What is the procedure of cct brkr selection in terms of overcurrent protection

How do you select a circuit breaker?
There are some unusual situations, but for simplicity
let's assume we are looking at typical residential style
panels and circuit breakers.
First, what is the load or the power requirements of the item to be
connected? Most household items can be connected to a 15 ampere or 20 ampere
receptacle so the breakers would be 15 ampere and 20 ampere also.
Electric dryers usually take 240 volt, 30 ampere receptacles which would require
a two-pole 30 ampere circuit breaker...

Now the question It appears you are heading for, is the short circuit or fault current rating of the circuit breakers. Usually you will need to contact the serving utility for that information. Available fault current is related to the size of the transformer supply and the length and type cable from the supply to the panel.
Most residential services are adequately protected by 22,000 AIC rated breakers. If you live in the city with underground networks the utility fault current can be much higher so you will need to consult them for a safe rating.
Note that the above is assuming location in the USA.

Did this answer your question?

Thanks for the answer its given me an insight into the calculative process but I am in south Africa so does this same process apply ?

In a general way, yes...
Can you give me some background for why this
question has come up? The more I know the better
the answers can be.