120 V ripple-free DC) and there is no mains power in the conduit. CCTV coax and alarm signal cables are both SELV. However, if the CCTV cables carry 12 V or 24 V DC power (PoC — power over coax), keep them away from sensitive alarm signal cables due to potential interference.

What type of conduit should I use for external runs?

External cable runs should use UV-resistant HDPE or PVC conduit (minimum 20 mm diameter for small cables). Metal conduit provides better tamper resistance but corrodes unless stainless steel or galvanised and sealed at penetrations. Where conduit is buried, use HDPE duct rated for direct burial. All conduit entries to the building must be sealed against moisture ingress.

How should I label cables?

Use heat-shrink labels or adhesive labels rated for the temperature range (typically -30°C to +80°C). Label at each end of the cable and at each junction box or accessible point. Label format should match the cable schedule: cable reference, origin, and destination. Avoid labels that rely on handwriting — thermal printed labels are far more legible after years in a roof void.

What is LSZH cable and when is it required?

LSZH (Low Smoke Zero Halogen) cable is specified where, in a fire, toxic halogen gases from burning cable insulation pose a risk to evacuees. Standard PVC cable releases hydrogen chloride on burning — corrosive and toxic in enclosed spaces. LSZH is mandatory for fire alarm systems (BS 5839-1 requires FP or PH fire-performance cable on circuits requiring cable integrity). It is good practice for security cables in occupied buildings, hospitals, schools, and transport infrastructure. Confirm with the building's fire risk assessment and fire alarm designer.

Regulations & Standards

  • BS 7671:2018+A2:2022 (IET Wiring Regulations) — Section 528: co-location and segregation of circuits; Section 411 for SELV protection

  • BS EN 50131-1:2006+A3:2022 — General requirements; tamper protection and cable monitoring for intruder alarm systems

  • BS 5839-1:2017 — Fire detection and fire alarm systems; Part 1: Code of practice for design, installation, commissioning and maintenance; cable routing and integrity requirements

  • PD 6662:2017 — UK application document for BS EN 50131; cable and wiring requirements for graded intruder alarm systems

  • NSI Code of Practice (NCP 109) — NSI installation standard referencing cable documentation requirements

  • BSIA Form 180 — As-installed drawings guidance from the British Security Industry Association

  • IET Wiring Regulations BS 7671 — IET publication, 18th Edition Amendment 2

  • NSI Codes of Practice — NSI technical installation standards

  • BSIA Technical Notes — British Security Industry Association cable and installation guidance

  • Belden Cable Selection Guide — Reference for screened cable selection for security/data applications

  • SWA Cable Technical Guide — Steel Wire Armoured cable and conduit wiring guidance

  • security system commissioning — Commissioning tests that verify cable integrity

  • intruder alarm grades — Cable requirements by system grade

  • access control systems guide — RS-485 and data cable requirements for access control

  • nvr dvr storage sizing — Cat6 infrastructure for IP CCTV systems

  • earthing bonding — Earthing principles relevant to screen earthing