Why upgrade
London Fire Brigade data 2014–2018: 1,500+ fires in plastic consumer units — overheating connections igniting enclosure. BS 7671:2018 (17th Edition Amendment 3, July 2015) mandated non-combustible enclosure (metal — typically steel) for new + replacement CUs in domestic. Plastic CUs installed pre-July 2015: not unsafe per se but lower fire-resistance margin; remove + replace with metal at next renovation. RCBO (residual current breaker with overcurrent) per circuit — current best practice (BS 7671:2018 Amendment 2, 2022) — each circuit individually protected; fault on kitchen ring does not trip lights. Older RCD-per-block: 1 RCD covers 5–6 circuits; nuisance tripping common. Surge Protection Device (SPD): protects against transient overvoltage from lightning/grid switching — increasingly recommended (mandated in new dwellings by BS 7671:2018 Amendment 2). Arc Fault Detection Device (AFDD): detects arcing faults (frayed flex, damaged cable) — recommended in bedrooms + lounges per BS 7671:2018 Amendment 2.
Specification
Metal CU per BS EN 61439-3 — typically MK Sentry, Hager Design 10/30, Wylex Amendment 3, Schneider Easy9 + Wiser; 10-way to 24-way modular. RCBO per circuit (typically 6A lighting, 16A immersion, 20A radial, 32A ring + cooker, 32A EV charger). SPD Type 2 mandatory for new + altered domestic (BS 7671:2018 A2 2022). AFDD: 1 per bedroom + 1 per lounge circuit (BS 7671:2018 A2 recommended). Main switch 100A (double pole isolator). Henley block + isolator outside CU recommended (allows CU swap without DNO disconnection). Earth bonding: 10mm² main earth to incoming gas + water + structural steel + extraneous metalwork; 4mm² supplementary bonding kitchen + bathroom. Labelling: every circuit identified + ratings + cable size; load schedule + earth-loop impedance values recorded on Electrical Installation Certificate.
Install process
Pre-install: EICR (Electrical Installation Condition Report) — £185–£385 — assesses existing wiring before CU upgrade. EICR findings may extend scope (replacement bonding, supplementary earthing, circuit identification). DNO notification for service-fuse swap if 60A → 100A upgrade needed (free via UK Power Networks). Install: power off via service fuse pull (electrician courtesy not permitted — usually DNO attend or installer manages with isolator above CU); existing CU removed; new CU mounted + connected; circuits one-by-one tested + commissioned; Earth-loop impedance + RCD trip times + insulation resistance measured. Re-energise + test all circuits. Documentation: Electrical Installation Certificate (EIC) covering CU + circuits affected, Minor Works Certificate for circuit alterations, Part P self-certificate filed with Building Control via competent-persons scheme. Typical 4–8 hours; full day if extensive rectification. Keep certificate — required at sale + insurance + future renovation.
