The consumer units have changed over the last twenty years by the move from fused circuits controlling the system for the fail safe position to the introduction of the circuit breaker. The advantage is that the fused circuit when it blows a fuse needs a replacement fuse or fuse wire. This was difficult and time consuming when it had to be done without or with little light. The circuit breaker just needs to be reset and provided the electrical fault has been solved then the electrical supply can easily be restored.
Whilst they have only recently been manufactured into the modern consumer units they have been around for a long time. The first circuit breaker was designed by Thomas Alva Edison in a patent in 1879, although his lighting circuits were protected by fuses the circuit breaker was used for accidental shorting and overloaded circuits.
The designs changed over the years and there are a myriad of different designs now that are made to deal with a complete range of problems from small domestic systems to very large current and voltage systems.
The typical circuit breaker requires a number of design features, firstly it must be able to detect an overload or fault and hence must have some sort of measuring system, secondly it must have a system of forcing two contacts apart in order to break the circuit.
The third problem and one which probably caused the most serious thought was to do with arc suppression. When two contacts are forced apart with a current still flowing they draw an arc across the contact gap. It is necessary to extinguish that gap quickly and efficiently without causing arc damage to the contacts. The ways of doing this are many up to the introduction of high pressure gases. Other ways are lengthening the arc, splitting the arc and intensive cooling. The smaller breakers have a solenoid to break the system an arc suppressor and or divider to get rid of the arc and a bimetal strip system to measure the fault.
The original problem was how to get all this into a small space not much bigger than the old fuse cartridge, the circuit breaker also had to be designed of materials that would not deform of break under the heating loads during fault conditions. This has been very successfully carried out and the modern circuit breakers are in operation in virtually every new consumer unit.