# Cabling/Wiring Rules ## Overview Correct cabling practices are fundamental to the reliability of life safety, security, and electrical systems. Poor segregation, inadequate fire resistance, or unsuitable fixings can compromise both system performance and occupant safety. The principal reference standards are: * **BS 5839-1:2025** - Fire detection and fire alarm systems (Clauses 25, 27, 28, 35). * **BS 7671:2018+A3 (IET Wiring Regulations)** - Electrical installation requirements. * **BS 5266-1** - Emergency lighting. * **BS EN 50174** - Information technology cabling (data networks). --- ## Fire Resistance Requirements Life safety systems (fire alarms, smoke control, emergency lighting, evacuation comms) must remain operational during a fire. * **Standard fire-resisting cable**: 30 min survival (BS EN 50200 PH30 + water spray). * **Enhanced fire-resisting cable**: 120 min survival (BS EN 50200 PH120 + BS 8434-2). * Required for unsprinklered buildings >30 m, phased evacuation, hospitals, and other high-risk sites. * Acceptable types: * Mineral insulated copper cable (MI, BS EN 60702-1). * LSZH fire-resistant (BS 7629-1). * Armoured fire-resistant (BS 7846). Security and communications systems do not normally require enhanced fire resistance unless they are part of the **life safety strategy** (e.g. smoke control fans, firefighter telephones). --- ## Segregation of Circuits ### General (BS 7671 Section 528) * LV and ELV circuits must be segregated or insulated for the highest voltage present. * Data and signal cables should be segregated from power to reduce electromagnetic interference. ### Life Safety Systems (BS 5839-1:2025, Clauses 25 & 27) * Fire alarm circuits must be routed independently of other services. * Shared containment only allowed with earthed metallic dividers. * Multicore cables must not mix fire alarm conductors with non-fire circuits. ### Security Systems * Access control and intruder cabling should be segregated from mains to reduce interference and prevent induced voltages. * CCTV coax and Cat6 runs should be routed separately from mains unless screened. --- ## Cable Support and Fixings * **Non-combustible supports required**: All fire alarm and escape-route cabling must be secured with metal fixings (BS 5839-1:2025 Clause 35.3, BS 7671 Reg 521.10.202). * Plastic clips/ties/trunking cannot be sole means of support. * Prevents premature collapse in fire, protecting escape routes and firefighters. --- ## Earthing and Identification * Fire alarm circuits must follow BS 7671 earthing rules (Clause 28 of BS 5839-1:2025). * Functional Earth (FE) conductors must be pink or labelled “FE” (BS 7671 Table 51). * Screened data/security cables should be earthed in accordance with BS EN 50174. --- ## Voltage Drop and Conductor Size * **Fire alarm circuits**: min 1 mm² CSA (BS 5839-1:2025 25.7). * **Emergency lighting**: voltage drop must not exceed 5% of nominal supply (BS 5266-1). * **Security/data cabling**: follow manufacturer recommendations; PoE circuits must meet BS EN 50174-2 design current ratings. --- ## Practical Design Rules | System | Fire Resistance | Segregation | Support | Notes | | ------------------------- | ----------------------------- | ---------------------------------- | ------------------------------------------------------ | --------------------------------------------------------------------------- | | Fire alarm | Standard or Enhanced FR cable | Dedicated containment | Metal fixings only | Enhanced cable in high-rise/phased evacuation. | | Emergency lighting | Standard FR cable | Separate from power | Metal fixings only | Central battery feeds must be protected. | | Smoke control | Enhanced FR cable | Segregated, monitored | Metal fixings | BS 7273-6 applies. | | Security - Access control | LSZH recommended | Segregated from mains | Not fire-rated unless doors form part of fire strategy | If interfaced with fire alarm, follow BS 7273-4. | | Security - CCTV | LSZH coax or Cat6 | Segregated from LV unless screened | Plastic allowed, metal preferred | Fire rating not required unless evacuation-critical. | | Data/IT | LSZH Cat6/Cat6A | Segregated per BS EN 50174 | Any fixings, but avoid plastic-only in escape routes | PoE current limits must be observed. | | Power circuits | PVC/XLPE | General segregation | Any fixings, except in escape routes | Escape route wiring must also be fire-resistant if powering safety systems. | --- ## References * BS 5839-1:2025 Clauses 25, 27, 28, 35. * BS 7671:2018+A3 (Regs 528, 521.10.202, 560.8). * BS 5266-1 Emergency lighting. * BS EN 50174 Information technology cabling.