Inside an 'iron box'

[Guest Fresh]

Dear gentlemen, I need a help but then it could be a bit out of the topic at this forum but I still hope many of you can help me... Thanks I ever seen the machines which usually comes with an iron box in which inside is containing a lot of breakers, contactors, noise filter, or more precisely is electrically based components. Well, I am wondering of how to get these components been build up and how they get it. For eg, how to get a 10A breaker, 3A and different type of current breaker. Also, there is some different types of breaker like class a, b etc type breaker. Then, how to calculate the total current of the m/c using and then selecting the breaker rating. Also, like how is the concept of doing these when eg, there is incoming power supply comes in then how to divide the power into many breakers and then when to use noise filter, current leakage breaker/relay, etc..... Is there any help or website out there that explain or introduce this kind of wyring inside an iron box? I sincerely need the 'help' and want to know more about this. Hope someone might be able to show me some 'guides' or 'references'... Thank you, thank you...