Inside the Consumer Unit

The consumer unit (CU), sometimes called a fuse box or distribution board, is the heart of your home's electrical system. It receives the mains supply and distributes it to individual circuits, each protected by its own overcurrent device.

Anatomy of a Modern CU

Key Components

Main switch — Isolates all circuits simultaneously. Required in all modern CUs.

RCD (Residual Current Device) — Detects current leaking to earth (e.g. through a person) and disconnects in milliseconds. Modern CUs use split-load design with two RCDs so a trip on one doesn't cut all circuits.

MCB (Miniature Circuit Breaker) — Each circuit has its own MCB rated for that circuit's cable size. If current exceeds the rating, the MCB trips.

RCBO — Combines RCD and MCB in one unit. Used in high-risk circuits (kitchens, bathrooms, outdoors).

MCB Ratings

| Rating | Typical circuit | |--------|----------------| | 6A | Lighting | | 16A | Immersion heater | | 20A | Radial sockets | | 32A | Ring final, shower (small) | | 40A+ | Electric shower, cooker |