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.

Important: Since 2016, all new and replacement consumer units must have a metal enclosure — plastic units are no longer permitted. Replacing a consumer unit is notifiable work that must be carried out by a registered electrician.

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. Older boards split circuits between two RCDs so a trip doesn't kill everything. Modern installations use RCBOs instead.

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, giving each circuit its own individual RCD protection. Now the standard approach in new domestic installations — a fault on one circuit trips only that circuit.

AFDD (Arc Fault Detection Device) — A newer device that detects dangerous arcing in cables and wiring — the kind of fault that can start a fire but won't trip an MCB or RCD. Required in new wiring in some premises under current regulations, and increasingly fitted as standard in modern installations.

MCB Ratings

Showers: Most domestic showers are 8.5kW or higher — these need a 40A or 45A MCB, not 32A. Always check the shower's rating plate and match the MCB and cable size to it. An undersized MCB is a fire risk.