The distribution box, MCBs and fuses
The distribution box
The distribution box shares the supply between the lighting circuits and the socket circuits. A lighting circuit may take at most 6 A — enough for the lamps of ordinary rooms. Appliances that take more, such as kitchen heaters and ovens, go on socket circuits, which provide about 13 A.
Figure to be added
Miniature circuit breakers (MCBs)
Each circuit is fed through its own MCB. If more than its rated current flows, its lever drops and it switches that circuit off automatically — only that circuit, not the whole house. Lighting circuits use 6 A MCBs and socket circuits 13 A. It stops the cables overheating and catching fire when a fault draws too much current, but it gives no protection from a shock.
Old circuits used a 5 A fuse for a lighting circuit and a 15 A fuse for a socket circuit. Replacing a blown fuse wire is fiddly, so fuses are going out of use. An MCB or fuse always goes in the live wire.
The consumer unit
In new circuits the isolator, RCCB and distribution box sit together in one case, the consumer unit.
Figure to be added