Most of the Consumer Units I install have a combination of MCB’s and RCD’s
An MCB, Miniature Circuit Breaker, provides overload protection for an individual circuit, say for example a circuit supplying power to your ground floor sockets. If there are too many appliances plugged into the sockets drawing power at the same time, the MCB will trip in order to protect the wires. Otherwise the wires will heat up until they burn up!
To solve this it would be neccessary to split the circuit, so that there are less sockets and therefore less demands for power. This is why the ground floor sockets are split between two circuits, the kitchen being one with all the other rooms on the a seperate circuit. Sometimes it is also necessary to create a third circuit for the utility as well.
An RCD, Residual Current Device, detects an imbalance in the flow of current or power, such as when a cable is cut, or an appliance develops a fault, and trips very quickly (less than 0.4 seconds). This is to prevent the user from getting a potentially deadly electric shock.
RCD’s are generally connected to a group of MCB’s, so provide additional protection to a number of MCB’s at the same time.
In short, MCB’s prevent fires, and RCD’s save lives!